# The Unstable Book

Welcome to the Unstable Book! This book consists of a number of chapters, each one organized by a "feature flag." That is, when using an unstable feature of Rust, you must use a flag, like this:

    #![feature(box_syntax)]

    fn main() {
        let five = box 5;
    }

The `box_syntax` feature has a chapter describing how to use it.

Because this documentation relates to unstable features, we make no guarantees that what is contained here is accurate or up to date. It's developed on a best-effort basis. Each page will have a link to its tracking issue with the latest developments; you might want to check those as well.

# `abi_msp430_interrupt`

The tracking issue for this feature is: #38487

# `abi_ptx`

The tracking issue for this feature is: None.

# `abi_sysv64`

The tracking issue for this feature is: #36167

# `abi_unadjusted`

The tracking issue for this feature is: none.

# `abi_vectorcall`

The tracking issue for this feature is: none.

# `abi_x86_interrupt`

The tracking issue for this feature is: #40180

# `advanced_slice_patterns`

The tracking issue for this feature is: #23121

See also `slice_patterns`.

The `advanced_slice_patterns` gate lets you use `..` to indicate any number of elements inside a pattern matching a slice. This wildcard can only be used once for a given array. If there's an identifier before the `..`, the result of the slice will be bound to that name. For example:

    #![feature(advanced_slice_patterns, slice_patterns)]

    fn is_symmetric(list: &[u32]) -> bool {
        match list {
            &[] | &[_] => true,
            &[x, ref inside.., y] if x == y => is_symmetric(inside),
            _ => false
        }
    }

    fn main() {
        let sym = &[0, 1, 4, 2, 4, 1, 0];
        assert!(is_symmetric(sym));

        let not_sym = &[0, 1, 7, 2, 4, 1, 0];
        assert!(!is_symmetric(not_sym));
    }

# `alloc_jemalloc`

The tracking issue for this feature is: #33082

See also `alloc_system`.

The compiler currently ships two default allocators: `alloc_system` and `alloc_jemalloc` (some targets don't have jemalloc, however). These allocators are normal Rust crates and contain an implementation of the routines to allocate and deallocate memory. The standard library is not compiled assuming either one, and the compiler will decide which allocator is in use at compile-time depending on the type of output artifact being produced.

Binaries generated by the compiler will use `alloc_jemalloc` by default (where available). In this situation the compiler "controls the world" in the sense of it has power over the final link. Primarily this means that the allocator decision can be left up the compiler.

Dynamic and static libraries, however, will use `alloc_system` by default. Here Rust is typically a 'guest' in another application or another world where it cannot authoritatively decide what allocator is in use. As a result it resorts back to the standard APIs (e.g. `malloc` and `free`) for acquiring and releasing memory.

# Switching Allocators

Although the compiler's default choices may work most of the time, it's often necessary to tweak certain aspects. Overriding the compiler's decision about which allocator is in use is done simply by linking to the desired allocator:

    #![feature(alloc_system)]

    extern crate alloc_system;

    fn main() {
        let a = Box::new(4); // Allocates from the system allocator.
        println!("{}", a);
    }

In this example the binary generated will not link to jemalloc by default but instead use the system allocator. Conversely to generate a dynamic library which uses jemalloc by default one would write:

    #![feature(alloc_jemalloc)]
    #![crate_type = "dylib"]

    extern crate alloc_jemalloc;

    pub fn foo() {
        let a = Box::new(4); // Allocates from jemalloc.
        println!("{}", a);
    }
    # fn main() {}

# `alloc_system`

The tracking issue for this feature is: #33082

See also `alloc_jemalloc`.

The compiler currently ships two default allocators: `alloc_system` and `alloc_jemalloc` (some targets don't have jemalloc, however). These allocators are normal Rust crates and contain an implementation of the routines to allocate and deallocate memory. The standard library is not compiled assuming either one, and the compiler will decide which allocator is in use at compile-time depending on the type of output artifact being produced.

Binaries generated by the compiler will use `alloc_jemalloc` by default (where available). In this situation the compiler "controls the world" in the sense of it has power over the final link. Primarily this means that the allocator decision can be left up the compiler.

