    rustc ^1.37.0

Determine if a type implements a logical trait
expression[**?**](#logical-trait-expression), brought to you by
[@NikolaiVazquez]!

This library defines [`impls!`], a macro[**?**](#macro) that returns
a [`bool`] indicating whether a type implements a boolean-like expression over a
set of traits[**?**](#trait).

```rust
assert!(impls!(String: Clone & !Copy & Send & Sync));
```

See ["Examples"](#examples) for detailed use cases and, if you're brave, see
["Trait-Dependent Type Sizes"](#trait-dependent-type-sizes) for some cursed
code.

## Index

- [Reasoning](#reasoning)
- [Usage](#usage)
- [Vocabulary](#vocabulary)
  - [Macro](#macro)
  - [Trait](#trait)
  - [Logical Trait Expression](#logical-trait-expression)
- [Examples](#examples)
  - [Constant Evaluation](#constant-evaluation)
  - [Precedence and Nesting](#precedence-and-nesting)
  - [Mutual Exclusion](#mutual-exclusion)
  - [Reference Types](#reference-types)
  - [Unsized Types](#unsized-types)
  - [Generic Types](#generic-types)
  - [Lifetimes](#lifetimes)
  - [Trait-Dependent Type Sizes](#trait-dependent-type-sizes)
- [Authors](#authors)
- [License](#license)

## Reasoning

As a library author, it's important to ensure that your API remains stable.
Trait implementations are part of API stability. For example: if you
accidentally introduce an inner type that makes your publicly-exposed type no
longer be [`Send`] or [`Sync`], you've now broken your API without noticing it!
The most common case of this happening is adding a [raw pointer][ptr]
(i.e. `*const T`, `*mut T`) as a type field.

By checking situations like this with [`impls!`], either at [compile-time] or in
a unit test, you can ensure that no API-breaking changes are made without
noticing until it's too late.

## Usage

This crate is available [on crates.io][crate] and can be used by adding the
following to your project's [`Cargo.toml`]:

```toml
[dependencies]
impls = "1"
```

and this to your crate root (`main.rs` or `lib.rs`):

```rust
#[macro_use]
extern crate impls;
```

When using [Rust 2018 edition][2018], the following import can help if
having `#[macro_use]` is undesirable.

```rust
use impls::impls;
```

## Vocabulary

This documentation uses jargon that may be new to inexperienced Rust users.
This section exists to make these terms easier to understand. Feel free to
skip this section if these are already familiar to you.

### Macro

In Rust, macros are functions over the [abstract syntax tree (AST)][AST].
They map input tokens to output tokens by performing some operation over
them through a set of rules. Because of this, only their outputs are ever
type-checked.

If you wish to learn about implementing macros, I recommend:
- [The Little Book of Rust Macros](https://danielkeep.github.io/tlborm/book/index.html)
- ["Macros" - The Rust Programming Language](https://doc.rust-lang.org/book/ch19-06-macros.html)
- ["Macros" - The Rust Reference](https://doc.rust-lang.org/stable/reference/macros.html)
- ["Macros By Example" - The Rust Reference](https://doc.rust-lang.org/stable/reference/macros-by-example.html)

To use this crate, you do not need to know how macros are defined.

### Trait

In Rust, traits are a way of defining a generalized property. They should be
thought of expressing what a type is capable of doing. For example: if a
type implements [`Into`] for some type `T`, then we know it can be converted
into `T` by just calling the `.into()` method on it.

If you wish to learn about traits in detail, I recommend:
- ["Traits: Defining Shared Behavior" - The Rust Programming Language](https://doc.rust-lang.org/book/ch10-02-traits.html)
- ["Traits" - The Rust Reference](https://doc.rust-lang.org/stable/reference/items/traits.html)

### Logical Trait Expression

In this crate, traits should be thought of as [`bool`]s where the condition
is whether the given type implements the trait or not.

