# Rust Compiler Error Index
## E0001

This error suggests that the expression arm corresponding to the noted pattern
will never be reached as for all possible values of the expression being
matched, one of the preceding patterns will match.

This means that perhaps some of the preceding patterns are too general, this
one is too specific or the ordering is incorrect.

For example, the following `match` block has too many arms:

```compile_fail,E0001
match Some(0) {
    Some(bar) => {/* ... */}
    None => {/* ... */}
    _ => {/* ... */} // All possible cases have already been handled
}
```

`match` blocks have their patterns matched in order, so, for example, putting
a wildcard arm above a more specific arm will make the latter arm irrelevant.

Ensure the ordering of the match arm is correct and remove any superfluous
arms.

## E0002

## Note: this error code is no longer emitted by the compiler.

This error indicates that an empty match expression is invalid because the type
it is matching on is non-empty (there exist values of this type). In safe code
it is impossible to create an instance of an empty type, so empty match
expressions are almost never desired. This error is typically fixed by adding
one or more cases to the match expression.

An example of an empty type is `enum Empty { }`. So, the following will work:

```
enum Empty {}

fn foo(x: Empty) {
    match x {
        // empty
    }
}
```

However, this won't:

```compile_fail
fn foo(x: Option<String>) {
    match x {
        // empty
    }
}
```

## E0003

## Note: this error code is no longer emitted by the compiler.

Not-a-Number (NaN) values cannot be compared for equality and hence can never
match the input to a match expression. So, the following will not compile:

```compile_fail
const NAN: f32 = 0.0 / 0.0;

let number = 0.1f32;

match number {
    NAN => { /* ... */ },
    _ => {}
}
```

To match against NaN values, you should instead use the `is_nan()` method in a
guard, like so:

```
let number = 0.1f32;

match number {
    x if x.is_nan() => { /* ... */ }
    _ => {}
}
```

## E0004

This error indicates that the compiler cannot guarantee a matching pattern for
one or more possible inputs to a match expression. Guaranteed matches are
required in order to assign values to match expressions, or alternatively,
determine the flow of execution. Erroneous code example:

```compile_fail,E0004
enum Terminator {
    HastaLaVistaBaby,
    TalkToMyHand,
}

let x = Terminator::HastaLaVistaBaby;

match x { // error: non-exhaustive patterns: `HastaLaVistaBaby` not covered
    Terminator::TalkToMyHand => {}
}
```

If you encounter this error you must alter your patterns so that every possible
value of the input type is matched. For types with a small number of variants
(like enums) you should probably cover all cases explicitly. Alternatively, the
underscore `_` wildcard pattern can be added after all other patterns to match
"anything else". Example:

```
enum Terminator {
    HastaLaVistaBaby,
    TalkToMyHand,
}

let x = Terminator::HastaLaVistaBaby;

match x {
    Terminator::TalkToMyHand => {}
    Terminator::HastaLaVistaBaby => {}
}

// or:

match x {
    Terminator::TalkToMyHand => {}
    _ => {}
}
```

## E0005

Patterns used to bind names must be irrefutable, that is, they must guarantee
that a name will be extracted in all cases. Erroneous code example:

```compile_fail,E0005
let x = Some(1);
let Some(y) = x;
// error: refutable pattern in local binding: `None` not covered
```

If you encounter this error you probably need to use a `match` or `if let` to
deal with the possibility of failure. Example:

```
let x = Some(1);

match x {
    Some(y) => {
        // do something
    },
    None => {}
}

// or:

if let Some(y) = x {
    // do something
}
```

## E0007

This error indicates that the bindings in a match arm would require a value to
be moved into more than one location, thus violating unique ownership. Code
like the following is invalid as it requires the entire `Option<String>` to be
moved into a variable called `op_string` while simultaneously requiring the
inner `String` to be moved into a variable called `s`.

```compile_fail,E0007
let x = Some("s".to_string());

match x {
    op_string @ Some(s) => {}, // error: cannot bind by-move with sub-bindings
    None => {},
}
```

See also the error E0303.

## E0008

Names bound in match arms retain their type in pattern guards. As such, if a
name is bound by move in a pattern, it should also be moved to wherever it is
referenced in the pattern guard code. Doing so however would prevent the name
from being available in the body of the match arm. Consider the following:

```compile_fail,E0008
match Some("hi".to_string()) {
    Some(s) if s.len() == 0 => {}, // use s.
    _ => {},
}
```

The variable `s` has type `String`, and its use in the guard is as a variable of
type `String`. The guard code effectively executes in a separate scope to the
body of the arm, so the value would be moved into this anonymous scope and
therefore becomes unavailable in the body of the arm.

The problem above can be solved by using the `ref` keyword.

```
match Some("hi".to_string()) {
    Some(ref s) if s.len() == 0 => {},
    _ => {},
}
```

Though this example seems innocuous and easy to solve, the problem becomes clear
when it encounters functions which consume the value:

```compile_fail,E0008
struct A{}

impl A {
    fn consume(self) -> usize {
        0
    }
}

fn main() {
    let a = Some(A{});
    match a {
        Some(y) if y.consume() > 0 => {}
        _ => {}
    }
}
```

In this situation, even the `ref` keyword cannot solve it, since borrowed
content cannot be moved. This problem cannot be solved generally. If the value
can be cloned, here is a not-so-specific solution:

```
#[derive(Clone)]
struct A{}

impl A {
    fn consume(self) -> usize {
        0
    }
}

fn main() {
    let a = Some(A{});
    match a{
        Some(ref y) if y.clone().consume() > 0 => {}
        _ => {}
    }
}
```

If the value will be consumed in the pattern guard, using its clone will not
move its ownership, so the code works.

## E0009

In a pattern, all values that don't implement the `Copy` trait have to be bound
the same way. The goal here is to avoid binding simultaneously by-move and
by-ref.

This limitation may be removed in a future version of Rust.

Erroneous code example:

```compile_fail,E0009
struct X { x: (), }

let x = Some((X { x: () }, X { x: () }));
match x {
    Some((y, ref z)) => {}, // error: cannot bind by-move and by-ref in the
                            //        same pattern
    None => panic!()
}
```

You have two solutions:

Solution #1: Bind the pattern's values the same way.

```
struct X { x: (), }

let x = Some((X { x: () }, X { x: () }));
match x {
    Some((ref y, ref z)) => {},
    // or Some((y, z)) => {}
    None => panic!()
}
```

Solution #2: Implement the `Copy` trait for the `X` structure.

However, please keep in mind that the first solution should be preferred.

```
#[derive(Clone, Copy)]
struct X { x: (), }

let x = Some((X { x: () }, X { x: () }));
match x {
    Some((y, ref z)) => {},
    None => panic!()
}
```

## E0020

This error indicates that an attempt was made to divide by zero (or take the
remainder of a zero divisor) in a static or constant expression. Erroneous
code example:

```compile_fail
#[deny(const_err)]

const X: i32 = 42 / 0;
// error: attempt to divide by zero in a constant expression
```

## E0023

A pattern used to match against an enum variant must provide a sub-pattern for
each field of the enum variant. This error indicates that a pattern attempted to
extract an incorrect number of fields from a variant.

```
enum Fruit {
    Apple(String, String),
    Pear(u32),
}
```

Here the `Apple` variant has two fields, and should be matched against like so:

```
enum Fruit {
    Apple(String, String),
    Pear(u32),
}

let x = Fruit::Apple(String::new(), String::new());

// Correct.
match x {
    Fruit::Apple(a, b) => {},
    _ => {}
}
```

Matching with the wrong number of fields has no sensible interpretation:

```compile_fail,E0023
enum Fruit {
    Apple(String, String),
    Pear(u32),
}

let x = Fruit::Apple(String::new(), String::new());

// Incorrect.
match x {
    Fruit::Apple(a) => {},
    Fruit::Apple(a, b, c) => {},
}
```

Check how many fields the enum was declared with and ensure that your pattern
uses the same number.

## E0025

Each field of a struct can only be bound once in a pattern. Erroneous code
example:

```compile_fail,E0025
struct Foo {
    a: u8,
    b: u8,
}

fn main(){
    let x = Foo { a:1, b:2 };

    let Foo { a: x, a: y } = x;
    // error: field `a` bound multiple times in the pattern
}
```

Each occurrence of a field name binds the value of that field, so to fix this
error you will have to remove or alter the duplicate uses of the field name.
Perhaps you misspelled another field name? Example:

```
struct Foo {
    a: u8,
    b: u8,
}

fn main(){
    let x = Foo { a:1, b:2 };

    let Foo { a: x, b: y } = x; // ok!
}
```

## E0026

This error indicates that a struct pattern attempted to extract a non-existent
field from a struct. Struct fields are identified by the name used before the
colon `:` so struct patterns should resemble the declaration of the struct type
being matched.

```
// Correct matching.
struct Thing {
    x: u32,
    y: u32
}

let thing = Thing { x: 1, y: 2 };

match thing {
    Thing { x: xfield, y: yfield } => {}
}
```

If you are using shorthand field patterns but want to refer to the struct field
by a different name, you should rename it explicitly.

Change this:

```compile_fail,E0026
struct Thing {
    x: u32,
    y: u32
}

let thing = Thing { x: 0, y: 0 };

match thing {
    Thing { x, z } => {}
}
```

To this:

```
struct Thing {
    x: u32,
    y: u32
}

let thing = Thing { x: 0, y: 0 };

match thing {
    Thing { x, y: z } => {}
}
```

## E0027

This error indicates that a pattern for a struct fails to specify a sub-pattern
for every one of the struct's fields. Ensure that each field from the struct's
definition is mentioned in the pattern, or use `..` to ignore unwanted fields.

For example:

```compile_fail,E0027
struct Dog {
    name: String,
    age: u32,
}

let d = Dog { name: "Rusty".to_string(), age: 8 };

// This is incorrect.
match d {
    Dog { age: x } => {}
}
```

This is correct (explicit):

```
struct Dog {
    name: String,
    age: u32,
}

let d = Dog { name: "Rusty".to_string(), age: 8 };

match d {
    Dog { name: ref n, age: x } => {}
}

// This is also correct (ignore unused fields).
match d {
    Dog { age: x, .. } => {}
}
```

## E0029

In a match expression, only numbers and characters can be matched against a
range. This is because the compiler checks that the range is non-empty at
compile-time, and is unable to evaluate arbitrary comparison functions. If you
want to capture values of an orderable type between two end-points, you can use
a guard.

```compile_fail,E0029
let string = "salutations !";

// The ordering relation for strings can't be evaluated at compile time,
// so this doesn't work:
match string {
    "hello" ... "world" => {}
    _ => {}
}

// This is a more general version, using a guard:
match string {
    s if s >= "hello" && s <= "world" => {}
    _ => {}
}
```

## E0033

This error indicates that a pointer to a trait type cannot be implicitly
dereferenced by a pattern. Every trait defines a type, but because the
size of trait implementors isn't fixed, this type has no compile-time size.
Therefore, all accesses to trait types must be through pointers. If you
encounter this error you should try to avoid dereferencing the pointer.

```ignore
let trait_obj: &SomeTrait = ...;

// This tries to implicitly dereference to create an unsized local variable.
let &invalid = trait_obj;

// You can call methods without binding to the value being pointed at.
trait_obj.method_one();
trait_obj.method_two();
```

You can read more about trait objects in the Trait Object section of the
Reference:

https://doc.rust-lang.org/reference.html#trait-objects

## E0034

The compiler doesn't know what method to call because more than one method
has the same prototype. Erroneous code example:

```compile_fail,E0034
struct Test;

trait Trait1 {
    fn foo();
}

trait Trait2 {
    fn foo();
}

impl Trait1 for Test { fn foo() {} }
impl Trait2 for Test { fn foo() {} }

fn main() {
    Test::foo() // error, which foo() to call?
}
```

To avoid this error, you have to keep only one of them and remove the others.
So let's take our example and fix it:

```
struct Test;

trait Trait1 {
    fn foo();
}

impl Trait1 for Test { fn foo() {} }

fn main() {
    Test::foo() // and now that's good!
}
```

However, a better solution would be using fully explicit naming of type and
trait:

```
struct Test;

trait Trait1 {
    fn foo();
}

trait Trait2 {
    fn foo();
}

impl Trait1 for Test { fn foo() {} }
impl Trait2 for Test { fn foo() {} }

fn main() {
    <Test as Trait1>::foo()
}
```

One last example:

```
trait F {
    fn m(&self);
}

trait G {
    fn m(&self);
}

struct X;

impl F for X { fn m(&self) { println!("I am F"); } }
impl G for X { fn m(&self) { println!("I am G"); } }

fn main() {
    let f = X;

    F::m(&f); // it displays "I am F"
    G::m(&f); // it displays "I am G"
}
```

## E0035

You tried to give a type parameter where it wasn't needed. Erroneous code
example:

```compile_fail,E0035
struct Test;

impl Test {
    fn method(&self) {}
}

fn main() {
    let x = Test;

    x.method::<i32>(); // Error: Test::method doesn't need type parameter!
}
```

To fix this error, just remove the type parameter:

```
struct Test;

impl Test {
    fn method(&self) {}
}

fn main() {
    let x = Test;

    x.method(); // OK, we're good!
}
```

## E0036

This error occurrs when you pass too many or not enough type parameters to
a method. Erroneous code example:

```compile_fail,E0036
struct Test;

impl Test {
    fn method<T>(&self, v: &[T]) -> usize {
        v.len()
    }
}

fn main() {
    let x = Test;
    let v = &[0];

    x.method::<i32, i32>(v); // error: only one type parameter is expected!
}
```

To fix it, just specify a correct number of type parameters:

```
struct Test;

impl Test {
    fn method<T>(&self, v: &[T]) -> usize {
        v.len()
    }
}

fn main() {
    let x = Test;
    let v = &[0];

    x.method::<i32>(v); // OK, we're good!
}
```

Please note on the last example that we could have called `method` like this:

```ignore
x.method(v);
```

## E0038

Trait objects like `Box<Trait>` can only be constructed when certain
requirements are satisfied by the trait in question.

Trait objects are a form of dynamic dispatch and use a dynamically sized type
for the inner type. So, for a given trait `Trait`, when `Trait` is treated as a
type, as in `Box<Trait>`, the inner type is 'unsized'. In such cases the boxed
pointer is a 'fat pointer' that contains an extra pointer to a table of methods
(among other things) for dynamic dispatch. This design mandates some
restrictions on the types of traits that are allowed to be used in trait
objects, which are collectively termed as 'object safety' rules.

Attempting to create a trait object for a non object-safe trait will trigger
this error.

There are various rules:

### The trait cannot require `Self: Sized`

When `Trait` is treated as a type, the type does not implement the special
`Sized` trait, because the type does not have a known size at compile time and
can only be accessed behind a pointer. Thus, if we have a trait like the
following:

```
trait Foo where Self: Sized {

}
```

We cannot create an object of type `Box<Foo>` or `&Foo` since in this case
`Self` would not be `Sized`.

Generally, `Self : Sized` is used to indicate that the trait should not be used
as a trait object. If the trait comes from your own crate, consider removing
this restriction.

### Method references the `Self` type in its arguments or return type

This happens when a trait has a method like the following:

```
trait Trait {
    fn foo(&self) -> Self;
}

impl Trait for String {
    fn foo(&self) -> Self {
        "hi".to_owned()
    }
}

impl Trait for u8 {
    fn foo(&self) -> Self {
        1
    }
}
```

(Note that `&self` and `&mut self` are okay, it's additional `Self` types which
cause this problem.)

In such a case, the compiler cannot predict the return type of `foo()` in a
situation like the following:

```compile_fail
trait Trait {
    fn foo(&self) -> Self;
}

fn call_foo(x: Box<Trait>) {
    let y = x.foo(); // What type is y?
    // ...
}
```

If only some methods aren't object-safe, you can add a `where Self: Sized` bound
on them to mark them as explicitly unavailable to trait objects. The
functionality will still be available to all other implementers, including
`Box<Trait>` which is itself sized (assuming you `impl Trait for Box<Trait>`).

```
trait Trait {
    fn foo(&self) -> Self where Self: Sized;
    // more functions
}
```

Now, `foo()` can no longer be called on a trait object, but you will now be
allowed to make a trait object, and that will be able to call any object-safe
methods. With such a bound, one can still call `foo()` on types implementing
that trait that aren't behind trait objects.

### Method has generic type parameters

As mentioned before, trait objects contain pointers to method tables. So, if we
have:

```
trait Trait {
    fn foo(&self);
}

impl Trait for String {
    fn foo(&self) {
        // implementation 1
    }
}

impl Trait for u8 {
    fn foo(&self) {
        // implementation 2
    }
}
// ...
```

At compile time each implementation of `Trait` will produce a table containing
the various methods (and other items) related to the implementation.

This works fine, but when the method gains generic parameters, we can have a
problem.

Usually, generic parameters get _monomorphized_. For example, if I have

```
fn foo<T>(x: T) {
    // ...
}
```

The machine code for `foo::<u8>()`, `foo::<bool>()`, `foo::<String>()`, or any
other type substitution is different. Hence the compiler generates the
implementation on-demand. If you call `foo()` with a `bool` parameter, the
compiler will only generate code for `foo::<bool>()`. When we have additional
type parameters, the number of monomorphized implementations the compiler
generates does not grow drastically, since the compiler will only generate an
implementation if the function is called with unparametrized substitutions
(i.e., substitutions where none of the substituted types are themselves
parametrized).

However, with trait objects we have to make a table containing _every_ object
that implements the trait. Now, if it has type parameters, we need to add
implementations for every type that implements the trait, and there could
theoretically be an infinite number of types.

For example, with:

```
trait Trait {
    fn foo<T>(&self, on: T);
    // more methods
}

impl Trait for String {
    fn foo<T>(&self, on: T) {
        // implementation 1
    }
}

impl Trait for u8 {
    fn foo<T>(&self, on: T) {
        // implementation 2
    }
}

// 8 more implementations
```

Now, if we have the following code:

```ignore
fn call_foo(thing: Box<Trait>) {
    thing.foo(true); // this could be any one of the 8 types above
    thing.foo(1);
    thing.foo("hello");
}
```

We don't just need to create a table of all implementations of all methods of
`Trait`, we need to create such a table, for each different type fed to
`foo()`. In this case this turns out to be (10 types implementing `Trait`)*(3
types being fed to `foo()`) = 30 implementations!

With real world traits these numbers can grow drastically.

To fix this, it is suggested to use a `where Self: Sized` bound similar to the
fix for the sub-error above if you do not intend to call the method with type
parameters:

```
trait Trait {
    fn foo<T>(&self, on: T) where Self: Sized;
    // more methods
}
```

If this is not an option, consider replacing the type parameter with another
trait object (e.g. if `T: OtherTrait`, use `on: Box<OtherTrait>`). If the number
of types you intend to feed to this method is limited, consider manually listing
out the methods of different types.

### Method has no receiver

Methods that do not take a `self` parameter can't be called since there won't be
a way to get a pointer to the method table for them.

```
trait Foo {
    fn foo() -> u8;
}
```

This could be called as `<Foo as Foo>::foo()`, which would not be able to pick
an implementation.

Adding a `Self: Sized` bound to these methods will generally make this compile.

```
trait Foo {
    fn foo() -> u8 where Self: Sized;
}
```

### The trait cannot use `Self` as a type parameter in the supertrait listing

This is similar to the second sub-error, but subtler. It happens in situations
like the following:

```compile_fail
trait Super<A> {}

trait Trait: Super<Self> {
}

struct Foo;

impl Super<Foo> for Foo{}

impl Trait for Foo {}
```

Here, the supertrait might have methods as follows:

```
trait Super<A> {
    fn get_a(&self) -> A; // note that this is object safe!
}
```

If the trait `Foo` was deriving from something like `Super<String>` or
`Super<T>` (where `Foo` itself is `Foo<T>`), this is okay, because given a type
`get_a()` will definitely return an object of that type.

However, if it derives from `Super<Self>`, even though `Super` is object safe,
the method `get_a()` would return an object of unknown type when called on the
function. `Self` type parameters let us make object safe traits no longer safe,
so they are forbidden when specifying supertraits.

There's no easy fix for this, generally code will need to be refactored so that
you no longer need to derive from `Super<Self>`.

## E0040

It is not allowed to manually call destructors in Rust. It is also not
necessary to do this since `drop` is called automatically whenever a value goes
out of scope.

Here's an example of this error:

```compile_fail,E0040
struct Foo {
    x: i32,
}

impl Drop for Foo {
    fn drop(&mut self) {
        println!("kaboom");
    }
}

fn main() {
    let mut x = Foo { x: -7 };
    x.drop(); // error: explicit use of destructor method
}
```

## E0044

You can't use type parameters on foreign items. Example of erroneous code:

```compile_fail,E0044
extern { fn some_func<T>(x: T); }
```

To fix this, replace the type parameter with the specializations that you
need:

```
extern { fn some_func_i32(x: i32); }
extern { fn some_func_i64(x: i64); }
```

## E0045

Rust only supports variadic parameters for interoperability with C code in its
FFI. As such, variadic parameters can only be used with functions which are
using the C ABI. Examples of erroneous code:

```compile_fail
#![feature(unboxed_closures)]

extern "rust-call" { fn foo(x: u8, ...); }

// or

fn foo(x: u8, ...) {}
```

To fix such code, put them in an extern "C" block:

```
extern "C" {
    fn foo (x: u8, ...);
}
```

## E0046

Items are missing in a trait implementation. Erroneous code example:

```compile_fail,E0046
trait Foo {
    fn foo();
}

struct Bar;

impl Foo for Bar {}
// error: not all trait items implemented, missing: `foo`
```

When trying to make some type implement a trait `Foo`, you must, at minimum,
provide implementations for all of `Foo`'s required methods (meaning the
methods that do not have default implementations), as well as any required
trait items like associated types or constants. Example:

```
trait Foo {
    fn foo();
}

struct Bar;

impl Foo for Bar {
    fn foo() {} // ok!
}
```

## E0049

This error indicates that an attempted implementation of a trait method
has the wrong number of type parameters.

For example, the trait below has a method `foo` with a type parameter `T`,
but the implementation of `foo` for the type `Bar` is missing this parameter:

```compile_fail,E0049
trait Foo {
    fn foo<T: Default>(x: T) -> Self;
}

struct Bar;

// error: method `foo` has 0 type parameters but its trait declaration has 1
// type parameter
impl Foo for Bar {
    fn foo(x: bool) -> Self { Bar }
}
```

## E0050

This error indicates that an attempted implementation of a trait method
has the wrong number of function parameters.

For example, the trait below has a method `foo` with two function parameters
(`&self` and `u8`), but the implementation of `foo` for the type `Bar` omits
the `u8` parameter:

```compile_fail,E0050
trait Foo {
    fn foo(&self, x: u8) -> bool;
}

struct Bar;

// error: method `foo` has 1 parameter but the declaration in trait `Foo::foo`
// has 2
impl Foo for Bar {
    fn foo(&self) -> bool { true }
}
```

## E0053

The parameters of any trait method must match between a trait implementation
and the trait definition.

Here are a couple examples of this error:

```compile_fail,E0053
trait Foo {
    fn foo(x: u16);
    fn bar(&self);
}

struct Bar;

impl Foo for Bar {
    // error, expected u16, found i16
    fn foo(x: i16) { }

    // error, types differ in mutability
    fn bar(&mut self) { }
}
```

## E0054

It is not allowed to cast to a bool. If you are trying to cast a numeric type
to a bool, you can compare it with zero instead:

```compile_fail,E0054
let x = 5;

// Not allowed, won't compile
let x_is_nonzero = x as bool;
```

```
let x = 5;

// Ok
let x_is_nonzero = x != 0;
```

## E0055

During a method call, a value is automatically dereferenced as many times as
needed to make the value's type match the method's receiver. The catch is that
the compiler will only attempt to dereference a number of times up to the
recursion limit (which can be set via the `recursion_limit` attribute).

For a somewhat artificial example:

```compile_fail,E0055
#![recursion_limit="2"]

struct Foo;

impl Foo {
    fn foo(&self) {}
}

fn main() {
    let foo = Foo;
    let ref_foo = &&Foo;

    // error, reached the recursion limit while auto-dereferencing &&Foo
    ref_foo.foo();
}
```

One fix may be to increase the recursion limit. Note that it is possible to
create an infinite recursion of dereferencing, in which case the only fix is to
somehow break the recursion.