Dynamic and static libraries, however, will use `alloc_system` by default. Here Rust is typically a 'guest' in another application or another world where it cannot authoritatively decide what allocator is in use. As a result it resorts back to the standard APIs (e.g. `malloc` and `free`) for acquiring and releasing memory.

# Switching Allocators

Although the compiler's default choices may work most of the time, it's often necessary to tweak certain aspects. Overriding the compiler's decision about which allocator is in use is done simply by linking to the desired allocator:

    #![feature(alloc_system)]

    extern crate alloc_system;

    fn main() {
        let a = Box::new(4); // Allocates from the system allocator.
        println!("{}", a);
    }

In this example the binary generated will not link to jemalloc by default but instead use the system allocator. Conversely to generate a dynamic library which uses jemalloc by default one would write:

    #![feature(alloc_jemalloc)]
    #![crate_type = "dylib"]

    extern crate alloc_jemalloc;

    pub fn foo() {
        let a = Box::new(4); // Allocates from jemalloc.
        println!("{}", a);
    }
    # fn main() {}

# `allocator`

The tracking issue for this feature is: #27389

Sometimes even the choices of jemalloc vs the system allocator aren't enough and an entirely new custom allocator is required. In this you'll write your own crate which implements the allocator API (e.g. the same as `alloc_system` or `alloc_jemalloc`). As an example, let's take a look at a simplified and annotated version of `alloc_system`

    # // Only needed for rustdoc --test down below.
    # #![feature(lang_items)]
    // The compiler needs to be instructed that this crate is an allocator in order
    // to realize that when this is linked in another allocator like jemalloc should
    // not be linked in.
    #![feature(allocator)]
    #![allocator]

    // Allocators are not allowed to depend on the standard library which in turn
    // requires an allocator in order to avoid circular dependencies. This crate,
    // however, can use all of libcore.
    #![no_std]

    // Let's give a unique name to our custom allocator:
    #![crate_name = "my_allocator"]
    #![crate_type = "rlib"]

    // Our system allocator will use the in-tree libc crate for FFI bindings. Note
    // that currently the external (crates.io) libc cannot be used because it links
    // to the standard library (e.g. `#![no_std]` isn't stable yet), so that's why
    // this specifically requires the in-tree version.
    #![feature(libc)]
    extern crate libc;

    // Listed below are the five allocation functions currently required by custom
    // allocators. Their signatures and symbol names are not currently typechecked
    // by the compiler, but this is a future extension and are required to match
    // what is found below.
    //
    // Note that the standard `malloc` and `realloc` functions do not provide a way
    // to communicate alignment so this implementation would need to be improved
    // with respect to alignment in that aspect.

    #[no_mangle]
    pub extern fn __rust_allocate(size: usize, _align: usize) -> *mut u8 {
        unsafe { libc::malloc(size as libc::size_t) as *mut u8 }
    }

    #[no_mangle]
    pub extern fn __rust_deallocate(ptr: *mut u8, _old_size: usize, _align: usize) {
        unsafe { libc::free(ptr as *mut libc::c_void) }
    }

    #[no_mangle]
    pub extern fn __rust_reallocate(ptr: *mut u8, _old_size: usize, size: usize,
                                    _align: usize) -> *mut u8 {
        unsafe {
            libc::realloc(ptr as *mut libc::c_void, size as libc::size_t) as *mut u8
        }
    }

    #[no_mangle]
    pub extern fn __rust_reallocate_inplace(_ptr: *mut u8, old_size: usize,
                                            _size: usize, _align: usize) -> usize {
        old_size // This api is not supported by libc.
    }

    #[no_mangle]
    pub extern fn __rust_usable_size(size: usize, _align: usize) -> usize {
        size
    }

    # // Only needed to get rustdoc to test this:
    # fn main() {}
    # #[lang = "panic_fmt"] fn panic_fmt() {}
    # #[lang = "eh_personality"] fn eh_personality() {}
    # #[lang = "eh_unwind_resume"] extern fn eh_unwind_resume() {}
    # #[no_mangle] pub extern fn rust_eh_register_frames () {}
    # #[no_mangle] pub extern fn rust_eh_unregister_frames () {}