An expression can be formed from these trait operations:

- And (`&`): also known as [logical conjunction], this returns `true` if
  **both** operands are `true`. This is usually defined in Rust via the
  [`BitAnd`] trait.

- Or (`|`): also known as [logical disjunction], this returns `true` if
  **either** of two operands is `true`. This is usually defined in Rust via
  the [`BitOr`] trait.

- Exclusive-or (`^`): also known as [exclusive disjunction], this returns
  `true` if **only one** of two operands is `true`. This is usually defined
  in Rust via the [`BitXor`] trait.

- Not (`!`): a negation that returns `false` if the operand is `true`, or
  `true` if the operand is `false`. This is usually defined in Rust via the
  [`Not`] trait.

See ["Precedence and Nesting"](#precedence-and-nesting) for information
about the order in which these operations are performed.

## Examples

This macro works in every type context. See below for use cases.

### Constant Evaluation

Because types are [compile-time] constructs, the result of this macro can be
used as a `const` value:

```rust
const IMPLS: bool = impls!(u8: From<u32>);
```

Using [`static_assertions`], we can fail to compile if the trait expression
evaluates to `false`:

```rust
const_assert!(impls!(*const u8: Send | Sync));
```

### Precedence and Nesting

Trait operations abide by [Rust's expression precedence][precedence]. To
define a custom order of operations (e.g. left-to-right), simply nest the
expressions with parentheses.

```rust
let pre = impls!(u64:   From<u8> | From<u16>  ^ From<u32>  & From<u64>);
let ltr = impls!(u64: ((From<u8> | From<u16>) ^ From<u32>) & From<u64>);

assert_eq!(pre, true | true ^ true & true);
assert_ne!(pre, ltr);
```

### Mutual Exclusion

Because exclusive-or (`^`) is a trait operation, we can check that a type
implements one of two traits, but not both:

```rust
struct T;

trait Foo {}
trait Bar {}

impl Foo for T {}

assert!(impls!(T: Foo ^ Bar));
```

### Reference Types

Something that's surprising to many Rust users is that [`&mut T`] _does not_
implement [`Copy`] _nor_ [`Clone`]:

```rust
assert!(impls!(&mut u32: !Copy & !Clone));
```

Surely you're thinking now that this macro must be broken, because you've
been able to reuse `&mut T` throughout your lifetime with Rust. This works
because, in certain contexts, the compiler silently adds "re-borrows"
(`&mut *ref`) with a shorter lifetime and shadows the original. In reality,
`&mut T` is a move-only type.

### Unsized Types

There's a variety of types in Rust that don't implement [`Sized`]:

```rust
// Slices store their size with their pointer.
assert!(impls!(str:  !Sized));
assert!(impls!([u8]: !Sized));

// Trait objects store their size in a vtable.
trait Foo {}
assert!(impls!(dyn Foo: !Sized));

// Wrappers around unsized types are also unsized themselves.
struct Bar([u8]);
assert!(impls!(Bar: !Sized));
```

### Generic Types

When called from a generic function, the returned value is based on the
constraints of the generic type:

```rust
use std::cell::Cell;

struct Value<T> {
    // ...
}

impl<T: Send> Value<T> {
    fn do_stuff() {
        assert!(impls!(Cell<T>: Send));
        // ...
    }
}
```

Keep in mind that this can result in false negatives:

```rust
const fn is_copy<T>() -> bool {
    impls!(T: Copy)
}

assert_ne!(is_copy::<u32>(), impls!(u32: Copy));
```

[precedence]: https://doc.rust-lang.org/reference/expressions.html#expression-precedence
[`static_assertions`]: https://docs.rs/static_assertions

### Lifetimes

Traits with lifetimes are also supported:

```rust
trait Ref<'a> {}
impl<'a, T: ?Sized> Ref<'a> for &'a T {}
impl<'a, T: ?Sized> Ref<'a> for &'a mut T {}

assert!(impls!(&'static str:      Ref<'static>));
assert!(impls!(&'static mut [u8]: Ref<'static>));
assert!(impls!(String:           !Ref<'static>));
```

### Trait-Dependent Type Sizes

This macro enables something really cool (read cursed) that couldn't be done
before: making a type's size dependent on what traits it implements! Note that
this probably is a bad idea and shouldn't be used in production.