## E0057

When invoking closures or other implementations of the function traits `Fn`,
`FnMut` or `FnOnce` using call notation, the number of parameters passed to the
function must match its definition.

An example using a closure:

```compile_fail,E0057
let f = |x| x * 3;
let a = f();        // invalid, too few parameters
let b = f(4);       // this works!
let c = f(2, 3);    // invalid, too many parameters
```

A generic function must be treated similarly:

```
fn foo<F: Fn()>(f: F) {
    f(); // this is valid, but f(3) would not work
}
```

## E0059

The built-in function traits are generic over a tuple of the function arguments.
If one uses angle-bracket notation (`Fn<(T,), Output=U>`) instead of parentheses
(`Fn(T) -> U`) to denote the function trait, the type parameter should be a
tuple. Otherwise function call notation cannot be used and the trait will not be
implemented by closures.

The most likely source of this error is using angle-bracket notation without
wrapping the function argument type into a tuple, for example:

```compile_fail,E0059
#![feature(unboxed_closures)]

fn foo<F: Fn<i32>>(f: F) -> F::Output { f(3) }
```

It can be fixed by adjusting the trait bound like this:

```
#![feature(unboxed_closures)]

fn foo<F: Fn<(i32,)>>(f: F) -> F::Output { f(3) }
```

Note that `(T,)` always denotes the type of a 1-tuple containing an element of
type `T`. The comma is necessary for syntactic disambiguation.

## E0060

External C functions are allowed to be variadic. However, a variadic function
takes a minimum number of arguments. For example, consider C's variadic `printf`
function:

```ignore
extern crate libc;
use libc::{ c_char, c_int };

extern "C" {
    fn printf(_: *const c_char, ...) -> c_int;
}
```

Using this declaration, it must be called with at least one argument, so
simply calling `printf()` is invalid. But the following uses are allowed:

```ignore
unsafe {
    use std::ffi::CString;

    printf(CString::new("test\n").unwrap().as_ptr());
    printf(CString::new("number = %d\n").unwrap().as_ptr(), 3);
    printf(CString::new("%d, %d\n").unwrap().as_ptr(), 10, 5);
}
```

## E0061

The number of arguments passed to a function must match the number of arguments
specified in the function signature.

For example, a function like:

```
fn f(a: u16, b: &str) {}
```

Must always be called with exactly two arguments, e.g. `f(2, "test")`.

Note that Rust does not have a notion of optional function arguments or
variadic functions (except for its C-FFI).

## E0062

This error indicates that during an attempt to build a struct or struct-like
enum variant, one of the fields was specified more than once. Erroneous code
example:

```compile_fail,E0062
struct Foo {
    x: i32,
}

fn main() {
    let x = Foo {
                x: 0,
                x: 0, // error: field `x` specified more than once
            };
}
```

Each field should be specified exactly one time. Example:

```
struct Foo {
    x: i32,
}

fn main() {
    let x = Foo { x: 0 }; // ok!
}
```

## E0063

This error indicates that during an attempt to build a struct or struct-like
enum variant, one of the fields was not provided. Erroneous code example:

```compile_fail,E0063
struct Foo {
    x: i32,
    y: i32,
}

fn main() {
    let x = Foo { x: 0 }; // error: missing field: `y`
}
```

Each field should be specified exactly once. Example:

```
struct Foo {
    x: i32,
    y: i32,
}

fn main() {
    let x = Foo { x: 0, y: 0 }; // ok!
}
```

## E0066

Box placement expressions (like C++'s "placement new") do not yet support any
place expression except the exchange heap (i.e. `std::boxed::HEAP`).
Furthermore, the syntax is changing to use `in` instead of `box`. See [RFC 470]
and [RFC 809] for more details.

[RFC 470]: https://github.com/rust-lang/rfcs/pull/470
[RFC 809]: https://github.com/rust-lang/rfcs/pull/809

## E0067

The left-hand side of a compound assignment expression must be an lvalue
expression. An lvalue expression represents a memory location and includes
item paths (ie, namespaced variables), dereferences, indexing expressions,
and field references.

Let's start with some erroneous code examples:

```compile_fail,E0067
use std::collections::LinkedList;

// Bad: assignment to non-lvalue expression
LinkedList::new() += 1;

// ...

fn some_func(i: &mut i32) {
    i += 12; // Error : '+=' operation cannot be applied on a reference !
}
```

And now some working examples:

```
let mut i : i32 = 0;

i += 12; // Good !

// ...

fn some_func(i: &mut i32) {
    *i += 12; // Good !
}
```

## E0069

The compiler found a function whose body contains a `return;` statement but
whose return type is not `()`. An example of this is:

```compile_fail,E0069
// error
fn foo() -> u8 {
    return;
}
```

Since `return;` is just like `return ();`, there is a mismatch between the
function's return type and the value being returned.

## E0070

The left-hand side of an assignment operator must be an lvalue expression. An
lvalue expression represents a memory location and can be a variable (with
optional namespacing), a dereference, an indexing expression or a field
reference.

More details can be found here:
https://doc.rust-lang.org/reference.html#lvalues-rvalues-and-temporaries

Now, we can go further. Here are some erroneous code examples:

```compile_fail,E0070
struct SomeStruct {
    x: i32,
    y: i32
}

const SOME_CONST : i32 = 12;

fn some_other_func() {}

fn some_function() {
    SOME_CONST = 14; // error : a constant value cannot be changed!
    1 = 3; // error : 1 isn't a valid lvalue!
    some_other_func() = 4; // error : we can't assign value to a function!
    SomeStruct.x = 12; // error : SomeStruct a structure name but it is used
                       // like a variable!
}
```

And now let's give working examples:

```
struct SomeStruct {
    x: i32,
    y: i32
}
let mut s = SomeStruct {x: 0, y: 0};

s.x = 3; // that's good !

// ...

fn some_func(x: &mut i32) {
    *x = 12; // that's good !
}
```

## E0071

You tried to use structure-literal syntax to create an item that is
not a structure or enum variant.

Example of erroneous code:

```compile_fail,E0071
type U32 = u32;
let t = U32 { value: 4 }; // error: expected struct, variant or union type,
                          // found builtin type `u32`
```

To fix this, ensure that the name was correctly spelled, and that
the correct form of initializer was used.

For example, the code above can be fixed to:

```
enum Foo {
    FirstValue(i32)
}

fn main() {
    let u = Foo::FirstValue(0i32);

    let t = 4;
}
```

## E0072

When defining a recursive struct or enum, any use of the type being defined
from inside the definition must occur behind a pointer (like `Box` or `&`).
This is because structs and enums must have a well-defined size, and without
the pointer, the size of the type would need to be unbounded.

Consider the following erroneous definition of a type for a list of bytes:

```compile_fail,E0072
// error, invalid recursive struct type
struct ListNode {
    head: u8,
    tail: Option<ListNode>,
}
```

This type cannot have a well-defined size, because it needs to be arbitrarily
large (since we would be able to nest `ListNode`s to any depth). Specifically,

```plain
size of `ListNode` = 1 byte for `head`
                   + 1 byte for the discriminant of the `Option`
                   + size of `ListNode`
```

One way to fix this is by wrapping `ListNode` in a `Box`, like so:

```
struct ListNode {
    head: u8,
    tail: Option<Box<ListNode>>,
}
```

This works because `Box` is a pointer, so its size is well-known.

## E0073

You cannot define a struct (or enum) `Foo` that requires an instance of `Foo`
in order to make a new `Foo` value. This is because there would be no way a
first instance of `Foo` could be made to initialize another instance!

Here's an example of a struct that has this problem:

```ignore
struct Foo { x: Box<Foo> } // error
```

One fix is to use `Option`, like so:

```
struct Foo { x: Option<Box<Foo>> }
```

Now it's possible to create at least one instance of `Foo`: `Foo { x: None }`.

## E0074

When using the `#[simd]` attribute on a tuple struct, the components of the
tuple struct must all be of a concrete, nongeneric type so the compiler can
reason about how to use SIMD with them. This error will occur if the types
are generic.

This will cause an error:

```ignore
#![feature(repr_simd)]

#[repr(simd)]
struct Bad<T>(T, T, T);
```

This will not:

```
#![feature(repr_simd)]

#[repr(simd)]
struct Good(u32, u32, u32);
```

## E0075

The `#[simd]` attribute can only be applied to non empty tuple structs, because
it doesn't make sense to try to use SIMD operations when there are no values to
operate on.

This will cause an error:

```compile_fail,E0075
#![feature(repr_simd)]

#[repr(simd)]
struct Bad;
```

This will not:

```
#![feature(repr_simd)]

#[repr(simd)]
struct Good(u32);
```

## E0076

When using the `#[simd]` attribute to automatically use SIMD operations in tuple
struct, the types in the struct must all be of the same type, or the compiler
will trigger this error.

This will cause an error:

```compile_fail,E0076
#![feature(repr_simd)]

#[repr(simd)]
struct Bad(u16, u32, u32);
```

This will not:

```
#![feature(repr_simd)]

#[repr(simd)]
struct Good(u32, u32, u32);
```

## E0077

When using the `#[simd]` attribute on a tuple struct, the elements in the tuple
must be machine types so SIMD operations can be applied to them.

This will cause an error:

```compile_fail,E0077
#![feature(repr_simd)]

#[repr(simd)]
struct Bad(String);
```

This will not:

```
#![feature(repr_simd)]

#[repr(simd)]
struct Good(u32, u32, u32);
```

## E0079

Enum variants which contain no data can be given a custom integer
representation. This error indicates that the value provided is not an integer
literal and is therefore invalid.

For example, in the following code:

```compile_fail,E0079
enum Foo {
    Q = "32",
}
```

We try to set the representation to a string.

There's no general fix for this; if you can work with an integer then just set
it to one:

```
enum Foo {
    Q = 32,
}
```

However if you actually wanted a mapping between variants and non-integer
objects, it may be preferable to use a method with a match instead:

```
enum Foo { Q }
impl Foo {
    fn get_str(&self) -> &'static str {
        match *self {
            Foo::Q => "32",
        }
    }
}
```

## E0080

This error indicates that the compiler was unable to sensibly evaluate an
constant expression that had to be evaluated. Attempting to divide by 0
or causing integer overflow are two ways to induce this error. For example:

```compile_fail,E0080
enum Enum {
    X = (1 << 500),
    Y = (1 / 0)
}
```

Ensure that the expressions given can be evaluated as the desired integer type.
See the FFI section of the Reference for more information about using a custom
integer type:

https://doc.rust-lang.org/reference.html#ffi-attributes

## E0081

Enum discriminants are used to differentiate enum variants stored in memory.
This error indicates that the same value was used for two or more variants,
making them impossible to tell apart.

```compile_fail,E0081
// Bad.
enum Enum {
    P = 3,
    X = 3,
    Y = 5,
}
```

```
// Good.
enum Enum {
    P,
    X = 3,
    Y = 5,
}
```

Note that variants without a manually specified discriminant are numbered from
top to bottom starting from 0, so clashes can occur with seemingly unrelated
variants.

```compile_fail,E0081
enum Bad {
    X,
    Y = 0
}
```

Here `X` will have already been specified the discriminant 0 by the time `Y` is
encountered, so a conflict occurs.

## E0082

When you specify enum discriminants with `=`, the compiler expects `isize`
values by default. Or you can add the `repr` attibute to the enum declaration
for an explicit choice of the discriminant type. In either cases, the
discriminant values must fall within a valid range for the expected type;
otherwise this error is raised. For example:

```ignore
#[repr(u8)]
enum Thing {
    A = 1024,
    B = 5,
}
```

Here, 1024 lies outside the valid range for `u8`, so the discriminant for `A` is
invalid. Here is another, more subtle example which depends on target word size:

```ignore
enum DependsOnPointerSize {
    A = 1 << 32,
}
```

Here, `1 << 32` is interpreted as an `isize` value. So it is invalid for 32 bit
target (`target_pointer_width = "32"`) but valid for 64 bit target.

You may want to change representation types to fix this, or else change invalid
discriminant values so that they fit within the existing type.

## E0084

An unsupported representation was attempted on a zero-variant enum.

Erroneous code example:

```compile_fail,E0084
#[repr(i32)]
enum NightsWatch {} // error: unsupported representation for zero-variant enum
```

It is impossible to define an integer type to be used to represent zero-variant
enum values because there are no zero-variant enum values. There is no way to
construct an instance of the following type using only safe code. So you have
two solutions. Either you add variants in your enum:

```
#[repr(i32)]
enum NightsWatch {
    JonSnow,
    Commander,
}
```

or you remove the integer represention of your enum:

```
enum NightsWatch {}
```

## E0087

Too many type parameters were supplied for a function. For example:

```compile_fail,E0087
fn foo<T>() {}

fn main() {
    foo::<f64, bool>(); // error, expected 1 parameter, found 2 parameters
}
```

The number of supplied parameters must exactly match the number of defined type
parameters.

## E0088

You gave too many lifetime parameters. Erroneous code example:

```compile_fail,E0088
fn f() {}

fn main() {
    f::<'static>() // error: too many lifetime parameters provided
}
```

Please check you give the right number of lifetime parameters. Example:

```
fn f() {}

fn main() {
    f() // ok!
}
```

It's also important to note that the Rust compiler can generally
determine the lifetime by itself. Example:

```
struct Foo {
    value: String
}

impl Foo {
    // it can be written like this
    fn get_value<'a>(&'a self) -> &'a str { &self.value }
    // but the compiler works fine with this too:
    fn without_lifetime(&self) -> &str { &self.value }
}

fn main() {
    let f = Foo { value: "hello".to_owned() };

    println!("{}", f.get_value());
    println!("{}", f.without_lifetime());
}
```

## E0089

Not enough type parameters were supplied for a function. For example:

```compile_fail,E0089
fn foo<T, U>() {}

fn main() {
    foo::<f64>(); // error, expected 2 parameters, found 1 parameter
}
```

Note that if a function takes multiple type parameters but you want the compiler
to infer some of them, you can use type placeholders:

```compile_fail,E0089
fn foo<T, U>(x: T) {}

fn main() {
    let x: bool = true;
    foo::<f64>(x);    // error, expected 2 parameters, found 1 parameter
    foo::<_, f64>(x); // same as `foo::<bool, f64>(x)`
}
```

## E0091

You gave an unnecessary type parameter in a type alias. Erroneous code
example:

```compile_fail,E0091
type Foo<T> = u32; // error: type parameter `T` is unused
// or:
type Foo<A,B> = Box<A>; // error: type parameter `B` is unused
```

Please check you didn't write too many type parameters. Example:

```
type Foo = u32; // ok!
type Foo2<A> = Box<A>; // ok!
```

## E0092

You tried to declare an undefined atomic operation function.
Erroneous code example:

```compile_fail,E0092
#![feature(intrinsics)]

extern "rust-intrinsic" {
    fn atomic_foo(); // error: unrecognized atomic operation
                     //        function
}
```

Please check you didn't make a mistake in the function's name. All intrinsic
functions are defined in librustc_trans/trans/intrinsic.rs and in
libcore/intrinsics.rs in the Rust source code. Example:

```
#![feature(intrinsics)]

extern "rust-intrinsic" {
    fn atomic_fence(); // ok!
}
```

## E0093

You declared an unknown intrinsic function. Erroneous code example:

```compile_fail,E0093
#![feature(intrinsics)]

extern "rust-intrinsic" {
    fn foo(); // error: unrecognized intrinsic function: `foo`
}

fn main() {
    unsafe {
        foo();
    }
}
```

Please check you didn't make a mistake in the function's name. All intrinsic
functions are defined in librustc_trans/trans/intrinsic.rs and in
libcore/intrinsics.rs in the Rust source code. Example:

```
#![feature(intrinsics)]

extern "rust-intrinsic" {
    fn atomic_fence(); // ok!
}

fn main() {
    unsafe {
        atomic_fence();
    }
}
```

## E0094

You gave an invalid number of type parameters to an intrinsic function.
Erroneous code example:

```compile_fail,E0094
#![feature(intrinsics)]

extern "rust-intrinsic" {
    fn size_of<T, U>() -> usize; // error: intrinsic has wrong number
                                 //        of type parameters
}
```

Please check that you provided the right number of type parameters
and verify with the function declaration in the Rust source code.
Example:

```
#![feature(intrinsics)]

extern "rust-intrinsic" {
    fn size_of<T>() -> usize; // ok!
}
```

## E0101

You hit this error because the compiler lacks the information to
determine a type for this expression. Erroneous code example:

```compile_fail,E0101
let x = |_| {}; // error: cannot determine a type for this expression
```

You have two possibilities to solve this situation:

 * Give an explicit definition of the expression
 * Infer the expression

Examples:

```
let x = |_ : u32| {}; // ok!
// or:
let x = |_| {};
x(0u32);
```

## E0102

You hit this error because the compiler lacks the information to
determine the type of this variable. Erroneous code example:

```compile_fail,E0102
// could be an array of anything
let x = []; // error: cannot determine a type for this local variable
```

To solve this situation, constrain the type of the variable.
Examples:

```
#![allow(unused_variables)]

fn main() {
    let x: [u8; 0] = [];
}
```

## E0106

This error indicates that a lifetime is missing from a type. If it is an error
inside a function signature, the problem may be with failing to adhere to the
lifetime elision rules (see below).

Here are some simple examples of where you'll run into this error:

```compile_fail,E0106
struct Foo { x: &bool }        // error
struct Foo<'a> { x: &'a bool } // correct

enum Bar { A(u8), B(&bool), }        // error
enum Bar<'a> { A(u8), B(&'a bool), } // correct

type MyStr = &str;        // error
type MyStr<'a> = &'a str; // correct
```

Lifetime elision is a special, limited kind of inference for lifetimes in
function signatures which allows you to leave out lifetimes in certain cases.
For more background on lifetime elision see [the book][book-le].

The lifetime elision rules require that any function signature with an elided
output lifetime must either have

 - exactly one input lifetime
 - or, multiple input lifetimes, but the function must also be a method with a
   `&self` or `&mut self` receiver

In the first case, the output lifetime is inferred to be the same as the unique
input lifetime. In the second case, the lifetime is instead inferred to be the
same as the lifetime on `&self` or `&mut self`.

Here are some examples of elision errors:

```compile_fail,E0106
// error, no input lifetimes
fn foo() -> &str { }

// error, `x` and `y` have distinct lifetimes inferred
fn bar(x: &str, y: &str) -> &str { }

// error, `y`'s lifetime is inferred to be distinct from `x`'s
fn baz<'a>(x: &'a str, y: &str) -> &str { }
```

[book-le]: https://doc.rust-lang.org/nightly/book/lifetimes.html#lifetime-elision

## E0107

This error means that an incorrect number of lifetime parameters were provided
for a type (like a struct or enum) or trait.

Some basic examples include:

```compile_fail,E0107
struct Foo<'a>(&'a str);
enum Bar { A, B, C }

struct Baz<'a> {
    foo: Foo,     // error: expected 1, found 0
    bar: Bar<'a>, // error: expected 0, found 1
}
```

Here's an example that is currently an error, but may work in a future version
of Rust:

```compile_fail,E0107
struct Foo<'a>(&'a str);

trait Quux { }
impl Quux for Foo { } // error: expected 1, found 0
```

Lifetime elision in implementation headers was part of the lifetime elision
RFC. It is, however, [currently unimplemented][iss15872].

[iss15872]: https://github.com/rust-lang/rust/issues/15872

## E0109

You tried to give a type parameter to a type which doesn't need it. Erroneous
code example:

```compile_fail,E0109
type X = u32<i32>; // error: type parameters are not allowed on this type
```

Please check that you used the correct type and recheck its definition. Perhaps
it doesn't need the type parameter.

Example:

```
type X = u32; // this compiles
```

Note that type parameters for enum-variant constructors go after the variant,
not after the enum (Option::None::<u32>, not Option::<u32>::None).

## E0110

You tried to give a lifetime parameter to a type which doesn't need it.
Erroneous code example:

```compile_fail,E0110
type X = u32<'static>; // error: lifetime parameters are not allowed on
                       //        this type
```

Please check that the correct type was used and recheck its definition; perhaps
it doesn't need the lifetime parameter. Example:

```
type X = u32; // ok!
```

## E0116

You can only define an inherent implementation for a type in the same crate
where the type was defined. For example, an `impl` block as below is not allowed
since `Vec` is defined in the standard library:

```compile_fail,E0116
impl Vec<u8> { } // error
```

To fix this problem, you can do either of these things:

 - define a trait that has the desired associated functions/types/constants and
   implement the trait for the type in question
 - define a new type wrapping the type and define an implementation on the new
   type

Note that using the `type` keyword does not work here because `type` only
introduces a type alias:

```compile_fail,E0116
type Bytes = Vec<u8>;

impl Bytes { } // error, same as above
```

## E0117

This error indicates a violation of one of Rust's orphan rules for trait
implementations. The rule prohibits any implementation of a foreign trait (a
trait defined in another crate) where

 - the type that is implementing the trait is foreign
 - all of the parameters being passed to the trait (if there are any) are also
   foreign.

Here's one example of this error:

```compile_fail,E0117
impl Drop for u32 {}
```

To avoid this kind of error, ensure that at least one local type is referenced
by the `impl`:

```ignore
pub struct Foo; // you define your type in your crate

impl Drop for Foo { // and you can implement the trait on it!
    // code of trait implementation here
}

impl From<Foo> for i32 { // or you use a type from your crate as
                         // a type parameter
    fn from(i: Foo) -> i32 {
        0
    }
}
```

Alternatively, define a trait locally and implement that instead:

```
trait Bar {
    fn get(&self) -> usize;
}

impl Bar for u32 {
    fn get(&self) -> usize { 0 }
}
```

For information on the design of the orphan rules, see [RFC 1023].

[RFC 1023]: https://github.com/rust-lang/rfcs/pull/1023

## E0118

You're trying to write an inherent implementation for something which isn't a
struct nor an enum. Erroneous code example:

```compile_fail,E0118
impl (u8, u8) { // error: no base type found for inherent implementation
    fn get_state(&self) -> String {
        // ...
    }
}
```

To fix this error, please implement a trait on the type or wrap it in a struct.
Example:

```
// we create a trait here
trait LiveLongAndProsper {
    fn get_state(&self) -> String;
}

// and now you can implement it on (u8, u8)
impl LiveLongAndProsper for (u8, u8) {
    fn get_state(&self) -> String {
        "He's dead, Jim!".to_owned()
    }
}
```

Alternatively, you can create a newtype. A newtype is a wrapping tuple-struct.
For example, `NewType` is a newtype over `Foo` in `struct NewType(Foo)`.
Example:

```
struct TypeWrapper((u8, u8));

impl TypeWrapper {
    fn get_state(&self) -> String {
        "Fascinating!".to_owned()
    }
}
```

## E0119

There are conflicting trait implementations for the same type.
Example of erroneous code:

```compile_fail,E0119
trait MyTrait {
    fn get(&self) -> usize;
}

impl<T> MyTrait for T {
    fn get(&self) -> usize { 0 }
}

struct Foo {
    value: usize
}

impl MyTrait for Foo { // error: conflicting implementations of trait
                       //        `MyTrait` for type `Foo`
    fn get(&self) -> usize { self.value }
}
```

When looking for the implementation for the trait, the compiler finds
both the `impl<T> MyTrait for T` where T is all types and the `impl
MyTrait for Foo`. Since a trait cannot be implemented multiple times,
this is an error. So, when you write:

```
trait MyTrait {
    fn get(&self) -> usize;
}

impl<T> MyTrait for T {
    fn get(&self) -> usize { 0 }
}
```

This makes the trait implemented on all types in the scope. So if you
try to implement it on another one after that, the implementations will
conflict. Example:

```
trait MyTrait {
    fn get(&self) -> usize;
}

impl<T> MyTrait for T {
    fn get(&self) -> usize { 0 }
}

struct Foo;

fn main() {
    let f = Foo;

    f.get(); // the trait is implemented so we can use it
}
```

## E0120

An attempt was made to implement Drop on a trait, which is not allowed: only
structs and enums can implement Drop. An example causing this error:

```compile_fail,E0120
trait MyTrait {}

impl Drop for MyTrait {
    fn drop(&mut self) {}
}
```

A workaround for this problem is to wrap the trait up in a struct, and implement
Drop on that. An example is shown below:

```
trait MyTrait {}
struct MyWrapper<T: MyTrait> { foo: T }

impl <T: MyTrait> Drop for MyWrapper<T> {
    fn drop(&mut self) {}
}

```

Alternatively, wrapping trait objects requires something like the following:

```
trait MyTrait {}

//or Box<MyTrait>, if you wanted an owned trait object
struct MyWrapper<'a> { foo: &'a MyTrait }

impl <'a> Drop for MyWrapper<'a> {
    fn drop(&mut self) {}
}
```

## E0121

In order to be consistent with Rust's lack of global type inference, type
placeholders are disallowed by design in item signatures.