After we compile this crate, it can be used as follows:

    extern crate my_allocator;

    fn main() {
        let a = Box::new(8); // Allocates memory via our custom allocator crate.
        println!("{}", a);
    }

## Custom allocator limitations

There are a few restrictions when working with custom allocators which may cause compiler errors:

- Any one artifact may only be linked to at most one allocator. Binaries, dylibs, and staticlibs must link to exactly one allocator, and if none have been explicitly chosen the compiler will choose one. On the other hand rlibs do not need to link to an allocator (but still can).

- A consumer of an allocator is tagged with `#![needs_allocator]` (e.g. the `liballoc` crate currently) and an `#[allocator]` crate cannot transitively depend on a crate which needs an allocator (e.g. circular dependencies are not allowed). This basically means that allocators must restrict themselves to libcore currently.

# `allow_internal_unstable`

The tracking issue for this feature is: None.

# `asm`

The tracking issue for this feature is: #29722

For extremely low-level manipulations and performance reasons, one might wish to control the CPU directly. Rust supports using inline assembly to do this via the `asm!` macro.

    asm!(assembly template
       : output operands
       : input operands
       : clobbers
       : options
       );

Any use of `asm` is feature gated (requires `#![feature(asm)]` on the crate to allow) and of course requires an `unsafe` block.

> Note: the examples here are given in x86/x86-64 assembly, but all platforms are supported.

## Assembly template

The `assembly template` is the only required parameter and must be a literal string (i.e. `""`)

    #![feature(asm)]

    #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
    fn foo() {
        unsafe {
            asm!("NOP");
        }
    }

    // Other platforms:
    #[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]
    fn foo() { /* ... */ }

    fn main() {
        // ...
        foo();
        // ...
    }

(The `feature(asm)` and `#[cfg]`s are omitted from now on.)

Output operands, input operands, clobbers and options are all optional but you must add the right number of `:` if you skip them:

    # #![feature(asm)]
    # #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
    # fn main() { unsafe {
    asm!("xor %eax, %eax"
        :
        :
        : "eax"
       );
    # } }
    # #[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]
    # fn main() {}

Whitespace also doesn't matter:

    # #![feature(asm)]
    # #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
    # fn main() { unsafe {
    asm!("xor %eax, %eax" ::: "eax");
    # } }
    # #[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]
    # fn main() {}

## Operands

Input and output operands follow the same format: `: "constraints1"(expr1), "constraints2"(expr2), ..."`. Output operand expressions must be mutable lvalues, or not yet assigned:

    # #![feature(asm)]
    # #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
    fn add(a: i32, b: i32) -> i32 {
        let c: i32;
        unsafe {
            asm!("add $2, $0"
                 : "=r"(c)
                 : "0"(a), "r"(b)
                 );
        }
        c
    }
    # #[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]
    # fn add(a: i32, b: i32) -> i32 { a + b }

    fn main() {
        assert_eq!(add(3, 14159), 14162)
    }

If you would like to use real operands in this position, however, you are required to put curly braces `{}` around the register that you want, and you are required to put the specific size of the operand. This is useful for very low level programming, where which register you use is important:

    # #![allow(unused_variables)]
    #
    #fn main() {
    # #![feature(asm)]
    # #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
    # unsafe fn read_byte_in(port: u16) -> u8 {
    let result: u8;
    asm!("in %dx, %al" : "={al}"(result) : "{dx}"(port));
    result
    # }

    #}

## Clobbers

Some instructions modify registers which might otherwise have held different values so we use the clobbers list to indicate to the compiler not to assume any values loaded into those registers will stay valid.

    # #![feature(asm)]
    # #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
    # fn main() { unsafe {
    // Put the value 0x200 in eax:
    asm!("mov $$0x200, %eax" : /* no outputs */ : /* no inputs */ : "eax");
    # } }
    # #[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]
    # fn main() {}

Input and output registers need not be listed since that information is already communicated by the given constraints. Otherwise, any other registers used either implicitly or explicitly should be listed.