Here `Foo` becomes 32 bytes for no other reason than it implementing [`Clone`]:

```rust
const SIZE: usize = 32 * (impls!(Foo: Clone) as usize);

#[derive(Clone)]
struct Foo([u8; SIZE]);

assert_eq!(std::mem::size_of::<Foo>(), 32);
```

The [`bool`] returned from [`impls!`] gets casted to a [`usize`], becoming 1 or
0 depending on if it's `true` or `false` respectively. If `true`, this becomes
32 × 1, which is 32. This then becomes the length of the byte array in `Foo`.

## Authors

- Nikolai Vazquez
  (GitHub: [@nvzqz](https://github.com/nvzqz), Twitter: [@NikolaiVazquez])

  Implemented the `impls!` macro with support for all logical operators and
  without the limitations of the initial `does_impl!` macro by Nadrieril.

- Nadrieril Feneanar
  (GitHub: [@Nadrieril](https://github.com/Nadrieril))

  Implemented the initial `does_impl!` macro in
  [nvzqz/static-assertions-rs#28](https://github.com/nvzqz/static-assertions-rs/pull/28)
  upon which this crate was originally based.

## License

This project is released under either:

- [MIT License](https://github.com/nvzqz/impls/blob/master/LICENSE-MIT)
- [Apache License (Version 2.0)](https://github.com/nvzqz/impls/blob/master/LICENSE-APACHE)

at your choosing.

[@NikolaiVazquez]: https://twitter.com/NikolaiVazquez

[compile-time]: https://en.wikipedia.org/wiki/Compile_time

[`&mut T`]: https://doc.rust-lang.org/std/primitive.reference.html
[`bool`]:   https://doc.rust-lang.org/std/primitive.bool.html
[`Clone`]:  https://doc.rust-lang.org/std/clone/trait.Clone.html
[`Copy`]:   https://doc.rust-lang.org/std/marker/trait.Copy.html
[`Sized`]:  https://doc.rust-lang.org/std/marker/trait.Sized.html
[`usize`]:  https://doc.rust-lang.org/std/primitive.usize.html

[`Cargo.toml`]: https://doc.rust-lang.org/cargo/reference/manifest.html
[`impls!`]: https://docs.rs/impls/1.0.3/impls/macro.impls.html
[2018]: https://blog.rust-lang.org/2018/12/06/Rust-1.31-and-rust-2018.html#rust-2018
[crate]: https://crates.io/crates/impls

[`BitAnd`]: https://doc.rust-lang.org/std/ops/trait.BitAnd.html
[`BitOr`]:  https://doc.rust-lang.org/std/ops/trait.BitOr.html
[`BitXor`]: https://doc.rust-lang.org/std/ops/trait.BitXor.html
[`Into`]:   https://doc.rust-lang.org/std/convert/trait.Into.html
[`Not`]:    https://doc.rust-lang.org/std/ops/trait.Not.html
[`Send`]:   https://doc.rust-lang.org/std/marker/trait.Send.html
[`Sync`]:   https://doc.rust-lang.org/std/marker/trait.Send.html
[ptr]:      https://doc.rust-lang.org/std/primitive.pointer.html

[AST]:                   https://en.wikipedia.org/wiki/Abstract_syntax_tree
[exclusive disjunction]: https://en.wikipedia.org/wiki/Exclusive_disjunction
[logical conjunction]:   https://en.wikipedia.org/wiki/Logical_conjunction
[logical disjunction]:   https://en.wikipedia.org/wiki/Logical_disjunction