Examples of this error include:

```compile_fail,E0121
fn foo() -> _ { 5 } // error, explicitly write out the return type instead

static BAR: _ = "test"; // error, explicitly write out the type instead
```

## E0122

An attempt was made to add a generic constraint to a type alias. While Rust will
allow this with a warning, it will not currently enforce the constraint.
Consider the example below:

```
trait Foo{}

type MyType<R: Foo> = (R, ());

fn main() {
    let t: MyType<u32>;
}
```

We're able to declare a variable of type `MyType<u32>`, despite the fact that
`u32` does not implement `Foo`. As a result, one should avoid using generic
constraints in concert with type aliases.

## E0124

You declared two fields of a struct with the same name. Erroneous code
example:

```compile_fail,E0124
struct Foo {
    field1: i32,
    field1: i32, // error: field is already declared
}
```

Please verify that the field names have been correctly spelled. Example:

```
struct Foo {
    field1: i32,
    field2: i32, // ok!
}
```

## E0128

Type parameter defaults can only use parameters that occur before them.
Erroneous code example:

```compile_fail,E0128
struct Foo<T=U, U=()> {
    field1: T,
    filed2: U,
}
// error: type parameters with a default cannot use forward declared
// identifiers
```

Since type parameters are evaluated in-order, you may be able to fix this issue
by doing:

```
struct Foo<U=(), T=U> {
    field1: T,
    filed2: U,
}
```

Please also verify that this wasn't because of a name-clash and rename the type
parameter if so.

## E0131

It is not possible to define `main` with type parameters, or even with function
parameters. When `main` is present, it must take no arguments and return `()`.
Erroneous code example:

```compile_fail,E0131
fn main<T>() { // error: main function is not allowed to have type parameters
}
```

## E0132

A function with the `start` attribute was declared with type parameters.

Erroneous code example:

```compile_fail,E0132
#![feature(start)]

#[start]
fn f<T>() {}
```

It is not possible to declare type parameters on a function that has the `start`
attribute. Such a function must have the following type signature (for more
information: http://doc.rust-lang.org/stable/book/no-stdlib.html):

```ignore
fn(isize, *const *const u8) -> isize;
```

Example:

```
#![feature(start)]

#[start]
fn my_start(argc: isize, argv: *const *const u8) -> isize {
    0
}
```

## E0133

Unsafe code was used outside of an unsafe function or block.

Erroneous code example:

```compile_fail,E0133
unsafe fn f() { return; } // This is the unsafe code

fn main() {
    f(); // error: call to unsafe function requires unsafe function or block
}
```

Using unsafe functionality is potentially dangerous and disallowed by safety
checks. Examples:

* Dereferencing raw pointers
* Calling functions via FFI
* Calling functions marked unsafe

These safety checks can be relaxed for a section of the code by wrapping the
unsafe instructions with an `unsafe` block. For instance:

```
unsafe fn f() { return; }

fn main() {
    unsafe { f(); } // ok!
}
```

See also https://doc.rust-lang.org/book/unsafe.html

## E0136

A binary can only have one entry point, and by default that entry point is the
function `main()`. If there are multiple such functions, please rename one.

## E0137

More than one function was declared with the `#[main]` attribute.

Erroneous code example:

```compile_fail,E0137
#![feature(main)]

#[main]
fn foo() {}

#[main]
fn f() {} // error: multiple functions with a #[main] attribute
```

This error indicates that the compiler found multiple functions with the
`#[main]` attribute. This is an error because there must be a unique entry
point into a Rust program. Example:

```
#![feature(main)]

#[main]
fn f() {} // ok!
```

## E0138

More than one function was declared with the `#[start]` attribute.

Erroneous code example:

```compile_fail,E0138
#![feature(start)]

#[start]
fn foo(argc: isize, argv: *const *const u8) -> isize {}

#[start]
fn f(argc: isize, argv: *const *const u8) -> isize {}
// error: multiple 'start' functions
```

This error indicates that the compiler found multiple functions with the
`#[start]` attribute. This is an error because there must be a unique entry
point into a Rust program. Example:

```
#![feature(start)]

#[start]
fn foo(argc: isize, argv: *const *const u8) -> isize { 0 } // ok!
```

## E0139

There are various restrictions on transmuting between types in Rust; for example
types being transmuted must have the same size. To apply all these restrictions,
the compiler must know the exact types that may be transmuted. When type
parameters are involved, this cannot always be done.

So, for example, the following is not allowed:

```
use std::mem::transmute;

struct Foo<T>(Vec<T>);

fn foo<T>(x: Vec<T>) {
    // we are transmuting between Vec<T> and Foo<F> here
    let y: Foo<T> = unsafe { transmute(x) };
    // do something with y
}
```

In this specific case there's a good chance that the transmute is harmless (but
this is not guaranteed by Rust). However, when alignment and enum optimizations
come into the picture, it's quite likely that the sizes may or may not match
with different type parameter substitutions. It's not possible to check this for
_all_ possible types, so `transmute()` simply only accepts types without any
unsubstituted type parameters.

If you need this, there's a good chance you're doing something wrong. Keep in
mind that Rust doesn't guarantee much about the layout of different structs
(even two structs with identical declarations may have different layouts). If
there is a solution that avoids the transmute entirely, try it instead.

If it's possible, hand-monomorphize the code by writing the function for each
possible type substitution. It's possible to use traits to do this cleanly,
for example:

```ignore
struct Foo<T>(Vec<T>);

trait MyTransmutableType {
    fn transmute(Vec<Self>) -> Foo<Self>;
}

impl MyTransmutableType for u8 {
    fn transmute(x: Foo<u8>) -> Vec<u8> {
        transmute(x)
    }
}

impl MyTransmutableType for String {
    fn transmute(x: Foo<String>) -> Vec<String> {
        transmute(x)
    }
}

// ... more impls for the types you intend to transmute

fn foo<T: MyTransmutableType>(x: Vec<T>) {
    let y: Foo<T> = <T as MyTransmutableType>::transmute(x);
    // do something with y
}
```

Each impl will be checked for a size match in the transmute as usual, and since
there are no unbound type parameters involved, this should compile unless there
is a size mismatch in one of the impls.

It is also possible to manually transmute:

```ignore
ptr::read(&v as *const _ as *const SomeType) // `v` transmuted to `SomeType`
```

Note that this does not move `v` (unlike `transmute`), and may need a
call to `mem::forget(v)` in case you want to avoid destructors being called.

## E0152

A lang item was redefined.

Erroneous code example:

```compile_fail,E0152
#![feature(lang_items)]

#[lang = "panic_fmt"]
struct Foo; // error: duplicate lang item found: `panic_fmt`
```

Lang items are already implemented in the standard library. Unless you are
writing a free-standing application (e.g. a kernel), you do not need to provide
them yourself.

You can build a free-standing crate by adding `#![no_std]` to the crate
attributes:

```
#![no_std]
```

See also https://doc.rust-lang.org/book/no-stdlib.html

## E0154

## Note: this error code is no longer emitted by the compiler.

Imports (`use` statements) are not allowed after non-item statements, such as
variable declarations and expression statements.

Here is an example that demonstrates the error:

```ignore
fn f() {
    // Variable declaration before import
    let x = 0;
    use std::io::Read;
    // ...
}
```

The solution is to declare the imports at the top of the block, function, or
file.

Here is the previous example again, with the correct order:

```
fn f() {
    use std::io::Read;
    let x = 0;
    // ...
}
```

See the Declaration Statements section of the reference for more information
about what constitutes an Item declaration and what does not:

https://doc.rust-lang.org/reference.html#statements

## E0158

`const` and `static` mean different things. A `const` is a compile-time
constant, an alias for a literal value. This property means you can match it
directly within a pattern.

The `static` keyword, on the other hand, guarantees a fixed location in memory.
This does not always mean that the value is constant. For example, a global
mutex can be declared `static` as well.

If you want to match against a `static`, consider using a guard instead:

```
static FORTY_TWO: i32 = 42;

match Some(42) {
    Some(x) if x == FORTY_TWO => {}
    _ => {}
}
```

## E0162

An if-let pattern attempts to match the pattern, and enters the body if the
match was successful. If the match is irrefutable (when it cannot fail to
match), use a regular `let`-binding instead. For instance:

```compile_fail,E0162
struct Irrefutable(i32);
let irr = Irrefutable(0);

// This fails to compile because the match is irrefutable.
if let Irrefutable(x) = irr {
    // This body will always be executed.
    // ...
}
```

Try this instead:

```
struct Irrefutable(i32);
let irr = Irrefutable(0);

let Irrefutable(x) = irr;
println!("{}", x);
```

## E0164

This error means that an attempt was made to match a struct type enum
variant as a non-struct type:

```compile_fail,E0164
enum Foo { B { i: u32 } }

fn bar(foo: Foo) -> u32 {
    match foo {
        Foo::B(i) => i, // error E0164
    }
}
```

Try using `{}` instead:

```
enum Foo { B { i: u32 } }

fn bar(foo: Foo) -> u32 {
    match foo {
        Foo::B{i} => i,
    }
}
```

## E0165

A while-let pattern attempts to match the pattern, and enters the body if the
match was successful. If the match is irrefutable (when it cannot fail to
match), use a regular `let`-binding inside a `loop` instead. For instance:

```compile_fail,E0165
struct Irrefutable(i32);
let irr = Irrefutable(0);

// This fails to compile because the match is irrefutable.
while let Irrefutable(x) = irr {
    // ...
}
```

Try this instead:

```no_run
struct Irrefutable(i32);
let irr = Irrefutable(0);

loop {
    let Irrefutable(x) = irr;
    // ...
}
```

## E0170

Enum variants are qualified by default. For example, given this type:

```
enum Method {
    GET,
    POST,
}
```

You would match it using:

```
enum Method {
    GET,
    POST,
}

let m = Method::GET;

match m {
    Method::GET => {},
    Method::POST => {},
}
```

If you don't qualify the names, the code will bind new variables named "GET" and
"POST" instead. This behavior is likely not what you want, so `rustc` warns when
that happens.

Qualified names are good practice, and most code works well with them. But if
you prefer them unqualified, you can import the variants into scope:

```ignore
use Method::*;
enum Method { GET, POST }
```

If you want others to be able to import variants from your module directly, use
`pub use`:

```ignore
pub use Method::*;
enum Method { GET, POST }
```

## E0172

This error means that an attempt was made to specify the type of a variable with
a combination of a concrete type and a trait. Consider the following example:

```compile_fail,E0172
fn foo(bar: i32+std::fmt::Display) {}
```

The code is trying to specify that we want to receive a signed 32-bit integer
which also implements `Display`. This doesn't make sense: when we pass `i32`, a
concrete type, it implicitly includes all of the traits that it implements.
This includes `Display`, `Debug`, `Clone`, and a host of others.

If `i32` implements the trait we desire, there's no need to specify the trait
separately. If it does not, then we need to `impl` the trait for `i32` before
passing it into `foo`. Either way, a fixed definition for `foo` will look like
the following:

```
fn foo(bar: i32) {}
```

To learn more about traits, take a look at the Book:

https://doc.rust-lang.org/book/traits.html

## E0178

In types, the `+` type operator has low precedence, so it is often necessary
to use parentheses.

For example:

```compile_fail,E0178
trait Foo {}

struct Bar<'a> {
    w: &'a Foo + Copy,   // error, use &'a (Foo + Copy)
    x: &'a Foo + 'a,     // error, use &'a (Foo + 'a)
    y: &'a mut Foo + 'a, // error, use &'a mut (Foo + 'a)
    z: fn() -> Foo + 'a, // error, use fn() -> (Foo + 'a)
}
```

More details can be found in [RFC 438].

[RFC 438]: https://github.com/rust-lang/rfcs/pull/438

## E0182

You bound an associated type in an expression path which is not
allowed.

Erroneous code example:

```compile_fail,E0182
trait Foo {
    type A;
    fn bar() -> isize;
}

impl Foo for isize {
    type A = usize;
    fn bar() -> isize { 42 }
}

// error: unexpected binding of associated item in expression path
let x: isize = Foo::<A=usize>::bar();
```

To give a concrete type when using the Universal Function Call Syntax,
use "Type as Trait". Example:

```
trait Foo {
    type A;
    fn bar() -> isize;
}

impl Foo for isize {
    type A = usize;
    fn bar() -> isize { 42 }
}

let x: isize = <isize as Foo>::bar(); // ok!
```

## E0184

Explicitly implementing both Drop and Copy for a type is currently disallowed.
This feature can make some sense in theory, but the current implementation is
incorrect and can lead to memory unsafety (see [issue #20126][iss20126]), so
it has been disabled for now.

[iss20126]: https://github.com/rust-lang/rust/issues/20126

## E0185

An associated function for a trait was defined to be static, but an
implementation of the trait declared the same function to be a method (i.e. to
take a `self` parameter).

Here's an example of this error:

```compile_fail,E0185
trait Foo {
    fn foo();
}

struct Bar;

impl Foo for Bar {
    // error, method `foo` has a `&self` declaration in the impl, but not in
    // the trait
    fn foo(&self) {}
}
```

## E0186

An associated function for a trait was defined to be a method (i.e. to take a
`self` parameter), but an implementation of the trait declared the same function
to be static.

Here's an example of this error:

```compile_fail,E0186
trait Foo {
    fn foo(&self);
}

struct Bar;

impl Foo for Bar {
    // error, method `foo` has a `&self` declaration in the trait, but not in
    // the impl
    fn foo() {}
}
```

## E0191

Trait objects need to have all associated types specified. Erroneous code
example:

```compile_fail,E0191
trait Trait {
    type Bar;
}

type Foo = Trait; // error: the value of the associated type `Bar` (from
                  //        the trait `Trait`) must be specified
```

Please verify you specified all associated types of the trait and that you
used the right trait. Example:

```
trait Trait {
    type Bar;
}

type Foo = Trait<Bar=i32>; // ok!
```

## E0192

Negative impls are only allowed for traits with default impls. For more
information see the [opt-in builtin traits RFC](https://github.com/rust-lang/
rfcs/blob/master/text/0019-opt-in-builtin-traits.md).

## E0193

`where` clauses must use generic type parameters: it does not make sense to use
them otherwise. An example causing this error:

```ignore
trait Foo {
    fn bar(&self);
}

#[derive(Copy,Clone)]
struct Wrapper<T> {
    Wrapped: T
}

impl Foo for Wrapper<u32> where Wrapper<u32>: Clone {
    fn bar(&self) { }
}
```

This use of a `where` clause is strange - a more common usage would look
something like the following:

```
trait Foo {
    fn bar(&self);
}

#[derive(Copy,Clone)]
struct Wrapper<T> {
    Wrapped: T
}
impl <T> Foo for Wrapper<T> where Wrapper<T>: Clone {
    fn bar(&self) { }
}
```

Here, we're saying that the implementation exists on Wrapper only when the
wrapped type `T` implements `Clone`. The `where` clause is important because
some types will not implement `Clone`, and thus will not get this method.

In our erroneous example, however, we're referencing a single concrete type.
Since we know for certain that `Wrapper<u32>` implements `Clone`, there's no
reason to also specify it in a `where` clause.

## E0194

A type parameter was declared which shadows an existing one. An example of this
error:

```compile_fail,E0194
trait Foo<T> {
    fn do_something(&self) -> T;
    fn do_something_else<T: Clone>(&self, bar: T);
}
```

In this example, the trait `Foo` and the trait method `do_something_else` both
define a type parameter `T`. This is not allowed: if the method wishes to
define a type parameter, it must use a different name for it.

## E0195

Your method's lifetime parameters do not match the trait declaration.
Erroneous code example:

```compile_fail,E0195
trait Trait {
    fn bar<'a,'b:'a>(x: &'a str, y: &'b str);
}

struct Foo;

impl Trait for Foo {
    fn bar<'a,'b>(x: &'a str, y: &'b str) {
    // error: lifetime parameters or bounds on method `bar`
    // do not match the trait declaration
    }
}
```

The lifetime constraint `'b` for bar() implementation does not match the
trait declaration. Ensure lifetime declarations match exactly in both trait
declaration and implementation. Example:

```
trait Trait {
    fn t<'a,'b:'a>(x: &'a str, y: &'b str);
}

struct Foo;

impl Trait for Foo {
    fn t<'a,'b:'a>(x: &'a str, y: &'b str) { // ok!
    }
}
```

## E0197

Inherent implementations (one that do not implement a trait but provide
methods associated with a type) are always safe because they are not
implementing an unsafe trait. Removing the `unsafe` keyword from the inherent
implementation will resolve this error.

```compile_fail,E0197
struct Foo;

// this will cause this error
unsafe impl Foo { }
// converting it to this will fix it
impl Foo { }
```

## E0198

A negative implementation is one that excludes a type from implementing a
particular trait. Not being able to use a trait is always a safe operation,
so negative implementations are always safe and never need to be marked as
unsafe.

```compile_fail
#![feature(optin_builtin_traits)]

struct Foo;

// unsafe is unnecessary
unsafe impl !Clone for Foo { }
```

This will compile:

```
#![feature(optin_builtin_traits)]

struct Foo;

trait Enterprise {}

impl Enterprise for .. { }

impl !Enterprise for Foo { }
```

Please note that negative impls are only allowed for traits with default impls.

## E0199

Safe traits should not have unsafe implementations, therefore marking an
implementation for a safe trait unsafe will cause a compiler error. Removing
the unsafe marker on the trait noted in the error will resolve this problem.

```compile_fail,E0199
struct Foo;

trait Bar { }

// this won't compile because Bar is safe
unsafe impl Bar for Foo { }
// this will compile
impl Bar for Foo { }
```

## E0200

Unsafe traits must have unsafe implementations. This error occurs when an
implementation for an unsafe trait isn't marked as unsafe. This may be resolved
by marking the unsafe implementation as unsafe.

```compile_fail,E0200
struct Foo;

unsafe trait Bar { }

// this won't compile because Bar is unsafe and impl isn't unsafe
impl Bar for Foo { }
// this will compile
unsafe impl Bar for Foo { }
```

## E0201

It is an error to define two associated items (like methods, associated types,
associated functions, etc.) with the same identifier.

For example:

```compile_fail,E0201
struct Foo(u8);

impl Foo {
    fn bar(&self) -> bool { self.0 > 5 }
    fn bar() {} // error: duplicate associated function
}

trait Baz {
    type Quux;
    fn baz(&self) -> bool;
}

impl Baz for Foo {
    type Quux = u32;

    fn baz(&self) -> bool { true }

    // error: duplicate method
    fn baz(&self) -> bool { self.0 > 5 }

    // error: duplicate associated type
    type Quux = u32;
}
```

Note, however, that items with the same name are allowed for inherent `impl`
blocks that don't overlap:

```
struct Foo<T>(T);

impl Foo<u8> {
    fn bar(&self) -> bool { self.0 > 5 }
}

impl Foo<bool> {
    fn bar(&self) -> bool { self.0 }
}
```

## E0202

Inherent associated types were part of [RFC 195] but are not yet implemented.
See [the tracking issue][iss8995] for the status of this implementation.

[RFC 195]: https://github.com/rust-lang/rfcs/pull/195
[iss8995]: https://github.com/rust-lang/rust/issues/8995

## E0204

An attempt to implement the `Copy` trait for a struct failed because one of the
fields does not implement `Copy`. To fix this, you must implement `Copy` for the
mentioned field. Note that this may not be possible, as in the example of

```compile_fail,E0204
struct Foo {
    foo : Vec<u32>,
}

impl Copy for Foo { }
```

This fails because `Vec<T>` does not implement `Copy` for any `T`.

Here's another example that will fail:

```compile_fail,E0204
#[derive(Copy)]
struct Foo<'a> {
    ty: &'a mut bool,
}
```

This fails because `&mut T` is not `Copy`, even when `T` is `Copy` (this
differs from the behavior for `&T`, which is always `Copy`).

## E0205

An attempt to implement the `Copy` trait for an enum failed because one of the
variants does not implement `Copy`. To fix this, you must implement `Copy` for
the mentioned variant. Note that this may not be possible, as in the example of

```compile_fail,E0205
enum Foo {
    Bar(Vec<u32>),
    Baz,
}

impl Copy for Foo { }
```

This fails because `Vec<T>` does not implement `Copy` for any `T`.

Here's another example that will fail:

```compile_fail,E0205
#[derive(Copy)]
enum Foo<'a> {
    Bar(&'a mut bool),
    Baz,
}
```

This fails because `&mut T` is not `Copy`, even when `T` is `Copy` (this
differs from the behavior for `&T`, which is always `Copy`).

## E0206

You can only implement `Copy` for a struct or enum. Both of the following
examples will fail, because neither `i32` (primitive type) nor `&'static Bar`
(reference to `Bar`) is a struct or enum:

```compile_fail,E0206
type Foo = i32;
impl Copy for Foo { } // error

#[derive(Copy, Clone)]
struct Bar;
impl Copy for &'static Bar { } // error
```

## E0207

Any type parameter or lifetime parameter of an `impl` must meet at least one of
the following criteria:

 - it appears in the self type of the impl
 - for a trait impl, it appears in the trait reference
 - it is bound as an associated type

### Error example 1

Suppose we have a struct `Foo` and we would like to define some methods for it.
The following definition leads to a compiler error:

```compile_fail,E0207
struct Foo;

impl<T: Default> Foo {
// error: the type parameter `T` is not constrained by the impl trait, self
// type, or predicates [E0207]
    fn get(&self) -> T {
        <T as Default>::default()
    }
}
```

The problem is that the parameter `T` does not appear in the self type (`Foo`)
of the impl. In this case, we can fix the error by moving the type parameter
from the `impl` to the method `get`:


```
struct Foo;

// Move the type parameter from the impl to the method
impl Foo {
    fn get<T: Default>(&self) -> T {
        <T as Default>::default()
    }
}
```

### Error example 2

As another example, suppose we have a `Maker` trait and want to establish a
type `FooMaker` that makes `Foo`s:

```compile_fail,E0207
trait Maker {
    type Item;
    fn make(&mut self) -> Self::Item;
}

struct Foo<T> {
    foo: T
}

struct FooMaker;

impl<T: Default> Maker for FooMaker {
// error: the type parameter `T` is not constrained by the impl trait, self
// type, or predicates [E0207]
    type Item = Foo<T>;

    fn make(&mut self) -> Foo<T> {
        Foo { foo: <T as Default>::default() }
    }
}
```

This fails to compile because `T` does not appear in the trait or in the
implementing type.

One way to work around this is to introduce a phantom type parameter into
`FooMaker`, like so:

```
use std::marker::PhantomData;

trait Maker {
    type Item;
    fn make(&mut self) -> Self::Item;
}

struct Foo<T> {
    foo: T
}

// Add a type parameter to `FooMaker`
struct FooMaker<T> {
    phantom: PhantomData<T>,
}

impl<T: Default> Maker for FooMaker<T> {
    type Item = Foo<T>;

    fn make(&mut self) -> Foo<T> {
        Foo {
            foo: <T as Default>::default(),
        }
    }
}
```

Another way is to do away with the associated type in `Maker` and use an input
type parameter instead:

```
// Use a type parameter instead of an associated type here
trait Maker<Item> {
    fn make(&mut self) -> Item;
}

struct Foo<T> {
    foo: T
}

struct FooMaker;

impl<T: Default> Maker<Foo<T>> for FooMaker {
    fn make(&mut self) -> Foo<T> {
        Foo { foo: <T as Default>::default() }
    }
}
```

### Additional information

For more information, please see [RFC 447].