If the assembly changes the condition code register `cc` should be specified as one of the clobbers. Similarly, if the assembly modifies memory, `memory` should also be specified.

## Options

The last section, `options` is specific to Rust. The format is comma separated literal strings (i.e. `:"foo", "bar", "baz"`). It's used to specify some extra info about the inline assembly:

Current valid options are:

1. volatile - specifying this is analogous to `__asm__ __volatile__ (...)` in gcc/clang.
2. alignstack - certain instructions expect the stack to be aligned a certain way (i.e. SSE) and specifying this indicates to the compiler to insert its usual stack alignment code
3. intel - use intel syntax instead of the default AT&T.

    # #![feature(asm)]
    # #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
    # fn main() {
    let result: i32;
    unsafe {
       asm!("mov eax, 2" : "={eax}"(result) : : : "intel")
    }
    println!("eax is currently {}", result);
    # }
    # #[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]
    # fn main() {}

## More Information

The current implementation of the `asm!` macro is a direct binding to LLVM's inline assembler expressions, so be sure to check out their documentation as well for more information about clobbers, constraints, etc.

# `associated_consts`

The tracking issue for this feature is: #29646

With the `associated_consts` feature, you can define constants like this:

    #![feature(associated_consts)]

    trait Foo {
        const ID: i32;
    }

    impl Foo for i32 {
        const ID: i32 = 1;
    }

    fn main() {
        assert_eq!(1, i32::ID);
    }

Any implementor of `Foo` will have to define `ID`. Without the definition:

    #![feature(associated_consts)]

    trait Foo {
        const ID: i32;
    }

    impl Foo for i32 {
    }

gives

    error: not all trait items implemented, missing: `ID` [E0046]
         impl Foo for i32 {
         }

A default value can be implemented as well:

    #![feature(associated_consts)]

    trait Foo {
        const ID: i32 = 1;
    }

    impl Foo for i32 {
    }

    impl Foo for i64 {
        const ID: i32 = 5;
    }

    fn main() {
        assert_eq!(1, i32::ID);
        assert_eq!(5, i64::ID);
    }

As you can see, when implementing `Foo`, you can leave it unimplemented, as with `i32`. It will then use the default value. But, as in `i64`, we can also add our own definition.

Associated constants don’t have to be associated with a trait. An `impl` block for a `struct` or an `enum` works fine too:

    # #![allow(unused_variables)]
    #
    #fn main() {
    #![feature(associated_consts)]

    struct Foo;

    impl Foo {
        const FOO: u32 = 3;
    }

    #}

# `associated_type_defaults`

The tracking issue for this feature is: #29661

# `attr_literals`

The tracking issue for this feature is: #34981

# `box_patterns`

The tracking issue for this feature is: #29641

See also `box_syntax`

Box patterns let you match on `Box<T>`s:

    #![feature(box_patterns)]

    fn main() {
        let b = Some(Box::new(5));
        match b {
            Some(box n) if n < 0 => {
                println!("Box contains negative number {}", n);
            },
            Some(box n) if n >= 0 => {
                println!("Box contains non-negative number {}", n);
            },
            None => {
                println!("No box");
            },
            _ => unreachable!()
        }
    }

# `box_syntax`

The tracking issue for this feature is: #27779

See also `box_patterns`

Currently the only stable way to create a `Box` is via the `Box::new` method. Also it is not possible in stable Rust to destructure a `Box` in a match pattern. The unstable `box` keyword can be used to create a `Box`. An example usage would be:

    #![feature(box_syntax)]

    fn main() {
        let b = box 5;
    }

# `cfg_target_feature`

The tracking issue for this feature is: #29717

# `cfg_target_has_atomic`

The tracking issue for this feature is: #32976

# `cfg_target_thread_local`

The tracking issue for this feature is: #29594

# `cfg_target_feature`

The tracking issue for this feature is: #29717

# `compiler_builtins`

The tracking issue for this feature is: None.