[RFC 447]: https://github.com/rust-lang/rfcs/blob/master/text/0447-no-unused-impl-parameters.md

## E0210

This error indicates a violation of one of Rust's orphan rules for trait
implementations. The rule concerns the use of type parameters in an
implementation of a foreign trait (a trait defined in another crate), and
states that type parameters must be "covered" by a local type. To understand
what this means, it is perhaps easiest to consider a few examples.

If `ForeignTrait` is a trait defined in some external crate `foo`, then the
following trait `impl` is an error:

```compile_fail,E0210
extern crate collections;
use collections::range::RangeArgument;

impl<T> RangeArgument<T> for T { } // error

fn main() {}
```

To work around this, it can be covered with a local type, `MyType`:

```ignore
struct MyType<T>(T);
impl<T> ForeignTrait for MyType<T> { } // Ok
```

Please note that a type alias is not sufficient.

For another example of an error, suppose there's another trait defined in `foo`
named `ForeignTrait2` that takes two type parameters. Then this `impl` results
in the same rule violation:

```compile_fail
struct MyType2;
impl<T> ForeignTrait2<T, MyType<T>> for MyType2 { } // error
```

The reason for this is that there are two appearances of type parameter `T` in
the `impl` header, both as parameters for `ForeignTrait2`. The first appearance
is uncovered, and so runs afoul of the orphan rule.

Consider one more example:

```ignore
impl<T> ForeignTrait2<MyType<T>, T> for MyType2 { } // Ok
```

This only differs from the previous `impl` in that the parameters `T` and
`MyType<T>` for `ForeignTrait2` have been swapped. This example does *not*
violate the orphan rule; it is permitted.

To see why that last example was allowed, you need to understand the general
rule. Unfortunately this rule is a bit tricky to state. Consider an `impl`:

```ignore
impl<P1, ..., Pm> ForeignTrait<T1, ..., Tn> for T0 { ... }
```

where `P1, ..., Pm` are the type parameters of the `impl` and `T0, ..., Tn`
are types. One of the types `T0, ..., Tn` must be a local type (this is another
orphan rule, see the explanation for E0117). Let `i` be the smallest integer
such that `Ti` is a local type. Then no type parameter can appear in any of the
`Tj` for `j < i`.

For information on the design of the orphan rules, see [RFC 1023].

[RFC 1023]: https://github.com/rust-lang/rfcs/pull/1023

## E0214

A generic type was described using parentheses rather than angle brackets. For
example:

```compile_fail,E0214
fn main() {
    let v: Vec(&str) = vec!["foo"];
}
```

This is not currently supported: `v` should be defined as `Vec<&str>`.
Parentheses are currently only used with generic types when defining parameters
for `Fn`-family traits.

## E0220

You used an associated type which isn't defined in the trait.
Erroneous code example:

```compile_fail,E0220
trait T1 {
    type Bar;
}

type Foo = T1<F=i32>; // error: associated type `F` not found for `T1`

// or:

trait T2 {
    type Bar;

    // error: Baz is used but not declared
    fn return_bool(&self, &Self::Bar, &Self::Baz) -> bool;
}
```

Make sure that you have defined the associated type in the trait body.
Also, verify that you used the right trait or you didn't misspell the
associated type name. Example:

```
trait T1 {
    type Bar;
}

type Foo = T1<Bar=i32>; // ok!

// or:

trait T2 {
    type Bar;
    type Baz; // we declare `Baz` in our trait.

    // and now we can use it here:
    fn return_bool(&self, &Self::Bar, &Self::Baz) -> bool;
}
```

## E0221

An attempt was made to retrieve an associated type, but the type was ambiguous.
For example:

```compile_fail,E0221
trait T1 {}
trait T2 {}

trait Foo {
    type A: T1;
}

trait Bar : Foo {
    type A: T2;
    fn do_something() {
        let _: Self::A;
    }
}
```

In this example, `Foo` defines an associated type `A`. `Bar` inherits that type
from `Foo`, and defines another associated type of the same name. As a result,
when we attempt to use `Self::A`, it's ambiguous whether we mean the `A` defined
by `Foo` or the one defined by `Bar`.

There are two options to work around this issue. The first is simply to rename
one of the types. Alternatively, one can specify the intended type using the
following syntax:

```
trait T1 {}
trait T2 {}

trait Foo {
    type A: T1;
}

trait Bar : Foo {
    type A: T2;
    fn do_something() {
        let _: <Self as Bar>::A;
    }
}
```

## E0223

An attempt was made to retrieve an associated type, but the type was ambiguous.
For example:

```compile_fail,E0223
trait MyTrait {type X; }

fn main() {
    let foo: MyTrait::X;
}
```

The problem here is that we're attempting to take the type of X from MyTrait.
Unfortunately, the type of X is not defined, because it's only made concrete in
implementations of the trait. A working version of this code might look like:

```
trait MyTrait {type X; }
struct MyStruct;

impl MyTrait for MyStruct {
    type X = u32;
}

fn main() {
    let foo: <MyStruct as MyTrait>::X;
}
```

This syntax specifies that we want the X type from MyTrait, as made concrete in
MyStruct. The reason that we cannot simply use `MyStruct::X` is that MyStruct
might implement two different traits with identically-named associated types.
This syntax allows disambiguation between the two.

## E0225

You attempted to use multiple types as bounds for a closure or trait object.
Rust does not currently support this. A simple example that causes this error:

```compile_fail,E0225
fn main() {
    let _: Box<std::io::Read + std::io::Write>;
}
```

Send and Sync are an exception to this rule: it's possible to have bounds of
one non-builtin trait, plus either or both of Send and Sync. For example, the
following compiles correctly:

```
fn main() {
    let _: Box<std::io::Read + Send + Sync>;
}
```

## E0229

An associated type binding was done outside of the type parameter declaration
and `where` clause. Erroneous code example:

```compile_fail,E0229
pub trait Foo {
    type A;
    fn boo(&self) -> <Self as Foo>::A;
}

struct Bar;

impl Foo for isize {
    type A = usize;
    fn boo(&self) -> usize { 42 }
}

fn baz<I>(x: &<I as Foo<A=Bar>>::A) {}
// error: associated type bindings are not allowed here
```

To solve this error, please move the type bindings in the type parameter
declaration:

```ignore
fn baz<I: Foo<A=Bar>>(x: &<I as Foo>::A) {} // ok!
```

Or in the `where` clause:

```ignore
fn baz<I>(x: &<I as Foo>::A) where I: Foo<A=Bar> {}
```

## E0230

The trait has more type parameters specified than appear in its definition.

Erroneous example code:

```compile_fail,E0230
#![feature(on_unimplemented)]
#[rustc_on_unimplemented = "Trait error on `{Self}` with `<{A},{B},{C}>`"]
// error: there is no type parameter C on trait TraitWithThreeParams
trait TraitWithThreeParams<A,B>
{}
```

Include the correct number of type parameters and the compilation should
proceed:

```
#![feature(on_unimplemented)]
#[rustc_on_unimplemented = "Trait error on `{Self}` with `<{A},{B},{C}>`"]
trait TraitWithThreeParams<A,B,C> // ok!
{}
```

## E0232

The attribute must have a value. Erroneous code example:

```compile_fail,E0232
#![feature(on_unimplemented)]

#[rustc_on_unimplemented] // error: this attribute must have a value
trait Bar {}
```

Please supply the missing value of the attribute. Example:

```
#![feature(on_unimplemented)]

#[rustc_on_unimplemented = "foo"] // ok!
trait Bar {}
```

## E0243

This error indicates that not enough type parameters were found in a type or
trait.

For example, the `Foo` struct below is defined to be generic in `T`, but the
type parameter is missing in the definition of `Bar`:

```compile_fail,E0243
struct Foo<T> { x: T }

struct Bar { x: Foo }
```

## E0244

This error indicates that too many type parameters were found in a type or
trait.

For example, the `Foo` struct below has no type parameters, but is supplied
with two in the definition of `Bar`:

```compile_fail,E0244
struct Foo { x: bool }

struct Bar<S, T> { x: Foo<S, T> }
```

## E0248

This error indicates an attempt to use a value where a type is expected. For
example:

```compile_fail,E0248
enum Foo {
    Bar(u32)
}

fn do_something(x: Foo::Bar) { }
```

In this example, we're attempting to take a type of `Foo::Bar` in the
do_something function. This is not legal: `Foo::Bar` is a value of type `Foo`,
not a distinct static type. Likewise, it's not legal to attempt to
`impl Foo::Bar`: instead, you must `impl Foo` and then pattern match to specify
behavior for specific enum variants.

## E0251

## Note: this error code is no longer emitted by the compiler.

Two items of the same name cannot be imported without rebinding one of the
items under a new local name.

An example of this error:

```ignore
use foo::baz;
use bar::*; // error, do `use foo::baz as quux` instead on the previous line

fn main() {}

mod foo {
    pub struct baz;
}

mod bar {
    pub mod baz {}
}
```

## E0252

Two items of the same name cannot be imported without rebinding one of the
items under a new local name.

Erroneous code example:

```compile_fail,E0252
use foo::baz;
use bar::baz; // error, do `use bar::baz as quux` instead

fn main() {}

mod foo {
    pub struct baz;
}

mod bar {
    pub mod baz {}
}
```

You can use aliases in order to fix this error. Example:

```
use foo::baz as foo_baz;
use bar::baz; // ok!

fn main() {}

mod foo {
    pub struct baz;
}

mod bar {
    pub mod baz {}
}
```

Or you can reference the item with its parent:

```
use bar::baz;

fn main() {
    let x = foo::baz; // ok!
}

mod foo {
    pub struct baz;
}

mod bar {
    pub mod baz {}
}
```

## E0253

Attempt was made to import an unimportable value. This can happen when trying
to import a method from a trait.

Erroneous code example:

```compile_fail,E0253
mod foo {
    pub trait MyTrait {
        fn do_something();
    }
}

use foo::MyTrait::do_something;
// error: `do_something` is not directly importable

fn main() {}
```

It's invalid to directly import methods belonging to a trait or concrete type.

## E0254

Attempt was made to import an item whereas an extern crate with this name has
already been imported.

Erroneous code example:

```compile_fail,E0254
extern crate collections;

mod foo {
    pub trait collections {
        fn do_something();
    }
}

use foo::collections; // error: an extern crate named `collections` has already
                      //        been imported in this module

fn main() {}
```

To fix issue issue, you have to rename at least one of the two imports.
Example:

```ignore
extern crate collections as libcollections; // ok!

mod foo {
    pub trait collections {
        fn do_something();
    }
}

use foo::collections;

fn main() {}
```

## E0255

You can't import a value whose name is the same as another value defined in the
module.

Erroneous code example:

```compile_fail,E0255
use bar::foo; // error: an item named `foo` is already in scope

fn foo() {}

mod bar {
     pub fn foo() {}
}

fn main() {}
```

You can use aliases in order to fix this error. Example:

```
use bar::foo as bar_foo; // ok!

fn foo() {}

mod bar {
     pub fn foo() {}
}

fn main() {}
```

Or you can reference the item with its parent:

```
fn foo() {}

mod bar {
     pub fn foo() {}
}

fn main() {
    bar::foo(); // we get the item by referring to its parent
}
```

## E0256

## Note: this error code is no longer emitted by the compiler.

You can't import a type or module when the name of the item being imported is
the same as another type or submodule defined in the module.

An example of this error:

```compile_fail
use foo::Bar; // error

type Bar = u32;

mod foo {
    pub mod Bar { }
}

fn main() {}
```

## E0259

The name chosen for an external crate conflicts with another external crate
that has been imported into the current module.

Erroneous code example:

```compile_fail,E0259
extern crate std;
extern crate libc as std;

fn main() {}
```

The solution is to choose a different name that doesn't conflict with any
external crate imported into the current module.

Correct example:

```ignore
extern crate std;
extern crate libc as other_name;
```

## E0260

The name for an item declaration conflicts with an external crate's name.

Erroneous code example:

```ignore,E0260
extern crate abc;

struct abc;
```

There are two possible solutions:

Solution #1: Rename the item.

```ignore
extern crate abc;

struct xyz;
```

Solution #2: Import the crate with a different name.

```ignore
extern crate abc as xyz;

struct abc;
```

See the Declaration Statements section of the reference for more information
about what constitutes an Item declaration and what does not:

https://doc.rust-lang.org/reference.html#statements

## E0261

When using a lifetime like `'a` in a type, it must be declared before being
used.

These two examples illustrate the problem:

```compile_fail,E0261
// error, use of undeclared lifetime name `'a`
fn foo(x: &'a str) { }

struct Foo {
    // error, use of undeclared lifetime name `'a`
    x: &'a str,
}
```

These can be fixed by declaring lifetime parameters:

```
fn foo<'a>(x: &'a str) {}

struct Foo<'a> {
    x: &'a str,
}
```

## E0262

Declaring certain lifetime names in parameters is disallowed. For example,
because the `'static` lifetime is a special built-in lifetime name denoting
the lifetime of the entire program, this is an error:

```compile_fail,E0262
// error, invalid lifetime parameter name `'static`
fn foo<'static>(x: &'static str) { }
```

## E0263

A lifetime name cannot be declared more than once in the same scope. For
example:

```compile_fail,E0263
// error, lifetime name `'a` declared twice in the same scope
fn foo<'a, 'b, 'a>(x: &'a str, y: &'b str) { }
```

## E0264

An unknown external lang item was used. Erroneous code example:

```compile_fail,E0264
#![feature(lang_items)]

extern "C" {
    #[lang = "cake"] // error: unknown external lang item: `cake`
    fn cake();
}
```

A list of available external lang items is available in
`src/librustc/middle/weak_lang_items.rs`. Example:

```
#![feature(lang_items)]

extern "C" {
    #[lang = "panic_fmt"] // ok!
    fn cake();
}
```

## E0271

This is because of a type mismatch between the associated type of some
trait (e.g. `T::Bar`, where `T` implements `trait Quux { type Bar; }`)
and another type `U` that is required to be equal to `T::Bar`, but is not.
Examples follow.

Here is a basic example:

```compile_fail,E0271
trait Trait { type AssociatedType; }

fn foo<T>(t: T) where T: Trait<AssociatedType=u32> {
    println!("in foo");
}

impl Trait for i8 { type AssociatedType = &'static str; }

foo(3_i8);
```

Here is that same example again, with some explanatory comments:

```ignore
trait Trait { type AssociatedType; }

fn foo<T>(t: T) where T: Trait<AssociatedType=u32> {
//                    ~~~~~~~~ ~~~~~~~~~~~~~~~~~~
//                        |            |
//         This says `foo` can         |
//           only be used with         |
//              some type that         |
//         implements `Trait`.         |
//                                     |
//                             This says not only must
//                             `T` be an impl of `Trait`
//                             but also that the impl
//                             must assign the type `u32`
//                             to the associated type.
    println!("in foo");
}

impl Trait for i8 { type AssociatedType = &'static str; }
~~~~~~~~~~~~~~~~~   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
//      |                             |
// `i8` does have                     |
// implementation                     |
// of `Trait`...                      |
//                     ... but it is an implementation
//                     that assigns `&'static str` to
//                     the associated type.

foo(3_i8);
// Here, we invoke `foo` with an `i8`, which does not satisfy
// the constraint `<i8 as Trait>::AssociatedType=u32`, and
// therefore the type-checker complains with this error code.
```

Here is a more subtle instance of the same problem, that can
arise with for-loops in Rust:

```compile_fail
let vs: Vec<i32> = vec![1, 2, 3, 4];
for v in &vs {
    match v {
        1 => {},
        _ => {},
    }
}
```

The above fails because of an analogous type mismatch,
though may be harder to see. Again, here are some
explanatory comments for the same example:

```ignore
{
    let vs = vec![1, 2, 3, 4];

    // `for`-loops use a protocol based on the `Iterator`
    // trait. Each item yielded in a `for` loop has the
    // type `Iterator::Item` -- that is, `Item` is the
    // associated type of the concrete iterator impl.
    for v in &vs {
//      ~    ~~~
//      |     |
//      |    We borrow `vs`, iterating over a sequence of
//      |    *references* of type `&Elem` (where `Elem` is
//      |    vector's element type). Thus, the associated
//      |    type `Item` must be a reference `&`-type ...
//      |
//  ... and `v` has the type `Iterator::Item`, as dictated by
//  the `for`-loop protocol ...

        match v {
            1 => {}
//          ~
//          |
// ... but *here*, `v` is forced to have some integral type;
// only types like `u8`,`i8`,`u16`,`i16`, et cetera can
// match the pattern `1` ...

            _ => {}
        }

// ... therefore, the compiler complains, because it sees
// an attempt to solve the equations
// `some integral-type` = type-of-`v`
//                      = `Iterator::Item`
//                      = `&Elem` (i.e. `some reference type`)
//
// which cannot possibly all be true.

    }
}
```

To avoid those issues, you have to make the types match correctly.
So we can fix the previous examples like this:

```
// Basic Example:
trait Trait { type AssociatedType; }

fn foo<T>(t: T) where T: Trait<AssociatedType = &'static str> {
    println!("in foo");
}

impl Trait for i8 { type AssociatedType = &'static str; }

foo(3_i8);

// For-Loop Example:
let vs = vec![1, 2, 3, 4];
for v in &vs {
    match v {
        &1 => {}
        _ => {}
    }
}
```

## E0272

The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
message for when a particular trait isn't implemented on a type placed in a
position that needs that trait. For example, when the following code is
compiled:

```compile_fail
#![feature(on_unimplemented)]

fn foo<T: Index<u8>>(x: T){}

#[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
trait Index<Idx> { /* ... */ }

foo(true); // `bool` does not implement `Index<u8>`
```

There will be an error about `bool` not implementing `Index<u8>`, followed by a
note saying "the type `bool` cannot be indexed by `u8`".

As you can see, you can specify type parameters in curly braces for
substitution with the actual types (using the regular format string syntax) in
a given situation. Furthermore, `{Self}` will substitute to the type (in this
case, `bool`) that we tried to use.

This error appears when the curly braces contain an identifier which doesn't
match with any of the type parameters or the string `Self`. This might happen
if you misspelled a type parameter, or if you intended to use literal curly
braces. If it is the latter, escape the curly braces with a second curly brace
of the same type; e.g. a literal `{` is `{{`.

## E0273

The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
message for when a particular trait isn't implemented on a type placed in a
position that needs that trait. For example, when the following code is
compiled:

```compile_fail
#![feature(on_unimplemented)]

fn foo<T: Index<u8>>(x: T){}

#[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
trait Index<Idx> { /* ... */ }

foo(true); // `bool` does not implement `Index<u8>`
```

there will be an error about `bool` not implementing `Index<u8>`, followed by a
note saying "the type `bool` cannot be indexed by `u8`".

As you can see, you can specify type parameters in curly braces for
substitution with the actual types (using the regular format string syntax) in
a given situation. Furthermore, `{Self}` will substitute to the type (in this
case, `bool`) that we tried to use.

This error appears when the curly braces do not contain an identifier. Please
add one of the same name as a type parameter. If you intended to use literal
braces, use `{{` and `}}` to escape them.

## E0274

The `#[rustc_on_unimplemented]` attribute lets you specify a custom error
message for when a particular trait isn't implemented on a type placed in a
position that needs that trait. For example, when the following code is
compiled:

```compile_fail
#![feature(on_unimplemented)]

fn foo<T: Index<u8>>(x: T){}

#[rustc_on_unimplemented = "the type `{Self}` cannot be indexed by `{Idx}`"]
trait Index<Idx> { /* ... */ }

foo(true); // `bool` does not implement `Index<u8>`
```

there will be an error about `bool` not implementing `Index<u8>`, followed by a
note saying "the type `bool` cannot be indexed by `u8`".

For this to work, some note must be specified. An empty attribute will not do
anything, please remove the attribute or add some helpful note for users of the
trait.

## E0275

This error occurs when there was a recursive trait requirement that overflowed
before it could be evaluated. Often this means that there is unbounded
recursion in resolving some type bounds.

For example, in the following code:

```compile_fail,E0275
trait Foo {}

struct Bar<T>(T);

impl<T> Foo for T where Bar<T>: Foo {}
```

To determine if a `T` is `Foo`, we need to check if `Bar<T>` is `Foo`. However,
to do this check, we need to determine that `Bar<Bar<T>>` is `Foo`. To
determine this, we check if `Bar<Bar<Bar<T>>>` is `Foo`, and so on. This is
clearly a recursive requirement that can't be resolved directly.

Consider changing your trait bounds so that they're less self-referential.

## E0276

This error occurs when a bound in an implementation of a trait does not match
the bounds specified in the original trait. For example:

```compile_fail,E0276
trait Foo {
    fn foo<T>(x: T);
}

impl Foo for bool {
    fn foo<T>(x: T) where T: Copy {}
}
```

Here, all types implementing `Foo` must have a method `foo<T>(x: T)` which can
take any type `T`. However, in the `impl` for `bool`, we have added an extra
bound that `T` is `Copy`, which isn't compatible with the original trait.

Consider removing the bound from the method or adding the bound to the original
method definition in the trait.

## E0277

You tried to use a type which doesn't implement some trait in a place which
expected that trait. Erroneous code example:

```compile_fail,E0277
// here we declare the Foo trait with a bar method
trait Foo {
    fn bar(&self);
}

// we now declare a function which takes an object implementing the Foo trait
fn some_func<T: Foo>(foo: T) {
    foo.bar();
}

fn main() {
    // we now call the method with the i32 type, which doesn't implement
    // the Foo trait
    some_func(5i32); // error: the trait bound `i32 : Foo` is not satisfied
}
```

In order to fix this error, verify that the type you're using does implement
the trait. Example:

```
trait Foo {
    fn bar(&self);
}

fn some_func<T: Foo>(foo: T) {
    foo.bar(); // we can now use this method since i32 implements the
               // Foo trait
}

// we implement the trait on the i32 type
impl Foo for i32 {
    fn bar(&self) {}
}

fn main() {
    some_func(5i32); // ok!
}
```

Or in a generic context, an erroneous code example would look like:

```compile_fail,E0277
fn some_func<T>(foo: T) {
    println!("{:?}", foo); // error: the trait `core::fmt::Debug` is not
                           //        implemented for the type `T`
}

fn main() {
    // We now call the method with the i32 type,
    // which *does* implement the Debug trait.
    some_func(5i32);
}
```

Note that the error here is in the definition of the generic function: Although
we only call it with a parameter that does implement `Debug`, the compiler
still rejects the function: It must work with all possible input types. In
order to make this example compile, we need to restrict the generic type we're
accepting:

```
use std::fmt;

// Restrict the input type to types that implement Debug.
fn some_func<T: fmt::Debug>(foo: T) {
    println!("{:?}", foo);
}

fn main() {
    // Calling the method is still fine, as i32 implements Debug.
    some_func(5i32);

    // This would fail to compile now:
    // struct WithoutDebug;
    // some_func(WithoutDebug);
}
```

Rust only looks at the signature of the called function, as such it must
already specify all requirements that will be used for every type parameter.

## E0281

You tried to supply a type which doesn't implement some trait in a location
which expected that trait. This error typically occurs when working with
`Fn`-based types. Erroneous code example:

```compile_fail,E0281
fn foo<F: Fn()>(x: F) { }

fn main() {
    // type mismatch: the type ... implements the trait `core::ops::Fn<(_,)>`,
    // but the trait `core::ops::Fn<()>` is required (expected (), found tuple
    // [E0281]
    foo(|y| { });
}
```

The issue in this case is that `foo` is defined as accepting a `Fn` with no
arguments, but the closure we attempted to pass to it requires one argument.

## E0282

This error indicates that type inference did not result in one unique possible
type, and extra information is required. In most cases this can be provided
by adding a type annotation. Sometimes you need to specify a generic type
parameter manually.

A common example is the `collect` method on `Iterator`. It has a generic type
parameter with a `FromIterator` bound, which for a `char` iterator is
implemented by `Vec` and `String` among others. Consider the following snippet
that reverses the characters of a string:

```compile_fail,E0282
let x = "hello".chars().rev().collect();
```

In this case, the compiler cannot infer what the type of `x` should be:
`Vec<char>` and `String` are both suitable candidates. To specify which type to
use, you can use a type annotation on `x`:

```
let x: Vec<char> = "hello".chars().rev().collect();
```

It is not necessary to annotate the full type. Once the ambiguity is resolved,
the compiler can infer the rest:

```
let x: Vec<_> = "hello".chars().rev().collect();
```

Another way to provide the compiler with enough information, is to specify the
generic type parameter:

```
let x = "hello".chars().rev().collect::<Vec<char>>();
```

Again, you need not specify the full type if the compiler can infer it:

```
let x = "hello".chars().rev().collect::<Vec<_>>();
```

Apart from a method or function with a generic type parameter, this error can
occur when a type parameter of a struct or trait cannot be inferred. In that
case it is not always possible to use a type annotation, because all candidates
have the same return type. For instance:

```compile_fail,E0282
struct Foo<T> {
    num: T,
}

impl<T> Foo<T> {
    fn bar() -> i32 {
        0
    }

    fn baz() {
        let number = Foo::bar();
    }
}
```

This will fail because the compiler does not know which instance of `Foo` to
call `bar` on. Change `Foo::bar()` to `Foo::<T>::bar()` to resolve the error.

## E0283

This error occurs when the compiler doesn't have enough information
to unambiguously choose an implementation.

For example:

```compile_fail,E0283
trait Generator {
    fn create() -> u32;
}

struct Impl;

impl Generator for Impl {
    fn create() -> u32 { 1 }
}

struct AnotherImpl;

impl Generator for AnotherImpl {
    fn create() -> u32 { 2 }
}

fn main() {
    let cont: u32 = Generator::create();
    // error, impossible to choose one of Generator trait implementation
    // Impl or AnotherImpl? Maybe anything else?
}
```

To resolve this error use the concrete type:

```
trait Generator {
    fn create() -> u32;
}

struct AnotherImpl;

impl Generator for AnotherImpl {
    fn create() -> u32 { 2 }
}

fn main() {
    let gen1 = AnotherImpl::create();

    // if there are multiple methods with same name (different traits)
    let gen2 = <AnotherImpl as Generator>::create();
}
```

## E0296

This error indicates that the given recursion limit could not be parsed. Ensure
that the value provided is a positive integer between quotes.

Erroneous code example:

```compile_fail,E0296
#![recursion_limit]

fn main() {}
```

And a working example:

```
#![recursion_limit="1000"]

fn main() {}
```

## E0297

Patterns used to bind names must be irrefutable. That is, they must guarantee
that a name will be extracted in all cases. Instead of pattern matching the
loop variable, consider using a `match` or `if let` inside the loop body. For
instance:

```compile_fail,E0297
let xs : Vec<Option<i32>> = vec![Some(1), None];

// This fails because `None` is not covered.
for Some(x) in xs {
    // ...
}
```

Match inside the loop instead:

```
let xs : Vec<Option<i32>> = vec![Some(1), None];

for item in xs {
    match item {
        Some(x) => {},
        None => {},
    }
}
```

Or use `if let`:

```
let xs : Vec<Option<i32>> = vec![Some(1), None];

for item in xs {
    if let Some(x) = item {
        // ...
    }
}
```

## E0301

Mutable borrows are not allowed in pattern guards, because matching cannot have
side effects. Side effects could alter the matched object or the environment
on which the match depends in such a way, that the match would not be
exhaustive. For instance, the following would not match any arm if mutable
borrows were allowed:

```compile_fail,E0301
match Some(()) {
    None => { },
    option if option.take().is_none() => {
        /* impossible, option is `Some` */
    },
    Some(_) => { } // When the previous match failed, the option became `None`.
}
```

## E0302

Assignments are not allowed in pattern guards, because matching cannot have
side effects. Side effects could alter the matched object or the environment
on which the match depends in such a way, that the match would not be
exhaustive. For instance, the following would not match any arm if assignments
were allowed:

```compile_fail,E0302
match Some(()) {
    None => { },
    option if { option = None; false } => { },
    Some(_) => { } // When the previous match failed, the option became `None`.
}
```

## E0303

In certain cases it is possible for sub-bindings to violate memory safety.
Updates to the borrow checker in a future version of Rust may remove this
restriction, but for now patterns must be rewritten without sub-bindings.

Before:

```compile_fail,E0303
match Some("hi".to_string()) {
    ref op_string_ref @ Some(s) => {},
    None => {},
}
```

After:

```
match Some("hi".to_string()) {
    Some(ref s) => {
        let op_string_ref = &Some(s);
        // ...
    },
    None => {},
}
```

The `op_string_ref` binding has type `&Option<&String>` in both cases.

See also https://github.com/rust-lang/rust/issues/14587

## E0306

In an array literal `[x; N]`, `N` is the number of elements in the array. This
must be an unsigned integer. Erroneous code example:

```compile_fail,E0306
let x = [0i32; true]; // error: expected positive integer for repeat count,
                      //        found boolean
```

Working example:

```
let x = [0i32; 2];
```

## E0308

This error occurs when the compiler was unable to infer the concrete type of a
variable. It can occur for several cases, the most common of which is a
mismatch in the expected type that the compiler inferred for a variable's
initializing expression, and the actual type explicitly assigned to the
variable.

For example:

```compile_fail,E0308
let x: i32 = "I am not a number!";
//     ~~~   ~~~~~~~~~~~~~~~~~~~~
//      |             |
//      |    initializing expression;
//      |    compiler infers type `&str`
//      |
//    type `i32` assigned to variable `x`
```

## E0309

Types in type definitions have lifetimes associated with them that represent
how long the data stored within them is guaranteed to be live. This lifetime
must be as long as the data needs to be alive, and missing the constraint that
denotes this will cause this error.

```compile_fail,E0309
// This won't compile because T is not constrained, meaning the data
// stored in it is not guaranteed to last as long as the reference
struct Foo<'a, T> {
    foo: &'a T
}
```

This will compile, because it has the constraint on the type parameter:

```
struct Foo<'a, T: 'a> {
    foo: &'a T
}
```

## E0310

Types in type definitions have lifetimes associated with them that represent
how long the data stored within them is guaranteed to be live. This lifetime
must be as long as the data needs to be alive, and missing the constraint that
denotes this will cause this error.

```compile_fail,E0310
// This won't compile because T is not constrained to the static lifetime
// the reference needs
struct Foo<T> {
    foo: &'static T
}
```

This will compile, because it has the constraint on the type parameter:

```
struct Foo<T: 'static> {
    foo: &'static T
}
```

## E0312

A lifetime of reference outlives lifetime of borrowed content.

Erroneous code example:

```compile_fail,E0312
fn make_child<'human, 'elve>(x: &mut &'human isize, y: &mut &'elve isize) {
    *x = *y;
    // error: lifetime of reference outlives lifetime of borrowed content
}
```

The compiler cannot determine if the `human` lifetime will live long enough
to keep up on the elve one. To solve this error, you have to give an
explicit lifetime hierarchy:

```
fn make_child<'human, 'elve: 'human>(x: &mut &'human isize,
                                     y: &mut &'elve isize) {
    *x = *y; // ok!
}
```

Or use the same lifetime for every variable:

```
fn make_child<'elve>(x: &mut &'elve isize, y: &mut &'elve isize) {
    *x = *y; // ok!
}
```

## E0317

This error occurs when an `if` expression without an `else` block is used in a
context where a type other than `()` is expected, for example a `let`
expression:

```compile_fail,E0317
fn main() {
    let x = 5;
    let a = if x == 5 { 1 };
}
```

An `if` expression without an `else` block has the type `()`, so this is a type
error. To resolve it, add an `else` block having the same type as the `if`
block.

## E0318

Default impls for a trait must be located in the same crate where the trait was
defined. For more information see the [opt-in builtin traits RFC](https://github
.com/rust-lang/rfcs/blob/master/text/0019-opt-in-builtin-traits.md).

## E0321

A cross-crate opt-out trait was implemented on something which wasn't a struct
or enum type. Erroneous code example:

```compile_fail,E0321
#![feature(optin_builtin_traits)]

struct Foo;

impl !Sync for Foo {}

unsafe impl Send for &'static Foo {}
// error: cross-crate traits with a default impl, like `core::marker::Send`,
//        can only be implemented for a struct/enum type, not
//        `&'static Foo`
```

Only structs and enums are permitted to impl Send, Sync, and other opt-out
trait, and the struct or enum must be local to the current crate. So, for
example, `unsafe impl Send for Rc<Foo>` is not allowed.

## E0322

The `Sized` trait is a special trait built-in to the compiler for types with a
constant size known at compile-time. This trait is automatically implemented
for types as needed by the compiler, and it is currently disallowed to
explicitly implement it for a type.

## E0323

An associated const was implemented when another trait item was expected.
Erroneous code example:

```compile_fail,E0323
#![feature(associated_consts)]

trait Foo {
    type N;
}

struct Bar;

impl Foo for Bar {
    const N : u32 = 0;
    // error: item `N` is an associated const, which doesn't match its
    //        trait `<Bar as Foo>`
}
```

Please verify that the associated const wasn't misspelled and the correct trait
was implemented. Example:

```
struct Bar;

trait Foo {
    type N;
}

impl Foo for Bar {
    type N = u32; // ok!
}
```

Or:

```
#![feature(associated_consts)]

struct Bar;

trait Foo {
    const N : u32;
}

impl Foo for Bar {
    const N : u32 = 0; // ok!
}
```

## E0324

A method was implemented when another trait item was expected. Erroneous
code example:

```compile_fail,E0324
#![feature(associated_consts)]

struct Bar;

trait Foo {
    const N : u32;

    fn M();
}

impl Foo for Bar {
    fn N() {}
    // error: item `N` is an associated method, which doesn't match its
    //        trait `<Bar as Foo>`
}
```

To fix this error, please verify that the method name wasn't misspelled and
verify that you are indeed implementing the correct trait items. Example:

```
#![feature(associated_consts)]

struct Bar;

trait Foo {
    const N : u32;

    fn M();
}

impl Foo for Bar {
    const N : u32 = 0;

    fn M() {} // ok!
}
```

## E0325

An associated type was implemented when another trait item was expected.
Erroneous code example:

```compile_fail,E0325
#![feature(associated_consts)]

struct Bar;

trait Foo {
    const N : u32;
}

impl Foo for Bar {
    type N = u32;
    // error: item `N` is an associated type, which doesn't match its
    //        trait `<Bar as Foo>`
}
```

Please verify that the associated type name wasn't misspelled and your
implementation corresponds to the trait definition. Example:

```
struct Bar;

trait Foo {
    type N;
}

impl Foo for Bar {
    type N = u32; // ok!
}
```

Or:

```
#![feature(associated_consts)]

struct Bar;

trait Foo {
    const N : u32;
}

impl Foo for Bar {
    const N : u32 = 0; // ok!
}
```

## E0326

The types of any associated constants in a trait implementation must match the
types in the trait definition. This error indicates that there was a mismatch.

Here's an example of this error:

```compile_fail,E0326
#![feature(associated_consts)]

trait Foo {
    const BAR: bool;
}

struct Bar;

impl Foo for Bar {
    const BAR: u32 = 5; // error, expected bool, found u32
}
```

## E0329

An attempt was made to access an associated constant through either a generic
type parameter or `Self`. This is not supported yet. An example causing this
error is shown below:

```ignore
#![feature(associated_consts)]

trait Foo {
    const BAR: f64;
}

struct MyStruct;

impl Foo for MyStruct {
    const BAR: f64 = 0f64;
}

fn get_bar_bad<F: Foo>(t: F) -> f64 {
    F::BAR
}
```

Currently, the value of `BAR` for a particular type can only be accessed
through a concrete type, as shown below:

```ignore
#![feature(associated_consts)]

trait Foo {
    const BAR: f64;
}

struct MyStruct;

fn get_bar_good() -> f64 {
    <MyStruct as Foo>::BAR
}
```

## E0364

Private items cannot be publicly re-exported. This error indicates that you
attempted to `pub use` a type or value that was not itself public.

Erroneous code example:

```compile_fail
mod foo {
    const X: u32 = 1;
}

pub use foo::X;

fn main() {}
```

The solution to this problem is to ensure that the items that you are
re-exporting are themselves marked with `pub`:

```
mod foo {
    pub const X: u32 = 1;
}

pub use foo::X;

fn main() {}
```

See the 'Use Declarations' section of the reference for more information on
this topic:

https://doc.rust-lang.org/reference.html#use-declarations

## E0365

Private modules cannot be publicly re-exported. This error indicates that you
attempted to `pub use` a module that was not itself public.

Erroneous code example:

```compile_fail,E0365
mod foo {
    pub const X: u32 = 1;
}

pub use foo as foo2;

fn main() {}
```

The solution to this problem is to ensure that the module that you are
re-exporting is itself marked with `pub`:

```
pub mod foo {
    pub const X: u32 = 1;
}

pub use foo as foo2;

fn main() {}
```

See the 'Use Declarations' section of the reference for more information
on this topic:

https://doc.rust-lang.org/reference.html#use-declarations

## E0366

An attempt was made to implement `Drop` on a concrete specialization of a
generic type. An example is shown below:

```compile_fail,E0366
struct Foo<T> {
    t: T
}

impl Drop for Foo<u32> {
    fn drop(&mut self) {}
}
```

This code is not legal: it is not possible to specialize `Drop` to a subset of
implementations of a generic type. One workaround for this is to wrap the
generic type, as shown below:

```
struct Foo<T> {
    t: T
}

struct Bar {
    t: Foo<u32>
}

impl Drop for Bar {
    fn drop(&mut self) {}
}
```

## E0367

An attempt was made to implement `Drop` on a specialization of a generic type.
An example is shown below:

```compile_fail,E0367
trait Foo{}

struct MyStruct<T> {
    t: T
}

impl<T: Foo> Drop for MyStruct<T> {
    fn drop(&mut self) {}
}
```

This code is not legal: it is not possible to specialize `Drop` to a subset of
implementations of a generic type. In order for this code to work, `MyStruct`
must also require that `T` implements `Foo`. Alternatively, another option is
to wrap the generic type in another that specializes appropriately:

```
trait Foo{}

struct MyStruct<T> {
    t: T
}

struct MyStructWrapper<T: Foo> {
    t: MyStruct<T>
}

impl <T: Foo> Drop for MyStructWrapper<T> {
    fn drop(&mut self) {}
}
```

## E0368

This error indicates that a binary assignment operator like `+=` or `^=` was
applied to a type that doesn't support it. For example:

```compile_fail,E0368
let mut x = 12f32; // error: binary operation `<<` cannot be applied to
                   //        type `f32`

x <<= 2;
```

To fix this error, please check that this type implements this binary
operation. Example:

```
let mut x = 12u32; // the `u32` type does implement the `ShlAssign` trait

x <<= 2; // ok!
```

It is also possible to overload most operators for your own type by
implementing the `[OP]Assign` traits from `std::ops`.

Another problem you might be facing is this: suppose you've overloaded the `+`
operator for some type `Foo` by implementing the `std::ops::Add` trait for
`Foo`, but you find that using `+=` does not work, as in this example:

```compile_fail,E0368
use std::ops::Add;

struct Foo(u32);

impl Add for Foo {
    type Output = Foo;

    fn add(self, rhs: Foo) -> Foo {
        Foo(self.0 + rhs.0)
    }
}

fn main() {
    let mut x: Foo = Foo(5);
    x += Foo(7); // error, `+= cannot be applied to the type `Foo`
}
```

This is because `AddAssign` is not automatically implemented, so you need to
manually implement it for your type.

## E0369

A binary operation was attempted on a type which doesn't support it.
Erroneous code example:

```compile_fail,E0369
let x = 12f32; // error: binary operation `<<` cannot be applied to
               //        type `f32`

x << 2;
```

To fix this error, please check that this type implements this binary
operation. Example:

```
let x = 12u32; // the `u32` type does implement it:
               // https://doc.rust-lang.org/stable/std/ops/trait.Shl.html

x << 2; // ok!
```

It is also possible to overload most operators for your own type by
implementing traits from `std::ops`.

## E0370

The maximum value of an enum was reached, so it cannot be automatically
set in the next enum value. Erroneous code example:

```compile_fail
#[deny(overflowing_literals)]
enum Foo {
    X = 0x7fffffffffffffff,
    Y, // error: enum discriminant overflowed on value after
       //        9223372036854775807: i64; set explicitly via
       //        Y = -9223372036854775808 if that is desired outcome
}
```

To fix this, please set manually the next enum value or put the enum variant
with the maximum value at the end of the enum. Examples:

```
enum Foo {
    X = 0x7fffffffffffffff,
    Y = 0, // ok!
}
```

Or:

```
enum Foo {
    Y = 0, // ok!
    X = 0x7fffffffffffffff,
}
```

## E0371

When `Trait2` is a subtrait of `Trait1` (for example, when `Trait2` has a
definition like `trait Trait2: Trait1 { ... }`), it is not allowed to implement
`Trait1` for `Trait2`. This is because `Trait2` already implements `Trait1` by
definition, so it is not useful to do this.

Example:

```compile_fail,E0371
trait Foo { fn foo(&self) { } }
trait Bar: Foo { }
trait Baz: Bar { }

impl Bar for Baz { } // error, `Baz` implements `Bar` by definition
impl Foo for Baz { } // error, `Baz` implements `Bar` which implements `Foo`
impl Baz for Baz { } // error, `Baz` (trivially) implements `Baz`
impl Baz for Bar { } // Note: This is OK
```

## E0373

This error occurs when an attempt is made to use data captured by a closure,
when that data may no longer exist. It's most commonly seen when attempting to
return a closure:

```compile_fail,E0373
fn foo() -> Box<Fn(u32) -> u32> {
    let x = 0u32;
    Box::new(|y| x + y)
}
```

Notice that `x` is stack-allocated by `foo()`. By default, Rust captures
closed-over data by reference. This means that once `foo()` returns, `x` no
longer exists. An attempt to access `x` within the closure would thus be
unsafe.

Another situation where this might be encountered is when spawning threads:

```compile_fail,E0373
fn foo() {
    let x = 0u32;
    let y = 1u32;

    let thr = std::thread::spawn(|| {
        x + y
    });
}
```

Since our new thread runs in parallel, the stack frame containing `x` and `y`
may well have disappeared by the time we try to use them. Even if we call
`thr.join()` within foo (which blocks until `thr` has completed, ensuring the
stack frame won't disappear), we will not succeed: the compiler cannot prove
that this behaviour is safe, and so won't let us do it.

The solution to this problem is usually to switch to using a `move` closure.
This approach moves (or copies, where possible) data into the closure, rather
than taking references to it. For example:

```
fn foo() -> Box<Fn(u32) -> u32> {
    let x = 0u32;
    Box::new(move |y| x + y)
}
```

Now that the closure has its own copy of the data, there's no need to worry
about safety.

## E0374

A struct without a field containing an unsized type cannot implement
`CoerceUnsized`. An
[unsized type](https://doc.rust-lang.org/book/unsized-types.html)
is any type that the compiler doesn't know the length or alignment of at
compile time. Any struct containing an unsized type is also unsized.

Example of erroneous code:

```compile_fail,E0374
#![feature(coerce_unsized)]
use std::ops::CoerceUnsized;

struct Foo<T: ?Sized> {
    a: i32,
}

// error: Struct `Foo` has no unsized fields that need `CoerceUnsized`.
impl<T, U> CoerceUnsized<Foo<U>> for Foo<T>
    where T: CoerceUnsized<U> {}
```

`CoerceUnsized` is used to coerce one struct containing an unsized type
into another struct containing a different unsized type. If the struct
doesn't have any fields of unsized types then you don't need explicit
coercion to get the types you want. To fix this you can either
not try to implement `CoerceUnsized` or you can add a field that is
unsized to the struct.

Example:

```
#![feature(coerce_unsized)]
use std::ops::CoerceUnsized;

// We don't need to impl `CoerceUnsized` here.
struct Foo {
    a: i32,
}

// We add the unsized type field to the struct.
struct Bar<T: ?Sized> {
    a: i32,
    b: T,
}

// The struct has an unsized field so we can implement
// `CoerceUnsized` for it.
impl<T, U> CoerceUnsized<Bar<U>> for Bar<T>
    where T: CoerceUnsized<U> {}
```

Note that `CoerceUnsized` is mainly used by smart pointers like `Box`, `Rc`
and `Arc` to be able to mark that they can coerce unsized types that they
are pointing at.

## E0375

A struct with more than one field containing an unsized type cannot implement
`CoerceUnsized`. This only occurs when you are trying to coerce one of the
types in your struct to another type in the struct. In this case we try to
impl `CoerceUnsized` from `T` to `U` which are both types that the struct
takes. An [unsized type](https://doc.rust-lang.org/book/unsized-types.html)
is any type that the compiler doesn't know the length or alignment of at
compile time. Any struct containing an unsized type is also unsized.

Example of erroneous code:

```compile_fail,E0375
#![feature(coerce_unsized)]
use std::ops::CoerceUnsized;

struct Foo<T: ?Sized, U: ?Sized> {
    a: i32,
    b: T,
    c: U,
}

// error: Struct `Foo` has more than one unsized field.
impl<T, U> CoerceUnsized<Foo<U, T>> for Foo<T, U> {}
```

`CoerceUnsized` only allows for coercion from a structure with a single
unsized type field to another struct with a single unsized type field.
In fact Rust only allows for a struct to have one unsized type in a struct
and that unsized type must be the last field in the struct. So having two
unsized types in a single struct is not allowed by the compiler. To fix this
use only one field containing an unsized type in the struct and then use
multiple structs to manage each unsized type field you need.

Example:

```
#![feature(coerce_unsized)]
use std::ops::CoerceUnsized;

struct Foo<T: ?Sized> {
    a: i32,
    b: T,
}

impl <T, U> CoerceUnsized<Foo<U>> for Foo<T>
    where T: CoerceUnsized<U> {}

fn coerce_foo<T: CoerceUnsized<U>, U>(t: T) -> Foo<U> {
    Foo { a: 12i32, b: t } // we use coercion to get the `Foo<U>` type we need
}
```


## E0376

The type you are trying to impl `CoerceUnsized` for is not a struct.
`CoerceUnsized` can only be implemented for a struct. Unsized types are
already able to be coerced without an implementation of `CoerceUnsized`
whereas a struct containing an unsized type needs to know the unsized type
field it's containing is able to be coerced. An
[unsized type](https://doc.rust-lang.org/book/unsized-types.html)
is any type that the compiler doesn't know the length or alignment of at
compile time. Any struct containing an unsized type is also unsized.

Example of erroneous code:

```compile_fail,E0376
#![feature(coerce_unsized)]
use std::ops::CoerceUnsized;

struct Foo<T: ?Sized> {
    a: T,
}

// error: The type `U` is not a struct
impl<T, U> CoerceUnsized<U> for Foo<T> {}
```

The `CoerceUnsized` trait takes a struct type. Make sure the type you are
providing to `CoerceUnsized` is a struct with only the last field containing an
unsized type.

Example:

```
#![feature(coerce_unsized)]
use std::ops::CoerceUnsized;

struct Foo<T> {
    a: T,
}

// The `Foo<U>` is a struct so `CoerceUnsized` can be implemented
impl<T, U> CoerceUnsized<Foo<U>> for Foo<T> where T: CoerceUnsized<U> {}
```

Note that in Rust, structs can only contain an unsized type if the field
containing the unsized type is the last and only unsized type field in the
struct.

## E0380

Default impls are only allowed for traits with no methods or associated items.
For more information see the [opt-in builtin traits RFC](https://github.com/rust
-lang/rfcs/blob/master/text/0019-opt-in-builtin-traits.md).

## E0381

It is not allowed to use or capture an uninitialized variable. For example:

```compile_fail,E0381
fn main() {
    let x: i32;
    let y = x; // error, use of possibly uninitialized variable
}
```

To fix this, ensure that any declared variables are initialized before being
used. Example:

```
fn main() {
    let x: i32 = 0;
    let y = x; // ok!
}
```

## E0382

This error occurs when an attempt is made to use a variable after its contents
have been moved elsewhere. For example:

```compile_fail,E0382
struct MyStruct { s: u32 }

fn main() {
    let mut x = MyStruct{ s: 5u32 };
    let y = x;
    x.s = 6;
    println!("{}", x.s);
}
```

Since `MyStruct` is a type that is not marked `Copy`, the data gets moved out
of `x` when we set `y`. This is fundamental to Rust's ownership system: outside
of workarounds like `Rc`, a value cannot be owned by more than one variable.

If we own the type, the easiest way to address this problem is to implement
`Copy` and `Clone` on it, as shown below. This allows `y` to copy the
information in `x`, while leaving the original version owned by `x`. Subsequent
changes to `x` will not be reflected when accessing `y`.

```
#[derive(Copy, Clone)]
struct MyStruct { s: u32 }

fn main() {
    let mut x = MyStruct{ s: 5u32 };
    let y = x;
    x.s = 6;
    println!("{}", x.s);
}
```

Alternatively, if we don't control the struct's definition, or mutable shared
ownership is truly required, we can use `Rc` and `RefCell`:

```
use std::cell::RefCell;
use std::rc::Rc;

struct MyStruct { s: u32 }

fn main() {
    let mut x = Rc::new(RefCell::new(MyStruct{ s: 5u32 }));
    let y = x.clone();
    x.borrow_mut().s = 6;
    println!("{}", x.borrow().s);
}
```

With this approach, x and y share ownership of the data via the `Rc` (reference
count type). `RefCell` essentially performs runtime borrow checking: ensuring
that at most one writer or multiple readers can access the data at any one time.

If you wish to learn more about ownership in Rust, start with the chapter in the
Book:

https://doc.rust-lang.org/book/ownership.html

## E0383

This error occurs when an attempt is made to partially reinitialize a
structure that is currently uninitialized.

For example, this can happen when a drop has taken place:

```ignore
struct Foo {
    a: u32,
}

let mut x = Foo { a: 1 };
drop(x); // `x` is now uninitialized
x.a = 2; // error, partial reinitialization of uninitialized structure `t`
```

This error can be fixed by fully reinitializing the structure in question:

```
struct Foo {
    a: u32,
}

let mut x = Foo { a: 1 };
drop(x);
x = Foo { a: 2 };
```

## E0384

This error occurs when an attempt is made to reassign an immutable variable.
For example:

```compile_fail,E0384
fn main() {
    let x = 3;
    x = 5; // error, reassignment of immutable variable
}
```

By default, variables in Rust are immutable. To fix this error, add the keyword
`mut` after the keyword `let` when declaring the variable. For example:

```
fn main() {
    let mut x = 3;
    x = 5;
}
```

## E0386

This error occurs when an attempt is made to mutate the target of a mutable
reference stored inside an immutable container.

For example, this can happen when storing a `&mut` inside an immutable `Box`:

```compile_fail,E0386
let mut x: i64 = 1;
let y: Box<_> = Box::new(&mut x);
**y = 2; // error, cannot assign to data in an immutable container
```

This error can be fixed by making the container mutable:

```
let mut x: i64 = 1;
let mut y: Box<_> = Box::new(&mut x);
**y = 2;
```

It can also be fixed by using a type with interior mutability, such as `Cell`
or `RefCell`:

```
use std::cell::Cell;

let x: i64 = 1;
let y: Box<Cell<_>> = Box::new(Cell::new(x));
y.set(2);
```

## E0387

This error occurs when an attempt is made to mutate or mutably reference data
that a closure has captured immutably. Examples of this error are shown below:

```compile_fail,E0387
// Accepts a function or a closure that captures its environment immutably.
// Closures passed to foo will not be able to mutate their closed-over state.
fn foo<F: Fn()>(f: F) { }

// Attempts to mutate closed-over data. Error message reads:
// `cannot assign to data in a captured outer variable...`
fn mutable() {
    let mut x = 0u32;
    foo(|| x = 2);
}

// Attempts to take a mutable reference to closed-over data.  Error message
// reads: `cannot borrow data mutably in a captured outer variable...`
fn mut_addr() {
    let mut x = 0u32;
    foo(|| { let y = &mut x; });
}
```

The problem here is that foo is defined as accepting a parameter of type `Fn`.
Closures passed into foo will thus be inferred to be of type `Fn`, meaning that
they capture their context immutably.

If the definition of `foo` is under your control, the simplest solution is to
capture the data mutably. This can be done by defining `foo` to take FnMut
rather than Fn:

```
fn foo<F: FnMut()>(f: F) { }
```

Alternatively, we can consider using the `Cell` and `RefCell` types to achieve
interior mutability through a shared reference. Our example's `mutable`
function could be redefined as below:

```
use std::cell::Cell;

fn foo<F: Fn()>(f: F) { }

fn mutable() {
    let x = Cell::new(0u32);
    foo(|| x.set(2));
}
```

You can read more about cell types in the API documentation:

https://doc.rust-lang.org/std/cell/

## E0388

A mutable borrow was attempted in a static location.

Erroneous code example:

```compile_fail,E0388
static X: i32 = 1;

static STATIC_REF: &'static mut i32 = &mut X;
// error: cannot borrow data mutably in a static location

const CONST_REF: &'static mut i32 = &mut X;
// error: cannot borrow data mutably in a static location
```

To fix this error, you have to use constant borrow:

```
static X: i32 = 1;

static STATIC_REF: &'static i32 = &X;
```

## E0389

An attempt was made to mutate data using a non-mutable reference. This
commonly occurs when attempting to assign to a non-mutable reference of a
mutable reference (`&(&mut T)`).

Example of erroneous code:

```compile_fail,E0389
struct FancyNum {
    num: u8,
}

fn main() {
    let mut fancy = FancyNum{ num: 5 };
    let fancy_ref = &(&mut fancy);
    fancy_ref.num = 6; // error: cannot assign to data in a `&` reference
    println!("{}", fancy_ref.num);
}
```

Here, `&mut fancy` is mutable, but `&(&mut fancy)` is not. Creating an
immutable reference to a value borrows it immutably. There can be multiple
references of type `&(&mut T)` that point to the same value, so they must be
immutable to prevent multiple mutable references to the same value.

To fix this, either remove the outer reference:

```
struct FancyNum {
    num: u8,
}

fn main() {
    let mut fancy = FancyNum{ num: 5 };

    let fancy_ref = &mut fancy;
    // `fancy_ref` is now &mut FancyNum, rather than &(&mut FancyNum)

    fancy_ref.num = 6; // No error!

    println!("{}", fancy_ref.num);
}
```

Or make the outer reference mutable:

```
struct FancyNum {
    num: u8
}

fn main() {
    let mut fancy = FancyNum{ num: 5 };

    let fancy_ref = &mut (&mut fancy);
    // `fancy_ref` is now &mut(&mut FancyNum), rather than &(&mut FancyNum)

    fancy_ref.num = 6; // No error!

    println!("{}", fancy_ref.num);
}
```

## E0390

You tried to implement methods for a primitive type. Erroneous code example:

```compile_fail,E0390
struct Foo {
    x: i32
}

impl *mut Foo {}
// error: only a single inherent implementation marked with
//        `#[lang = "mut_ptr"]` is allowed for the `*mut T` primitive
```

This isn't allowed, but using a trait to implement a method is a good solution.
Example:

```
struct Foo {
    x: i32
}

trait Bar {
    fn bar();
}

impl Bar for *mut Foo {
    fn bar() {} // ok!
}
```

## E0391

This error indicates that some types or traits depend on each other
and therefore cannot be constructed.

The following example contains a circular dependency between two traits:

```compile_fail,E0391
trait FirstTrait : SecondTrait {

}

trait SecondTrait : FirstTrait {

}
```

## E0392

This error indicates that a type or lifetime parameter has been declared
but not actually used. Here is an example that demonstrates the error:

```compile_fail,E0392
enum Foo<T> {
    Bar,
}
```

If the type parameter was included by mistake, this error can be fixed
by simply removing the type parameter, as shown below:

```
enum Foo {
    Bar,
}
```

Alternatively, if the type parameter was intentionally inserted, it must be
used. A simple fix is shown below:

```
enum Foo<T> {
    Bar(T),
}
```

This error may also commonly be found when working with unsafe code. For
example, when using raw pointers one may wish to specify the lifetime for
which the pointed-at data is valid. An initial attempt (below) causes this
error:

```compile_fail,E0392
struct Foo<'a, T> {
    x: *const T,
}
```

We want to express the constraint that Foo should not outlive `'a`, because
the data pointed to by `T` is only valid for that lifetime. The problem is
that there are no actual uses of `'a`. It's possible to work around this
by adding a PhantomData type to the struct, using it to tell the compiler
to act as if the struct contained a borrowed reference `&'a T`:

```
use std::marker::PhantomData;

struct Foo<'a, T: 'a> {
    x: *const T,
    phantom: PhantomData<&'a T>
}
```

PhantomData can also be used to express information about unused type
parameters. You can read more about it in the API documentation:

https://doc.rust-lang.org/std/marker/struct.PhantomData.html

## E0393

A type parameter which references `Self` in its default value was not specified.
Example of erroneous code:

```compile_fail,E0393
trait A<T=Self> {}

fn together_we_will_rule_the_galaxy(son: &A) {}
// error: the type parameter `T` must be explicitly specified in an
//        object type because its default value `Self` references the
//        type `Self`
```

A trait object is defined over a single, fully-defined trait. With a regular
default parameter, this parameter can just be substituted in. However, if the
default parameter is `Self`, the trait changes for each concrete type; i.e.
`i32` will be expected to implement `A<i32>`, `bool` will be expected to
implement `A<bool>`, etc... These types will not share an implementation of a
fully-defined trait; instead they share implementations of a trait with
different parameters substituted in for each implementation. This is
irreconcilable with what we need to make a trait object work, and is thus
disallowed. Making the trait concrete by explicitly specifying the value of the
defaulted parameter will fix this issue. Fixed example:

```
trait A<T=Self> {}

fn together_we_will_rule_the_galaxy(son: &A<i32>) {} // Ok!
```

## E0398

In Rust 1.3, the default object lifetime bounds are expected to change, as
described in RFC #1156 [1]. You are getting a warning because the compiler
thinks it is possible that this change will cause a compilation error in your
code. It is possible, though unlikely, that this is a false alarm.

The heart of the change is that where `&'a Box<SomeTrait>` used to default to
`&'a Box<SomeTrait+'a>`, it now defaults to `&'a Box<SomeTrait+'static>` (here,
`SomeTrait` is the name of some trait type). Note that the only types which are
affected are references to boxes, like `&Box<SomeTrait>` or
`&[Box<SomeTrait>]`. More common types like `&SomeTrait` or `Box<SomeTrait>`
are unaffected.

To silence this warning, edit your code to use an explicit bound. Most of the
time, this means that you will want to change the signature of a function that
you are calling. For example, if the error is reported on a call like `foo(x)`,
and `foo` is defined as follows:

```ignore
fn foo(arg: &Box<SomeTrait>) { ... }
```

You might change it to:

```ignore
fn foo<'a>(arg: &Box<SomeTrait+'a>) { ... }
```

This explicitly states that you expect the trait object `SomeTrait` to contain
references (with a maximum lifetime of `'a`).

[1]: https://github.com/rust-lang/rfcs/pull/1156

## E0399

You implemented a trait, overriding one or more of its associated types but did
not reimplement its default methods.

Example of erroneous code:

```compile_fail,E0399
#![feature(associated_type_defaults)]

pub trait Foo {
    type Assoc = u8;
    fn bar(&self) {}
}

impl Foo for i32 {
    // error - the following trait items need to be reimplemented as
    //         `Assoc` was overridden: `bar`
    type Assoc = i32;
}
```

To fix this, add an implementation for each default method from the trait:

```
#![feature(associated_type_defaults)]

pub trait Foo {
    type Assoc = u8;
    fn bar(&self) {}
}

impl Foo for i32 {
    type Assoc = i32;
    fn bar(&self) {} // ok!
}
```

## E0401

Inner items do not inherit type parameters from the functions they are embedded
in.

Erroneous code example:

```compile_fail,E0401
fn foo<T>(x: T) {
    fn bar(y: T) { // T is defined in the "outer" function
        // ..
    }
    bar(x);
}
```

Nor will this:

```compile_fail,E0401
fn foo<T>(x: T) {
    type MaybeT = Option<T>;
    // ...
}
```

Or this:

```compile_fail,E0401
fn foo<T>(x: T) {
    struct Foo {
        x: T,
    }
    // ...
}
```

Items inside functions are basically just like top-level items, except
that they can only be used from the function they are in.

There are a couple of solutions for this.

If the item is a function, you may use a closure:

```
fn foo<T>(x: T) {
    let bar = |y: T| { // explicit type annotation may not be necessary
        // ..
    };
    bar(x);
}
```

For a generic item, you can copy over the parameters:

```
fn foo<T>(x: T) {
    fn bar<T>(y: T) {
        // ..
    }
    bar(x);
}
```

```
fn foo<T>(x: T) {
    type MaybeT<T> = Option<T>;
}
```

Be sure to copy over any bounds as well:

```
fn foo<T: Copy>(x: T) {
    fn bar<T: Copy>(y: T) {
        // ..
    }
    bar(x);
}
```

```
fn foo<T: Copy>(x: T) {
    struct Foo<T: Copy> {
        x: T,
    }
}
```

This may require additional type hints in the function body.

In case the item is a function inside an `impl`, defining a private helper
function might be easier:

```ignore
impl<T> Foo<T> {
    pub fn foo(&self, x: T) {
        self.bar(x);
    }

    fn bar(&self, y: T) {
        // ..
    }
}
```

For default impls in traits, the private helper solution won't work, however
closures or copying the parameters should still work.

## E0403

Some type parameters have the same name.

Erroneous code example:

```compile_fail,E0403
fn foo<T, T>(s: T, u: T) {} // error: the name `T` is already used for a type
                            //        parameter in this type parameter list
```

Please verify that none of the type parameterss are misspelled, and rename any
clashing parameters. Example:

```
fn foo<T, Y>(s: T, u: Y) {} // ok!
```

## E0404

You tried to implement something which was not a trait on an object.

Erroneous code example:

```compile_fail,E0404
struct Foo;
struct Bar;

impl Foo for Bar {} // error: `Foo` is not a trait
```

Please verify that you didn't misspell the trait's name or otherwise use the
wrong identifier. Example:

```
trait Foo {
    // some functions
}
struct Bar;

impl Foo for Bar { // ok!
    // functions implementation
}
```

## E0405

The code refers to a trait that is not in scope.

Erroneous code example:

```compile_fail,E0405
struct Foo;

impl SomeTrait for Foo {} // error: trait `SomeTrait` is not in scope
```

Please verify that the name of the trait wasn't misspelled and ensure that it
was imported. Example:

```ignore
// solution 1:
use some_file::SomeTrait;

// solution 2:
trait SomeTrait {
    // some functions
}

struct Foo;

impl SomeTrait for Foo { // ok!
    // implements functions
}
```

## E0407

A definition of a method not in the implemented trait was given in a trait
implementation.

Erroneous code example:

```compile_fail,E0407
trait Foo {
    fn a();
}

struct Bar;

impl Foo for Bar {
    fn a() {}
    fn b() {} // error: method `b` is not a member of trait `Foo`
}
```

Please verify you didn't misspell the method name and you used the correct
trait. First example:

```
trait Foo {
    fn a();
    fn b();
}

struct Bar;

impl Foo for Bar {
    fn a() {}
    fn b() {} // ok!
}
```

Second example:

```
trait Foo {
    fn a();
}

struct Bar;

impl Foo for Bar {
    fn a() {}
}

impl Bar {
    fn b() {}
}
```

## E0408

An "or" pattern was used where the variable bindings are not consistently bound
across patterns.

Erroneous code example:

```compile_fail,E0408
match x {
    Some(y) | None => { /* use y */ } // error: variable `y` from pattern #1 is
                                      //        not bound in pattern #2
    _ => ()
}
```

Here, `y` is bound to the contents of the `Some` and can be used within the
block corresponding to the match arm. However, in case `x` is `None`, we have
not specified what `y` is, and the block will use a nonexistent variable.

To fix this error, either split into multiple match arms:

```
let x = Some(1);
match x {
    Some(y) => { /* use y */ }
    None => { /* ... */ }
}
```

or, bind the variable to a field of the same type in all sub-patterns of the
or pattern:

```
let x = (0, 2);
match x {
    (0, y) | (y, 0) => { /* use y */}
    _ => {}
}
```

In this example, if `x` matches the pattern `(0, _)`, the second field is set
to `y`. If it matches `(_, 0)`, the first field is set to `y`; so in all
cases `y` is set to some value.

## E0409

An "or" pattern was used where the variable bindings are not consistently bound
across patterns.

Erroneous code example:

```compile_fail,E0409
let x = (0, 2);
match x {
    (0, ref y) | (y, 0) => { /* use y */} // error: variable `y` is bound with
                                          //        different mode in pattern #2
                                          //        than in pattern #1
    _ => ()
}
```

Here, `y` is bound by-value in one case and by-reference in the other.

To fix this error, just use the same mode in both cases.
Generally using `ref` or `ref mut` where not already used will fix this:

```ignore
let x = (0, 2);
match x {
    (0, ref y) | (ref y, 0) => { /* use y */}
    _ => ()
}
```

Alternatively, split the pattern:

```
let x = (0, 2);
match x {
    (y, 0) => { /* use y */ }
    (0, ref y) => { /* use y */}
    _ => ()
}
```

## E0411

The `Self` keyword was used outside an impl or a trait.

Erroneous code example:

```compile_fail,E0411
<Self>::foo; // error: use of `Self` outside of an impl or trait
```

The `Self` keyword represents the current type, which explains why it can only
be used inside an impl or a trait. It gives access to the associated items of a
type:

```
trait Foo {
    type Bar;
}

trait Baz : Foo {
    fn bar() -> Self::Bar; // like this
}
```

However, be careful when two types have a common associated type:

```compile_fail
trait Foo {
    type Bar;
}

trait Foo2 {
    type Bar;
}

trait Baz : Foo + Foo2 {
    fn bar() -> Self::Bar;
    // error: ambiguous associated type `Bar` in bounds of `Self`
}
```

This problem can be solved by specifying from which trait we want to use the
`Bar` type:

```
trait Foo {
    type Bar;
}

trait Foo2 {
    type Bar;
}

trait Baz : Foo + Foo2 {
    fn bar() -> <Self as Foo>::Bar; // ok!
}
```

## E0412

The type name used is not in scope.

Erroneous code examples:

```compile_fail,E0412
impl Something {} // error: type name `Something` is not in scope

// or:

trait Foo {
    fn bar(N); // error: type name `N` is not in scope
}

// or:

fn foo(x: T) {} // type name `T` is not in scope
```

To fix this error, please verify you didn't misspell the type name, you did
declare it or imported it into the scope. Examples:

```
struct Something;

impl Something {} // ok!

// or:

trait Foo {
    type N;

    fn bar(Self::N); // ok!
}

// or:

fn foo<T>(x: T) {} // ok!
```

## E0415

More than one function parameter have the same name.

Erroneous code example:

```compile_fail,E0415
fn foo(f: i32, f: i32) {} // error: identifier `f` is bound more than
                          //        once in this parameter list
```

Please verify you didn't misspell parameters' name. Example:

```
fn foo(f: i32, g: i32) {} // ok!
```

## E0416

An identifier is bound more than once in a pattern.

Erroneous code example:

```compile_fail,E0416
match (1, 2) {
    (x, x) => {} // error: identifier `x` is bound more than once in the
                 //        same pattern
}
```

Please verify you didn't misspell identifiers' name. Example:

```
match (1, 2) {
    (x, y) => {} // ok!
}
```

Or maybe did you mean to unify? Consider using a guard:

```ignore
match (A, B, C) {
    (x, x2, see) if x == x2 => { /* A and B are equal, do one thing */ }
    (y, z, see) => { /* A and B unequal; do another thing */ }
}
```

## E0423

A `struct` variant name was used like a function name.

Erroneous code example:

```compile_fail,E0423
struct Foo { a: bool };

let f = Foo();
// error: `Foo` is a struct variant name, but this expression uses
//        it like a function name
```

Please verify you didn't misspell the name of what you actually wanted to use
here. Example:

```
fn Foo() -> u32 { 0 }

let f = Foo(); // ok!
```

## E0424

The `self` keyword was used in a static method.

Erroneous code example:

```compile_fail,E0424
struct Foo;

impl Foo {
    fn bar(self) {}

    fn foo() {
        self.bar(); // error: `self` is not available in a static method.
    }
}
```

Please check if the method's argument list should have contained `self`,
`&self`, or `&mut self` (in case you didn't want to create a static
method), and add it if so. Example:

```
struct Foo;

impl Foo {
    fn bar(self) {}

    fn foo(self) {
        self.bar(); // ok!
    }
}
```

## E0425

An unresolved name was used.

Erroneous code examples:

```compile_fail,E0425
something_that_doesnt_exist::foo;
// error: unresolved name `something_that_doesnt_exist::foo`

// or:

trait Foo {
    fn bar() {
        Self; // error: unresolved name `Self`
    }
}

// or:

let x = unknown_variable;  // error: unresolved name `unknown_variable`
```

Please verify that the name wasn't misspelled and ensure that the
identifier being referred to is valid for the given situation. Example:

```
enum something_that_does_exist {
    Foo,
}
```

Or:

```
mod something_that_does_exist {
    pub static foo : i32 = 0i32;
}

something_that_does_exist::foo; // ok!
```

Or:

```
let unknown_variable = 12u32;
let x = unknown_variable; // ok!
```

If the item is not defined in the current module, it must be imported using a
`use` statement, like so:

```ignore
use foo::bar;
bar();
```

If the item you are importing is not defined in some super-module of the
current module, then it must also be declared as public (e.g., `pub fn`).

## E0426

An undeclared label was used.

Erroneous code example:

```compile_fail,E0426
loop {
    break 'a; // error: use of undeclared label `'a`
}
```

Please verify you spelt or declare the label correctly. Example:

```
'a: loop {
    break 'a; // ok!
}
```

## E0428

A type or module has been defined more than once.

Erroneous code example:

```compile_fail,E0428
struct Bar;
struct Bar; // error: duplicate definition of value `Bar`
```

Please verify you didn't misspell the type/module's name or remove/rename the
duplicated one. Example:

```
struct Bar;
struct Bar2; // ok!
```

## E0429

The `self` keyword cannot appear alone as the last segment in a `use`
declaration.

Erroneous code example:

```compile_fail,E0429
use std::fmt::self; // error: `self` imports are only allowed within a { } list
```

To use a namespace itself in addition to some of its members, `self` may appear
as part of a brace-enclosed list of imports:

```
use std::fmt::{self, Debug};
```

If you only want to import the namespace, do so directly:

```
use std::fmt;
```

## E0430

The `self` import appears more than once in the list.

Erroneous code example:

```compile_fail,E0430
use something::{self, self}; // error: `self` import can only appear once in
                             //        the list
```

Please verify you didn't misspell the import name or remove the duplicated
`self` import. Example:

```ignore
use something::self; // ok!
```

## E0431

An invalid `self` import was made.

Erroneous code example:

```compile_fail,E0431
use {self}; // error: `self` import can only appear in an import list with a
            //        non-empty prefix
```

You cannot import the current module into itself, please remove this import
or verify you didn't misspell it.

## E0432

An import was unresolved.

Erroneous code example:

```compile_fail,E0432
use something::Foo; // error: unresolved import `something::Foo`.
```

Paths in `use` statements are relative to the crate root. To import items
relative to the current and parent modules, use the `self::` and `super::`
prefixes, respectively. Also verify that you didn't misspell the import
name and that the import exists in the module from where you tried to
import it. Example:

```ignore
use self::something::Foo; // ok!

mod something {
    pub struct Foo;
}
```

Or, if you tried to use a module from an external crate, you may have missed
the `extern crate` declaration (which is usually placed in the crate root):

```ignore
extern crate homura; // Required to use the `homura` crate

use homura::Madoka;
```

## E0433

An undeclared type or module was used.

Erroneous code example:

```compile_fail,E0433
let map = HashMap::new();
// error: failed to resolve. Use of undeclared type or module `HashMap`
```

Please verify you didn't misspell the type/module's name or that you didn't
forgot to import it:


```
use std::collections::HashMap; // HashMap has been imported.
let map: HashMap<u32, u32> = HashMap::new(); // So it can be used!
```

## E0434

This error indicates that a variable usage inside an inner function is invalid
because the variable comes from a dynamic environment. Inner functions do not
have access to their containing environment.

Erroneous code example:

```compile_fail,E0434
fn foo() {
    let y = 5;
    fn bar() -> u32 {
        y // error: can't capture dynamic environment in a fn item; use the
          //        || { ... } closure form instead.
    }
}
```

Functions do not capture local variables. To fix this error, you can replace the
function with a closure:

```
fn foo() {
    let y = 5;
    let bar = || {
        y
    };
}
```

or replace the captured variable with a constant or a static item:

```
fn foo() {
    static mut X: u32 = 4;
    const Y: u32 = 5;
    fn bar() -> u32 {
        unsafe {
            X = 3;
        }
        Y
    }
}
```

## E0435

A non-constant value was used to initialise a constant.

Erroneous code example:

```compile_fail,E0435
let foo = 42u32;
const FOO : u32 = foo; // error: attempt to use a non-constant value in a
                       //        constant
```

To fix this error, please replace the value with a constant. Example:

```
const FOO : u32 = 42u32; // ok!
```

Or:

```
const OTHER_FOO : u32 = 42u32;
const FOO : u32 = OTHER_FOO; // ok!
```

## E0437

Trait implementations can only implement associated types that are members of
the trait in question. This error indicates that you attempted to implement
an associated type whose name does not match the name of any associated type
in the trait.

Erroneous code example:

```compile_fail,E0437
trait Foo {}

impl Foo for i32 {
    type Bar = bool;
}
```

The solution to this problem is to remove the extraneous associated type:

```
trait Foo {}

impl Foo for i32 {}
```

## E0438

Trait implementations can only implement associated constants that are
members of the trait in question. This error indicates that you
attempted to implement an associated constant whose name does not
match the name of any associated constant in the trait.

Erroneous code example:

```compile_fail,E0438
#![feature(associated_consts)]

trait Foo {}

impl Foo for i32 {
    const BAR: bool = true;
}
```

The solution to this problem is to remove the extraneous associated constant:

```
trait Foo {}

impl Foo for i32 {}
```

## E0439

The length of the platform-intrinsic function `simd_shuffle`
wasn't specified. Erroneous code example:

```compile_fail,E0439
#![feature(platform_intrinsics)]

extern "platform-intrinsic" {
    fn simd_shuffle<A,B>(a: A, b: A, c: [u32; 8]) -> B;
    // error: invalid `simd_shuffle`, needs length: `simd_shuffle`
}
```

The `simd_shuffle` function needs the length of the array passed as
last parameter in its name. Example:

```
#![feature(platform_intrinsics)]

extern "platform-intrinsic" {
    fn simd_shuffle8<A,B>(a: A, b: A, c: [u32; 8]) -> B;
}
```

## E0440

A platform-specific intrinsic function has the wrong number of type
parameters. Erroneous code example:

```compile_fail,E0440
#![feature(repr_simd)]
#![feature(platform_intrinsics)]

#[repr(simd)]
struct f64x2(f64, f64);

extern "platform-intrinsic" {
    fn x86_mm_movemask_pd<T>(x: f64x2) -> i32;
    // error: platform-specific intrinsic has wrong number of type
    //        parameters
}
```

Please refer to the function declaration to see if it corresponds
with yours. Example:

```
#![feature(repr_simd)]
#![feature(platform_intrinsics)]

#[repr(simd)]
struct f64x2(f64, f64);

extern "platform-intrinsic" {
    fn x86_mm_movemask_pd(x: f64x2) -> i32;
}
```

## E0441

An unknown platform-specific intrinsic function was used. Erroneous
code example:

```compile_fail,E0441
#![feature(repr_simd)]
#![feature(platform_intrinsics)]

#[repr(simd)]
struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);

extern "platform-intrinsic" {
    fn x86_mm_adds_ep16(x: i16x8, y: i16x8) -> i16x8;
    // error: unrecognized platform-specific intrinsic function
}
```

Please verify that the function name wasn't misspelled, and ensure
that it is declared in the rust source code (in the file
src/librustc_platform_intrinsics/x86.rs). Example:

```
#![feature(repr_simd)]
#![feature(platform_intrinsics)]

#[repr(simd)]
struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);

extern "platform-intrinsic" {
    fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i16x8; // ok!
}
```

## E0442

Intrinsic argument(s) and/or return value have the wrong type.
Erroneous code example:

```compile_fail,E0442
#![feature(repr_simd)]
#![feature(platform_intrinsics)]

#[repr(simd)]
struct i8x16(i8, i8, i8, i8, i8, i8, i8, i8,
             i8, i8, i8, i8, i8, i8, i8, i8);
#[repr(simd)]
struct i32x4(i32, i32, i32, i32);
#[repr(simd)]
struct i64x2(i64, i64);

extern "platform-intrinsic" {
    fn x86_mm_adds_epi16(x: i8x16, y: i32x4) -> i64x2;
    // error: intrinsic arguments/return value have wrong type
}
```

To fix this error, please refer to the function declaration to give
it the awaited types. Example:

```
#![feature(repr_simd)]
#![feature(platform_intrinsics)]

#[repr(simd)]
struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);

extern "platform-intrinsic" {
    fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i16x8; // ok!
}
```

## E0443

Intrinsic argument(s) and/or return value have the wrong type.
Erroneous code example:

```compile_fail,E0443
#![feature(repr_simd)]
#![feature(platform_intrinsics)]

#[repr(simd)]
struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);
#[repr(simd)]
struct i64x8(i64, i64, i64, i64, i64, i64, i64, i64);

extern "platform-intrinsic" {
    fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i64x8;
    // error: intrinsic argument/return value has wrong type
}
```

To fix this error, please refer to the function declaration to give
it the awaited types. Example:

```
#![feature(repr_simd)]
#![feature(platform_intrinsics)]

#[repr(simd)]
struct i16x8(i16, i16, i16, i16, i16, i16, i16, i16);

extern "platform-intrinsic" {
    fn x86_mm_adds_epi16(x: i16x8, y: i16x8) -> i16x8; // ok!
}
```

## E0444

A platform-specific intrinsic function has wrong number of arguments.
Erroneous code example:

```compile_fail,E0444
#![feature(repr_simd)]
#![feature(platform_intrinsics)]

#[repr(simd)]
struct f64x2(f64, f64);

extern "platform-intrinsic" {
    fn x86_mm_movemask_pd(x: f64x2, y: f64x2, z: f64x2) -> i32;
    // error: platform-specific intrinsic has invalid number of arguments
}
```

Please refer to the function declaration to see if it corresponds
with yours. Example:

```
#![feature(repr_simd)]
#![feature(platform_intrinsics)]

#[repr(simd)]
struct f64x2(f64, f64);

extern "platform-intrinsic" {
    fn x86_mm_movemask_pd(x: f64x2) -> i32; // ok!
}
```

## E0445

A private trait was used on a public type parameter bound. Erroneous code
examples:

```compile_fail,E0445
#![deny(private_in_public)]

trait Foo {
    fn dummy(&self) { }
}

pub trait Bar : Foo {} // error: private trait in public interface
pub struct Bar2<T: Foo>(pub T); // same error
pub fn foo<T: Foo> (t: T) {} // same error
```

To solve this error, please ensure that the trait is also public. The trait
can be made inaccessible if necessary by placing it into a private inner
module, but it still has to be marked with `pub`. Example:

```ignore
pub trait Foo { // we set the Foo trait public
    fn dummy(&self) { }
}

pub trait Bar : Foo {} // ok!
pub struct Bar2<T: Foo>(pub T); // ok!
pub fn foo<T: Foo> (t: T) {} // ok!
```

## E0446

A private type was used in a public type signature. Erroneous code example:

```compile_fail,E0446
#![deny(private_in_public)]

mod Foo {
    struct Bar(u32);

    pub fn bar() -> Bar { // error: private type in public interface
        Bar(0)
    }
}
```

To solve this error, please ensure that the type is also public. The type
can be made inaccessible if necessary by placing it into a private inner
module, but it still has to be marked with `pub`.
Example:

```
mod Foo {
    pub struct Bar(u32); // we set the Bar type public

    pub fn bar() -> Bar { // ok!
        Bar(0)
    }
}
```

## E0447

The `pub` keyword was used inside a function. Erroneous code example:

```ignore
fn foo() {
    pub struct Bar; // error: visibility has no effect inside functions
}
```

Since we cannot access items defined inside a function, the visibility of its
items does not impact outer code. So using the `pub` keyword in this context
is invalid.

## E0448

The `pub` keyword was used inside a public enum. Erroneous code example:

```compile_fail
pub enum Foo {
    pub Bar, // error: unnecessary `pub` visibility
}
```

Since the enum is already public, adding `pub` on one its elements is
unnecessary. Example:

```compile_fail,
enum Foo {
    pub Bar, // not ok!
}
```

This is the correct syntax:

```ignore
pub enum Foo {
    Bar, // ok!
}
```

## E0450

A tuple constructor was invoked while some of its fields are private. Erroneous
code example:

```compile_fail,E0450
mod Bar {
    pub struct Foo(isize);
}

let f = Bar::Foo(0); // error: cannot invoke tuple struct constructor with
                     //        private fields
```

To solve this issue, please ensure that all of the fields of the tuple struct
are public. Alternatively, provide a `new()` method to the tuple struct to
construct it from a given inner value. Example:

```
mod Bar {
    pub struct Foo(pub isize); // we set its field to public
}

let f = Bar::Foo(0); // ok!

// or:
mod bar {
    pub struct Foo(isize);

    impl Foo {
        pub fn new(x: isize) -> Foo {
            Foo(x)
        }
    }
}

let f = bar::Foo::new(1);
```

## E0451

A struct constructor with private fields was invoked. Erroneous code example:

```compile_fail,E0451
mod Bar {
    pub struct Foo {
        pub a: isize,
        b: isize,
    }
}

let f = Bar::Foo{ a: 0, b: 0 }; // error: field `b` of struct `Bar::Foo`
                                //        is private
```

To fix this error, please ensure that all the fields of the struct are public,
or implement a function for easy instantiation. Examples:

```
mod Bar {
    pub struct Foo {
        pub a: isize,
        pub b: isize, // we set `b` field public
    }
}

let f = Bar::Foo{ a: 0, b: 0 }; // ok!
```

Or:

```
mod Bar {
    pub struct Foo {
        pub a: isize,
        b: isize, // still private
    }

    impl Foo {
        pub fn new() -> Foo { // we create a method to instantiate `Foo`
            Foo { a: 0, b: 0 }
        }
    }
}

let f = Bar::Foo::new(); // ok!
```

## E0452

An invalid lint attribute has been given. Erroneous code example:

```compile_fail,E0452
#![allow(foo = "")] // error: malformed lint attribute
```

Lint attributes only accept a list of identifiers (where each identifier is a
lint name). Ensure the attribute is of this form:

```
#![allow(foo)] // ok!
// or:
#![allow(foo, foo2)] // ok!
```

## E0453

A lint check attribute was overruled by a `forbid` directive set as an
attribute on an enclosing scope, or on the command line with the `-F` option.

Example of erroneous code:

```compile_fail,E0453
#![forbid(non_snake_case)]

#[allow(non_snake_case)]
fn main() {
    let MyNumber = 2; // error: allow(non_snake_case) overruled by outer
                      //        forbid(non_snake_case)
}
```

The `forbid` lint setting, like `deny`, turns the corresponding compiler
warning into a hard error. Unlike `deny`, `forbid` prevents itself from being
overridden by inner attributes.

If you're sure you want to override the lint check, you can change `forbid` to
`deny` (or use `-D` instead of `-F` if the `forbid` setting was given as a
command-line option) to allow the inner lint check attribute:

```
#![deny(non_snake_case)]

#[allow(non_snake_case)]
fn main() {
    let MyNumber = 2; // ok!
}
```

Otherwise, edit the code to pass the lint check, and remove the overruled
attribute:

```
#![forbid(non_snake_case)]

fn main() {
    let my_number = 2;
}
```

## E0454

A link name was given with an empty name. Erroneous code example:

```compile_fail,E0454
#[link(name = "")] extern {} // error: #[link(name = "")] given with empty name
```

The rust compiler cannot link to an external library if you don't give it its
name. Example:

```ignore
#[link(name = "some_lib")] extern {} // ok!
```

## E0455

Linking with `kind=framework` is only supported when targeting OS X,
as frameworks are specific to that operating system.

Erroneous code example:

```ignore
#[link(name = "FooCoreServices", kind = "framework")] extern {}
// OS used to compile is Linux for example
```

To solve this error you can use conditional compilation:

```
#[cfg_attr(target="macos", link(name = "FooCoreServices", kind = "framework"))]
extern {}
```

See more: https://doc.rust-lang.org/book/conditional-compilation.html

## E0458

An unknown "kind" was specified for a link attribute. Erroneous code example:

```compile_fail,E0458
#[link(kind = "wonderful_unicorn")] extern {}
// error: unknown kind: `wonderful_unicorn`
```

Please specify a valid "kind" value, from one of the following:

 * static
 * dylib
 * framework


## E0459

A link was used without a name parameter. Erroneous code example:

```compile_fail,E0459
#[link(kind = "dylib")] extern {}
// error: #[link(...)] specified without `name = "foo"`
```

Please add the name parameter to allow the rust compiler to find the library
you want. Example:

```ignore
#[link(kind = "dylib", name = "some_lib")] extern {} // ok!
```

## E0463

A plugin/crate was declared but cannot be found. Erroneous code example:

```compile_fail,E0463
#![feature(plugin)]
#![plugin(cookie_monster)] // error: can't find crate for `cookie_monster`
extern crate cake_is_a_lie; // error: can't find crate for `cake_is_a_lie`
```

You need to link your code to the relevant crate in order to be able to use it
(through Cargo or the `-L` option of rustc example). Plugins are crates as
well, and you link to them the same way.

## E0466

Macro import declarations were malformed.

Erroneous code examples:

```compile_fail,E0466
#[macro_use(a_macro(another_macro))] // error: invalid import declaration
extern crate core as some_crate;

#[macro_use(i_want = "some_macros")] // error: invalid import declaration
extern crate core as another_crate;
```

This is a syntax error at the level of attribute declarations. The proper
syntax for macro imports is the following:

```ignore
// In some_crate:
#[macro_export]
macro_rules! get_tacos {
    ...
}

#[macro_export]
macro_rules! get_pimientos {
    ...
}

// In your crate:
#[macro_use(get_tacos, get_pimientos)] // It imports `get_tacos` and
extern crate some_crate;               // `get_pimientos` macros from some_crate
```

If you would like to import all exported macros, write `macro_use` with no
arguments.

## E0467

Macro reexport declarations were empty or malformed.

Erroneous code examples:

```compile_fail,E0467
#[macro_reexport]                    // error: no macros listed for export
extern crate core as macros_for_good;

#[macro_reexport(fun_macro = "foo")] // error: not a macro identifier
extern crate core as other_macros_for_good;
```

This is a syntax error at the level of attribute declarations.

Currently, `macro_reexport` requires at least one macro name to be listed.
Unlike `macro_use`, listing no names does not reexport all macros from the
given crate.

Decide which macros you would like to export and list them properly.

These are proper reexport declarations:

```ignore
#[macro_reexport(some_macro, another_macro)]
extern crate macros_for_good;
```

## E0468

A non-root module attempts to import macros from another crate.

Example of erroneous code:

```compile_fail,E0468
mod foo {
    #[macro_use(helpful_macro)] // error: must be at crate root to import
    extern crate core;          //        macros from another crate
    helpful_macro!(...);
}
```

Only `extern crate` imports at the crate root level are allowed to import
macros.

Either move the macro import to crate root or do without the foreign macros.
This will work:

```ignore
#[macro_use(helpful_macro)]
extern crate some_crate;

mod foo {
    helpful_macro!(...)
}
```

## E0469

A macro listed for import was not found.

Erroneous code example:

```compile_fail,E0469
#[macro_use(drink, be_merry)] // error: imported macro not found
extern crate collections;

fn main() {
    // ...
}
```

Either the listed macro is not contained in the imported crate, or it is not
exported from the given crate.

This could be caused by a typo. Did you misspell the macro's name?

Double-check the names of the macros listed for import, and that the crate
in question exports them.

A working version would be:

```ignore
// In some_crate crate:
#[macro_export]
macro_rules! eat {
    ...
}

#[macro_export]
macro_rules! drink {
    ...
}

// In your crate:
#[macro_use(eat, drink)]
extern crate some_crate; //ok!
```

## E0470

A macro listed for reexport was not found.

Erroneous code example:

```compile_fail,E0470
#[macro_reexport(drink, be_merry)]
extern crate collections;

fn main() {
    // ...
}
```

Either the listed macro is not contained in the imported crate, or it is not
exported from the given crate.

This could be caused by a typo. Did you misspell the macro's name?

Double-check the names of the macros listed for reexport, and that the crate
in question exports them.

A working version:

```ignore
// In some_crate crate:
#[macro_export]
macro_rules! eat {
    ...
}

#[macro_export]
macro_rules! drink {
    ...
}

// In your_crate:
#[macro_reexport(eat, drink)]
extern crate some_crate;
```

## E0478

A lifetime bound was not satisfied.

Erroneous code example:

```compile_fail,E0478
// Check that the explicit lifetime bound (`'SnowWhite`, in this example) must
// outlive all the superbounds from the trait (`'kiss`, in this example).

trait Wedding<'t>: 't { }

struct Prince<'kiss, 'SnowWhite> {
    child: Box<Wedding<'kiss> + 'SnowWhite>,
    // error: lifetime bound not satisfied
}
```

In this example, the `'SnowWhite` lifetime is supposed to outlive the `'kiss`
lifetime but the declaration of the `Prince` struct doesn't enforce it. To fix
this issue, you need to specify it:

```
trait Wedding<'t>: 't { }

struct Prince<'kiss, 'SnowWhite: 'kiss> { // You say here that 'kiss must live
                                          // longer than 'SnowWhite.
    child: Box<Wedding<'kiss> + 'SnowWhite>, // And now it's all good!
}
```

## E0496

A lifetime name is shadowing another lifetime name. Erroneous code example:

```compile_fail,E0496
struct Foo<'a> {
    a: &'a i32,
}

impl<'a> Foo<'a> {
    fn f<'a>(x: &'a i32) { // error: lifetime name `'a` shadows a lifetime
                           //        name that is already in scope
    }
}
```

Please change the name of one of the lifetimes to remove this error. Example:

```
struct Foo<'a> {
    a: &'a i32,
}

impl<'a> Foo<'a> {
    fn f<'b>(x: &'b i32) { // ok!
    }
}

fn main() {
}
```

## E0497

A stability attribute was used outside of the standard library. Erroneous code
example:

```compile_fail
#[stable] // error: stability attributes may not be used outside of the
          //        standard library
fn foo() {}
```

It is not possible to use stability attributes outside of the standard library.
Also, for now, it is not possible to write deprecation messages either.

## E0499

A variable was borrowed as mutable more than once. Erroneous code example:

```compile_fail,E0499
let mut i = 0;
let mut x = &mut i;
let mut a = &mut i;
// error: cannot borrow `i` as mutable more than once at a time
```

Please note that in rust, you can either have many immutable references, or one
mutable reference. Take a look at
https://doc.rust-lang.org/stable/book/references-and-borrowing.html for more
information. Example:


```
let mut i = 0;
let mut x = &mut i; // ok!

// or:
let mut i = 0;
let a = &i; // ok!
let b = &i; // still ok!
let c = &i; // still ok!
```

## E0500

A borrowed variable was used in another closure. Example of erroneous code:

```compile_fail
fn you_know_nothing(jon_snow: &mut i32) {
    let nights_watch = || {
        *jon_snow = 2;
    };
    let starks = || {
        *jon_snow = 3; // error: closure requires unique access to `jon_snow`
                       //        but it is already borrowed
    };
}
```

In here, `jon_snow` is already borrowed by the `nights_watch` closure, so it
cannot be borrowed by the `starks` closure at the same time. To fix this issue,
you can put the closure in its own scope:

```
fn you_know_nothing(jon_snow: &mut i32) {
    {
        let nights_watch = || {
            *jon_snow = 2;
        };
    } // At this point, `jon_snow` is free.
    let starks = || {
        *jon_snow = 3;
    };
}
```

Or, if the type implements the `Clone` trait, you can clone it between
closures:

```
fn you_know_nothing(jon_snow: &mut i32) {
    let mut jon_copy = jon_snow.clone();
    let nights_watch = || {
        jon_copy = 2;
    };
    let starks = || {
        *jon_snow = 3;
    };
}
```

## E0501

This error indicates that a mutable variable is being used while it is still
captured by a closure. Because the closure has borrowed the variable, it is not
available for use until the closure goes out of scope.

Note that a capture will either move or borrow a variable, but in this
situation, the closure is borrowing the variable. Take a look at
http://rustbyexample.com/fn/closures/capture.html for more information about
capturing.

Example of erroneous code:

```compile_fail,E0501
fn inside_closure(x: &mut i32) {
    // Actions which require unique access
}

fn outside_closure(x: &mut i32) {
    // Actions which require unique access
}

fn foo(a: &mut i32) {
    let bar = || {
        inside_closure(a)
    };
    outside_closure(a); // error: cannot borrow `*a` as mutable because previous
                        //        closure requires unique access.
}
```

To fix this error, you can place the closure in its own scope:

```
fn inside_closure(x: &mut i32) {}
fn outside_closure(x: &mut i32) {}

fn foo(a: &mut i32) {
    {
        let bar = || {
            inside_closure(a)
        };
    } // borrow on `a` ends.
    outside_closure(a); // ok!
}
```

Or you can pass the variable as a parameter to the closure:

```
fn inside_closure(x: &mut i32) {}
fn outside_closure(x: &mut i32) {}

fn foo(a: &mut i32) {
    let bar = |s: &mut i32| {
        inside_closure(s)
    };
    outside_closure(a);
    bar(a);
}
```

It may be possible to define the closure later:

```
fn inside_closure(x: &mut i32) {}
fn outside_closure(x: &mut i32) {}

fn foo(a: &mut i32) {
    outside_closure(a);
    let bar = || {
        inside_closure(a)
    };
}
```

## E0502

This error indicates that you are trying to borrow a variable as mutable when it
has already been borrowed as immutable.

Example of erroneous code:

```compile_fail,E0502
fn bar(x: &mut i32) {}
fn foo(a: &mut i32) {
    let ref y = a; // a is borrowed as immutable.
    bar(a); // error: cannot borrow `*a` as mutable because `a` is also borrowed
            //        as immutable
}
```

To fix this error, ensure that you don't have any other references to the
variable before trying to access it mutably:

```
fn bar(x: &mut i32) {}
fn foo(a: &mut i32) {
    bar(a);
    let ref y = a; // ok!
}
```

For more information on the rust ownership system, take a look at
https://doc.rust-lang.org/stable/book/references-and-borrowing.html.

## E0503

A value was used after it was mutably borrowed.

Example of erroneous code:

```compile_fail,E0503
fn main() {
    let mut value = 3;
    // Create a mutable borrow of `value`. This borrow
    // lives until the end of this function.
    let _borrow = &mut value;
    let _sum = value + 1; // error: cannot use `value` because
                          //        it was mutably borrowed
}
```

In this example, `value` is mutably borrowed by `borrow` and cannot be
used to calculate `sum`. This is not possible because this would violate
Rust's mutability rules.

You can fix this error by limiting the scope of the borrow:

```
fn main() {
    let mut value = 3;
    // By creating a new block, you can limit the scope
    // of the reference.
    {
        let _borrow = &mut value; // Use `_borrow` inside this block.
    }
    // The block has ended and with it the borrow.
    // You can now use `value` again.
    let _sum = value + 1;
}
```

Or by cloning `value` before borrowing it:

```
fn main() {
    let mut value = 3;
    // We clone `value`, creating a copy.
    let value_cloned = value.clone();
    // The mutable borrow is a reference to `value` and
    // not to `value_cloned`...
    let _borrow = &mut value;
    // ... which means we can still use `value_cloned`,
    let _sum = value_cloned + 1;
    // even though the borrow only ends here.
}
```

You can find more information about borrowing in the rust-book:
http://doc.rust-lang.org/stable/book/references-and-borrowing.html

## E0504

This error occurs when an attempt is made to move a borrowed variable into a
closure.

Example of erroneous code:

```compile_fail,E0504
struct FancyNum {
    num: u8,
}

fn main() {
    let fancy_num = FancyNum { num: 5 };
    let fancy_ref = &fancy_num;

    let x = move || {
        println!("child function: {}", fancy_num.num);
        // error: cannot move `fancy_num` into closure because it is borrowed
    };

    x();
    println!("main function: {}", fancy_ref.num);
}
```

Here, `fancy_num` is borrowed by `fancy_ref` and so cannot be moved into
the closure `x`. There is no way to move a value into a closure while it is
borrowed, as that would invalidate the borrow.

If the closure can't outlive the value being moved, try using a reference
rather than moving:

```
struct FancyNum {
    num: u8,
}

fn main() {
    let fancy_num = FancyNum { num: 5 };
    let fancy_ref = &fancy_num;

    let x = move || {
        // fancy_ref is usable here because it doesn't move `fancy_num`
        println!("child function: {}", fancy_ref.num);
    };

    x();

    println!("main function: {}", fancy_num.num);
}
```

If the value has to be borrowed and then moved, try limiting the lifetime of
the borrow using a scoped block:

```
struct FancyNum {
    num: u8,
}

fn main() {
    let fancy_num = FancyNum { num: 5 };

    {
        let fancy_ref = &fancy_num;
        println!("main function: {}", fancy_ref.num);
        // `fancy_ref` goes out of scope here
    }

    let x = move || {
        // `fancy_num` can be moved now (no more references exist)
        println!("child function: {}", fancy_num.num);
    };

    x();
}
```

If the lifetime of a reference isn't enough, such as in the case of threading,
consider using an `Arc` to create a reference-counted value:

```
use std::sync::Arc;
use std::thread;

struct FancyNum {
    num: u8,
}

fn main() {
    let fancy_ref1 = Arc::new(FancyNum { num: 5 });
    let fancy_ref2 = fancy_ref1.clone();

    let x = thread::spawn(move || {
        // `fancy_ref1` can be moved and has a `'static` lifetime
        println!("child thread: {}", fancy_ref1.num);
    });

    x.join().expect("child thread should finish");
    println!("main thread: {}", fancy_ref2.num);
}
```

## E0505

A value was moved out while it was still borrowed.

Erroneous code example:

```compile_fail,E0505
struct Value {}

fn eat(val: Value) {}

fn main() {
    let x = Value{};
    {
        let _ref_to_val: &Value = &x;
        eat(x);
    }
}
```

Here, the function `eat` takes the ownership of `x`. However,
`x` cannot be moved because it was borrowed to `_ref_to_val`.
To fix that you can do few different things:

* Try to avoid moving the variable.
* Release borrow before move.
* Implement the `Copy` trait on the type.

Examples:

```
struct Value {}

fn eat(val: &Value) {}

fn main() {
    let x = Value{};
    {
        let _ref_to_val: &Value = &x;
        eat(&x); // pass by reference, if it's possible
    }
}
```

Or:

```
struct Value {}

fn eat(val: Value) {}

fn main() {
    let x = Value{};
    {
        let _ref_to_val: &Value = &x;
    }
    eat(x); // release borrow and then move it.
}
```

Or:

```
#[derive(Clone, Copy)] // implement Copy trait
struct Value {}

fn eat(val: Value) {}

fn main() {
    let x = Value{};
    {
        let _ref_to_val: &Value = &x;
        eat(x); // it will be copied here.
    }
}
```

You can find more information about borrowing in the rust-book:
http://doc.rust-lang.org/stable/book/references-and-borrowing.html

## E0506

This error occurs when an attempt is made to assign to a borrowed value.

Example of erroneous code:

```compile_fail,E0506
struct FancyNum {
    num: u8,
}

fn main() {
    let mut fancy_num = FancyNum { num: 5 };
    let fancy_ref = &fancy_num;
    fancy_num = FancyNum { num: 6 };
    // error: cannot assign to `fancy_num` because it is borrowed

    println!("Num: {}, Ref: {}", fancy_num.num, fancy_ref.num);
}
```

Because `fancy_ref` still holds a reference to `fancy_num`, `fancy_num` can't
be assigned to a new value as it would invalidate the reference.

Alternatively, we can move out of `fancy_num` into a second `fancy_num`:

```
struct FancyNum {
    num: u8,
}

fn main() {
    let mut fancy_num = FancyNum { num: 5 };
    let moved_num = fancy_num;
    fancy_num = FancyNum { num: 6 };

    println!("Num: {}, Moved num: {}", fancy_num.num, moved_num.num);
}
```

If the value has to be borrowed, try limiting the lifetime of the borrow using
a scoped block:

```
struct FancyNum {
    num: u8,
}

fn main() {
    let mut fancy_num = FancyNum { num: 5 };

    {
        let fancy_ref = &fancy_num;
        println!("Ref: {}", fancy_ref.num);
    }

    // Works because `fancy_ref` is no longer in scope
    fancy_num = FancyNum { num: 6 };
    println!("Num: {}", fancy_num.num);
}
```

Or by moving the reference into a function:

```
struct FancyNum {
    num: u8,
}

fn main() {
    let mut fancy_num = FancyNum { num: 5 };

    print_fancy_ref(&fancy_num);

    // Works because function borrow has ended
    fancy_num = FancyNum { num: 6 };
    println!("Num: {}", fancy_num.num);
}

fn print_fancy_ref(fancy_ref: &FancyNum){
    println!("Ref: {}", fancy_ref.num);
}
```

## E0507

You tried to move out of a value which was borrowed. Erroneous code example:

```compile_fail,E0507
use std::cell::RefCell;

struct TheDarkKnight;

impl TheDarkKnight {
    fn nothing_is_true(self) {}
}

fn main() {
    let x = RefCell::new(TheDarkKnight);

    x.borrow().nothing_is_true(); // error: cannot move out of borrowed content
}
```

Here, the `nothing_is_true` method takes the ownership of `self`. However,
`self` cannot be moved because `.borrow()` only provides an `&TheDarkKnight`,
which is a borrow of the content owned by the `RefCell`. To fix this error,
you have three choices:

* Try to avoid moving the variable.
* Somehow reclaim the ownership.
* Implement the `Copy` trait on the type.

Examples:

```
use std::cell::RefCell;

struct TheDarkKnight;

impl TheDarkKnight {
    fn nothing_is_true(&self) {} // First case, we don't take ownership
}

fn main() {
    let x = RefCell::new(TheDarkKnight);

    x.borrow().nothing_is_true(); // ok!
}
```

Or:

```
use std::cell::RefCell;

struct TheDarkKnight;

impl TheDarkKnight {
    fn nothing_is_true(self) {}
}

fn main() {
    let x = RefCell::new(TheDarkKnight);
    let x = x.into_inner(); // we get back ownership

    x.nothing_is_true(); // ok!
}
```

Or:

```
use std::cell::RefCell;

#[derive(Clone, Copy)] // we implement the Copy trait
struct TheDarkKnight;

impl TheDarkKnight {
    fn nothing_is_true(self) {}
}

fn main() {
    let x = RefCell::new(TheDarkKnight);

    x.borrow().nothing_is_true(); // ok!
}
```

Moving out of a member of a mutably borrowed struct is fine if you put something
back. `mem::replace` can be used for that:

```ignore
struct TheDarkKnight;

impl TheDarkKnight {
    fn nothing_is_true(self) {}
}

struct Batcave {
    knight: TheDarkKnight
}

fn main() {
    use std::mem;

    let mut cave = Batcave {
        knight: TheDarkKnight
    };
    let borrowed = &mut cave;

    borrowed.knight.nothing_is_true(); // E0507
    mem::replace(&mut borrowed.knight, TheDarkKnight).nothing_is_true(); // ok!
}
```

You can find more information about borrowing in the rust-book:
http://doc.rust-lang.org/stable/book/references-and-borrowing.html

## E0508

A value was moved out of a non-copy fixed-size array.

Example of erroneous code:

```compile_fail,E0508
struct NonCopy;

fn main() {
    let array = [NonCopy; 1];
    let _value = array[0]; // error: cannot move out of type `[NonCopy; 1]`,
                           //        a non-copy fixed-size array
}
```

The first element was moved out of the array, but this is not
possible because `NonCopy` does not implement the `Copy` trait.

Consider borrowing the element instead of moving it:

```
struct NonCopy;

fn main() {
    let array = [NonCopy; 1];
    let _value = &array[0]; // Borrowing is allowed, unlike moving.
}
```

Alternatively, if your type implements `Clone` and you need to own the value,
consider borrowing and then cloning:

```
#[derive(Clone)]
struct NonCopy;

fn main() {
    let array = [NonCopy; 1];
    // Now you can clone the array element.
    let _value = array[0].clone();
}
```

## E0509

This error occurs when an attempt is made to move out of a value whose type
implements the `Drop` trait.

Example of erroneous code:

```compile_fail,E0509
struct FancyNum {
    num: usize
}

struct DropStruct {
    fancy: FancyNum
}

impl Drop for DropStruct {
    fn drop(&mut self) {
        // Destruct DropStruct, possibly using FancyNum
    }
}

fn main() {
    let drop_struct = DropStruct{fancy: FancyNum{num: 5}};
    let fancy_field = drop_struct.fancy; // Error E0509
    println!("Fancy: {}", fancy_field.num);
    // implicit call to `drop_struct.drop()` as drop_struct goes out of scope
}
```

Here, we tried to move a field out of a struct of type `DropStruct` which
implements the `Drop` trait. However, a struct cannot be dropped if one or
more of its fields have been moved.

Structs implementing the `Drop` trait have an implicit destructor that gets
called when they go out of scope. This destructor may use the fields of the
struct, so moving out of the struct could make it impossible to run the
destructor. Therefore, we must think of all values whose type implements the
`Drop` trait as single units whose fields cannot be moved.

This error can be fixed by creating a reference to the fields of a struct,
enum, or tuple using the `ref` keyword:

```
struct FancyNum {
    num: usize
}

struct DropStruct {
    fancy: FancyNum
}

impl Drop for DropStruct {
    fn drop(&mut self) {
        // Destruct DropStruct, possibly using FancyNum
    }
}

fn main() {
    let drop_struct = DropStruct{fancy: FancyNum{num: 5}};
    let ref fancy_field = drop_struct.fancy; // No more errors!
    println!("Fancy: {}", fancy_field.num);
    // implicit call to `drop_struct.drop()` as drop_struct goes out of scope
}
```

Note that this technique can also be used in the arms of a match expression:

```
struct FancyNum {
    num: usize
}

enum DropEnum {
    Fancy(FancyNum)
}

impl Drop for DropEnum {
    fn drop(&mut self) {
        // Destruct DropEnum, possibly using FancyNum
    }
}

fn main() {
    // Creates and enum of type `DropEnum`, which implements `Drop`
    let drop_enum = DropEnum::Fancy(FancyNum{num: 10});
    match drop_enum {
        // Creates a reference to the inside of `DropEnum::Fancy`
        DropEnum::Fancy(ref fancy_field) => // No error!
            println!("It was fancy-- {}!", fancy_field.num),
    }
    // implicit call to `drop_enum.drop()` as drop_enum goes out of scope
}
```

## E0511

Invalid monomorphization of an intrinsic function was used. Erroneous code
example:

```ignore
#![feature(platform_intrinsics)]

extern "platform-intrinsic" {
    fn simd_add<T>(a: T, b: T) -> T;
}

fn main() {
    unsafe { simd_add(0, 1); }
    // error: invalid monomorphization of `simd_add` intrinsic
}
```

The generic type has to be a SIMD type. Example:

```
#![feature(repr_simd)]
#![feature(platform_intrinsics)]

#[repr(simd)]
#[derive(Copy, Clone)]
struct i32x1(i32);

extern "platform-intrinsic" {
    fn simd_add<T>(a: T, b: T) -> T;
}

unsafe { simd_add(i32x1(0), i32x1(1)); } // ok!
```

## E0512

Transmute with two differently sized types was attempted. Erroneous code
example:

```compile_fail,E0512
fn takes_u8(_: u8) {}

fn main() {
    unsafe { takes_u8(::std::mem::transmute(0u16)); }
    // error: transmute called with differently sized types
}
```

Please use types with same size or use the expected type directly. Example:

```
fn takes_u8(_: u8) {}

fn main() {
    unsafe { takes_u8(::std::mem::transmute(0i8)); } // ok!
    // or:
    unsafe { takes_u8(0u8); } // ok!
}
```

## E0513

The type of the variable couldn't be found out.

Erroneous code example:

```compile_fail,E0513
use std::mem;

unsafe {
    let size = mem::size_of::<u32>();
    mem::transmute_copy::<u32, [u8; size]>(&8_8);
    // error: no type for local variable
}
```

To fix this error, please use a constant size instead of `size`. To make
this error more obvious, you could run:

```compile_fail,E0080
use std::mem;

unsafe {
    mem::transmute_copy::<u32, [u8; mem::size_of::<u32>()]>(&8_8);
    // error: constant evaluation error
}
```

So now, you can fix your code by setting the size directly:

```
use std::mem;

unsafe {
    mem::transmute_copy::<u32, [u8; 4]>(&8_8);
    // `u32` is 4 bytes so we replace the `mem::size_of` call with its size
}
```

## E0516

The `typeof` keyword is currently reserved but unimplemented.
Erroneous code example:

```compile_fail,E0516
fn main() {
    let x: typeof(92) = 92;
}
```

Try using type inference instead. Example:

```
fn main() {
    let x = 92;
}
```

## E0517

This error indicates that a `#[repr(..)]` attribute was placed on an
unsupported item.

Examples of erroneous code:

```compile_fail,E0517
#[repr(C)]
type Foo = u8;

#[repr(packed)]
enum Foo {Bar, Baz}

#[repr(u8)]
struct Foo {bar: bool, baz: bool}

#[repr(C)]
impl Foo {
    // ...
}
```

* The `#[repr(C)]` attribute can only be placed on structs and enums.
* The `#[repr(packed)]` and `#[repr(simd)]` attributes only work on structs.
* The `#[repr(u8)]`, `#[repr(i16)]`, etc attributes only work on enums.

These attributes do not work on typedefs, since typedefs are just aliases.

Representations like `#[repr(u8)]`, `#[repr(i64)]` are for selecting the
discriminant size for C-like enums (when there is no associated data, e.g.
`enum Color {Red, Blue, Green}`), effectively setting the size of the enum to
the size of the provided type. Such an enum can be cast to a value of the same
type as well. In short, `#[repr(u8)]` makes the enum behave like an integer
with a constrained set of allowed values.

Only C-like enums can be cast to numerical primitives, so this attribute will
not apply to structs.

`#[repr(packed)]` reduces padding to make the struct size smaller. The
representation of enums isn't strictly defined in Rust, and this attribute
won't work on enums.

`#[repr(simd)]` will give a struct consisting of a homogenous series of machine
types (i.e. `u8`, `i32`, etc) a representation that permits vectorization via
SIMD. This doesn't make much sense for enums since they don't consist of a
single list of data.

## E0518

This error indicates that an `#[inline(..)]` attribute was incorrectly placed
on something other than a function or method.

Examples of erroneous code:

```compile_fail,E0518
#[inline(always)]
struct Foo;

#[inline(never)]
impl Foo {
    // ...
}
```

`#[inline]` hints the compiler whether or not to attempt to inline a method or
function. By default, the compiler does a pretty good job of figuring this out
itself, but if you feel the need for annotations, `#[inline(always)]` and
`#[inline(never)]` can override or force the compiler's decision.

If you wish to apply this attribute to all methods in an impl, manually annotate
each method; it is not possible to annotate the entire impl with an `#[inline]`
attribute.

## E0520

A non-default implementation was already made on this type so it cannot be
specialized further. Erroneous code example:

```compile_fail,E0520
#![feature(specialization)]

trait SpaceLlama {
    fn fly(&self);
}

// applies to all T
impl<T> SpaceLlama for T {
    default fn fly(&self) {}
}

// non-default impl
// applies to all `Clone` T and overrides the previous impl
impl<T: Clone> SpaceLlama for T {
    fn fly(&self) {}
}

// since `i32` is clone, this conflicts with the previous implementation
impl SpaceLlama for i32 {
    default fn fly(&self) {}
    // error: item `fly` is provided by an `impl` that specializes
    //        another, but the item in the parent `impl` is not marked
    //        `default` and so it cannot be specialized.
}
```

Specialization only allows you to override `default` functions in
implementations.

To fix this error, you need to mark all the parent implementations as default.
Example:

```
#![feature(specialization)]

trait SpaceLlama {
    fn fly(&self);
}

// applies to all T
impl<T> SpaceLlama for T {
    default fn fly(&self) {} // This is a parent implementation.
}

// applies to all `Clone` T; overrides the previous impl
impl<T: Clone> SpaceLlama for T {
    default fn fly(&self) {} // This is a parent implementation but was
                             // previously not a default one, causing the error
}

// applies to i32, overrides the previous two impls
impl SpaceLlama for i32 {
    fn fly(&self) {} // And now that's ok!
}
```

## E0522

The lang attribute is intended for marking special items that are built-in to
Rust itself. This includes special traits (like `Copy` and `Sized`) that affect
how the compiler behaves, as well as special functions that may be automatically
invoked (such as the handler for out-of-bounds accesses when indexing a slice).
Erroneous code example:

```compile_fail,E0522
#![feature(lang_items)]

#[lang = "cookie"]
fn cookie() -> ! { // error: definition of an unknown language item: `cookie`
    loop {}
}
```

## E0525

A closure was attempted to get used whereas it doesn't implement the expected
trait.

Erroneous code example:

```compile_fail,E0525
struct X;

fn foo<T>(_: T) {}
fn bar<T: Fn(u32)>(_: T) {}

fn main() {
    let x = X;
    let closure = |_| foo(x); // error: expected a closure that implements
                              //        the `Fn` trait, but this closure only
                              //        implements `FnOnce`
    bar(closure);
}
```

In the example above, `closure` is an `FnOnce` closure whereas the `bar`
function expected an `Fn` closure. In this case, it's simple to fix the issue,
you just have to implement `Copy` and `Clone` traits on `struct X` and it'll
be ok:

```
#[derive(Clone, Copy)] // We implement `Clone` and `Copy` traits.
struct X;

fn foo<T>(_: T) {}
fn bar<T: Fn(u32)>(_: T) {}

fn main() {
    let x = X;
    let closure = |_| foo(x);
    bar(closure); // ok!
}
```

To understand better how closures work in Rust, read:
https://doc.rust-lang.org/book/closures.html

## E0527

The number of elements in an array or slice pattern differed from the number of
elements in the array being matched.

Example of erroneous code:

```compile_fail,E0527
#![feature(slice_patterns)]

let r = &[1, 2, 3, 4];
match r {
    &[a, b] => { // error: pattern requires 2 elements but array
                 //        has 4
        println!("a={}, b={}", a, b);
    }
}
```

Ensure that the pattern is consistent with the size of the matched
array. Additional elements can be matched with `..`:

```
#![feature(slice_patterns)]

let r = &[1, 2, 3, 4];
match r {
    &[a, b, ..] => { // ok!
        println!("a={}, b={}", a, b);
    }
}
```

## E0528

An array or slice pattern required more elements than were present in the
matched array.

Example of erroneous code:

```compile_fail,E0528
#![feature(slice_patterns)]

let r = &[1, 2];
match r {
    &[a, b, c, rest..] => { // error: pattern requires at least 3
                            //        elements but array has 2
        println!("a={}, b={}, c={} rest={:?}", a, b, c, rest);
    }
}
```

Ensure that the matched array has at least as many elements as the pattern
requires. You can match an arbitrary number of remaining elements with `..`:

```
#![feature(slice_patterns)]

let r = &[1, 2, 3, 4, 5];
match r {
    &[a, b, c, rest..] => { // ok!
        // prints `a=1, b=2, c=3 rest=[4, 5]`
        println!("a={}, b={}, c={} rest={:?}", a, b, c, rest);
    }
}
```

## E0529

An array or slice pattern was matched against some other type.

Example of erroneous code:

```compile_fail,E0529
#![feature(slice_patterns)]

let r: f32 = 1.0;
match r {
    [a, b] => { // error: expected an array or slice, found `f32`
        println!("a={}, b={}", a, b);
    }
}
```

Ensure that the pattern and the expression being matched on are of consistent
types:

```
#![feature(slice_patterns)]

let r = [1.0, 2.0];
match r {
    [a, b] => { // ok!
        println!("a={}, b={}", a, b);
    }
}
```

## E0530

A binding shadowed something it shouldn't.

Erroneous code example:

```compile_fail,E0530
static TEST: i32 = 0;

let r: (i32, i32) = (0, 0);
match r {
    TEST => {} // error: match bindings cannot shadow statics
}
```

To fix this error, just change the binding's name in order to avoid shadowing
one of the following:

* struct name
* struct/enum variant
* static
* const
* associated const

Fixed example:

```
static TEST: i32 = 0;

let r: (i32, i32) = (0, 0);
match r {
    something => {} // ok!
}
```

## E0532

Pattern arm did not match expected kind.

Erroneous code example:

```compile_fail,E0532
enum State {
    Succeeded,
    Failed(String),
}

fn print_on_failure(state: &State) {
    match *state {
        // error: expected unit struct/variant or constant, found tuple
        //        variant `State::Failed`
        State::Failed => println!("Failed"),
        _ => ()
    }
}
```

To fix this error, ensure the match arm kind is the same as the expression
matched.

Fixed example:

```
enum State {
    Succeeded,
    Failed(String),
}

fn print_on_failure(state: &State) {
    match *state {
        State::Failed(ref msg) => println!("Failed with {}", msg),
        _ => ()
    }
}
```

## E0559

An unknown field was specified into an enum's structure variant.

Erroneous code example:

```compile_fail,E0559
enum Field {
    Fool { x: u32 },
}

let s = Field::Fool { joke: 0 };
// error: struct variant `Field::Fool` has no field named `joke`
```

Verify you didn't misspell the field's name or that the field exists. Example:

```
enum Field {
    Fool { joke: u32 },
}

let s = Field::Fool { joke: 0 }; // ok!
```

## E0560

An unknown field was specified into a structure.

Erroneous code example:

```compile_fail,E0560
struct Simba {
    mother: u32,
}

let s = Simba { mother: 1, father: 0 };
// error: structure `Simba` has no field named `father`
```

Verify you didn't misspell the field's name or that the field exists. Example:

```
struct Simba {
    mother: u32,
    father: u32,
}

let s = Simba { mother: 1, father: 0 }; // ok!
```

## E0569

If an impl has a generic parameter with the `#[may_dangle]` attribute, then
that impl must be declared as an `unsafe impl. For example:

```compile_fail,E0569
#![feature(generic_param_attrs)]
#![feature(dropck_eyepatch)]

struct Foo<X>(X);
impl<#[may_dangle] X> Drop for Foo<X> {
    fn drop(&mut self) { }
}
```

In this example, we are asserting that the destructor for `Foo` will not
access any data of type `X`, and require this assertion to be true for
overall safety in our program. The compiler does not currently attempt to
verify this assertion; therefore we must tag this `impl` as unsafe.

## E0570

The requested ABI is unsupported by the current target.

The rust compiler maintains for each target a blacklist of ABIs unsupported on
that target. If an ABI is present in such a list this usually means that the
target / ABI combination is currently unsupported by llvm.

If necessary, you can circumvent this check using custom target specifications.

## E0572

A return statement was found outside of a function body.

Erroneous code example:

```compile_fail,E0572
const FOO: u32 = return 0; // error: return statement outside of function body

fn main() {}
```

To fix this issue, just remove the return keyword or move the expression into a
function. Example:

```
const FOO: u32 = 0;

fn some_fn() -> u32 {
    return FOO;
}

fn main() {
    some_fn();
}
```