# `concat_idents`

The tracking issue for this feature is: #29599

# `conservative_impl_trait`

The tracking issue for this feature is: #34511

# `const_fn`

The tracking issue for this feature is: #24111

# `const_indexing`

The tracking issue for this feature is: #29947

# `custom_attribute`

The tracking issue for this feature is: #29642

# `custom_derive`

The tracking issue for this feature is: #29644

# `default_type_parameter_fallback`

The tracking issue for this feature is: #27336

# `drop_types_in_const`

The tracking issue for this feature is: #33156

# `dropck_eyepatch`

The tracking issue for this feature is: #34761

# `dropck_parametricity`

The tracking issue for this feature is: #28498

# `exclusive_range_pattern`

The tracking issue for this feature is: #37854

# `field_init_shorthand`

The tracking issue for this feature is: #37340

# `fundamental`

The tracking issue for this feature is: #29635

# `generic_param_attrs`

The tracking issue for this feature is: #34761

# `i128_type`

The tracking issue for this feature is: #35118

# `inclusive_range_syntax`

The tracking issue for this feature is: #28237

# `intrinsics`

The tracking issue for this feature is: None.

Intrinsics are never intended to be stable directly, but intrinsics are often exported in some sort of stable manner. Prefer using the stable interfaces to the intrinsic directly when you can.

These are imported as if they were FFI functions, with the special `rust-intrinsic` ABI. For example, if one was in a freestanding context, but wished to be able to `transmute` between types, and perform efficient pointer arithmetic, one would import those functions via a declaration like

    #![feature(intrinsics)]
    # fn main() {}

    extern "rust-intrinsic" {
        fn transmute<T, U>(x: T) -> U;

        fn offset<T>(dst: *const T, offset: isize) -> *const T;
    }

As with any other FFI functions, these are always `unsafe` to call.

# `lang_items`

The tracking issue for this feature is: None.

The `rustc` compiler has certain pluggable operations, that is, functionality that isn't hard-coded into the language, but is implemented in libraries, with a special marker to tell the compiler it exists. The marker is the attribute `#[lang = "..."]` and there are various different values of `...`, i.e. various different 'lang items'.

For example, `Box` pointers require two lang items, one for allocation and one for deallocation. A freestanding program that uses the `Box` sugar for dynamic allocations via `malloc` and `free`:

    #![feature(lang_items, box_syntax, start, libc, core_intrinsics)]
    #![no_std]
    use core::intrinsics;

    extern crate libc;

    #[lang = "owned_box"]
    pub struct Box<T>(*mut T);

    #[lang = "exchange_malloc"]
    unsafe fn allocate(size: usize, _align: usize) -> *mut u8 {
        let p = libc::malloc(size as libc::size_t) as *mut u8;

        // Check if `malloc` failed:
        if p as usize == 0 {
            intrinsics::abort();
        }

        p
    }

    #[lang = "exchange_free"]
    unsafe fn deallocate(ptr: *mut u8, _size: usize, _align: usize) {
        libc::free(ptr as *mut libc::c_void)
    }

    #[lang = "box_free"]
    unsafe fn box_free<T: ?Sized>(ptr: *mut T) {
        deallocate(ptr as *mut u8, ::core::mem::size_of_val(&*ptr), ::core::mem::align_of_val(&*ptr));
    }

    #[start]
    fn main(argc: isize, argv: *const *const u8) -> isize {
        let x = box 1;

        0
    }

    #[lang = "eh_personality"] extern fn rust_eh_personality() {}
    #[lang = "panic_fmt"] extern fn rust_begin_panic() -> ! { unsafe { intrinsics::abort() } }
    # #[lang = "eh_unwind_resume"] extern fn rust_eh_unwind_resume() {}
    # #[no_mangle] pub extern fn rust_eh_register_frames () {}
    # #[no_mangle] pub extern fn rust_eh_unregister_frames () {}

Note the use of `abort`: the `exchange_malloc` lang item is assumed to return a valid pointer, and so needs to do the check internally.

Other features provided by lang items include:

- overloadable operators via traits: the traits corresponding to the `==`, `<`, dereferencing (`*`) and `+` (etc.) operators are all marked with lang items; those specific four are `eq`, `ord`, `deref`, and `add` respectively.
- stack unwinding and general failure; the `eh_personality`, `eh_unwind_resume`, `fail` and `fail_bounds_checks` lang items.
- the traits in `std::marker` used to indicate types of various kinds; lang items `send`, `sync` and `copy`.
- the marker types and variance indicators found in `std::marker`; lang items `covariant_type`, `contravariant_lifetime`, etc.

Lang items are loaded lazily by the compiler; e.g. if one never uses `Box` then there is no need to define functions for `exchange_malloc` and `exchange_free`. `rustc` will emit an error when an item is needed but not found in the current crate or any that it depends on.

Most lang items are defined by `libcore`, but if you're trying to build an executable without the standard library, you'll run into the need for lang items. The rest of this page focuses on this use-case, even though lang items are a bit broader than that.

### Using libc

In order to build a `#[no_std]` executable we will need libc as a dependency. We can specify this using our `Cargo.toml` file:

    [dependencies]
    libc = { version = "0.2.14", default-features = false }

Note that the default features have been disabled. This is a critical step - the default features of libc include the standard library and so must be disabled.

### Writing an executable without stdlib

Controlling the entry point is possible in two ways: the `#[start]` attribute, or overriding the default shim for the C `main` function with your own.

The function marked `#[start]` is passed the command line parameters in the same format as C:

    #![feature(lang_items, core_intrinsics)]
    #![feature(start)]
    #![no_std]
    use core::intrinsics;

    // Pull in the system libc library for what crt0.o likely requires.
    extern crate libc;

    // Entry point for this program.
    #[start]
    fn start(_argc: isize, _argv: *const *const u8) -> isize {
        0
    }

    // These functions are used by the compiler, but not
    // for a bare-bones hello world. These are normally
    // provided by libstd.
    #[lang = "eh_personality"]
    #[no_mangle]
    pub extern fn rust_eh_personality() {
    }

    // This function may be needed based on the compilation target.
    #[lang = "eh_unwind_resume"]
    #[no_mangle]
    pub extern fn rust_eh_unwind_resume() {
    }

    #[lang = "panic_fmt"]
    #[no_mangle]
    pub extern fn rust_begin_panic(_msg: core::fmt::Arguments,
                                   _file: &'static str,
                                   _line: u32) -> ! {
        unsafe { intrinsics::abort() }
    }

To override the compiler-inserted `main` shim, one has to disable it with `#![no_main]` and then create the appropriate symbol with the correct ABI and the correct name, which requires overriding the compiler's name mangling too:

    #![feature(lang_items, core_intrinsics)]
    #![feature(start)]
    #![no_std]
    #![no_main]
    use core::intrinsics;

    // Pull in the system libc library for what crt0.o likely requires.
    extern crate libc;

    // Entry point for this program.
    #[no_mangle] // ensure that this symbol is called `main` in the output
    pub extern fn main(_argc: i32, _argv: *const *const u8) -> i32 {
        0
    }

    // These functions are used by the compiler, but not
    // for a bare-bones hello world. These are normally
    // provided by libstd.
    #[lang = "eh_personality"]
    #[no_mangle]
    pub extern fn rust_eh_personality() {
    }

    // This function may be needed based on the compilation target.
    #[lang = "eh_unwind_resume"]
    #[no_mangle]
    pub extern fn rust_eh_unwind_resume() {
    }

    #[lang = "panic_fmt"]
    #[no_mangle]
    pub extern fn rust_begin_panic(_msg: core::fmt::Arguments,
                                   _file: &'static str,
                                   _line: u32) -> ! {
        unsafe { intrinsics::abort() }
    }

## More about the language items

The compiler currently makes a few assumptions about symbols which are available in the executable to call. Normally these functions are provided by the standard library, but without it you must define your own. These symbols are called "language items", and they each have an internal name, and then a signature that an implementation must conform to.

The first of these functions, `rust_eh_personality`, is used by the failure mechanisms of the compiler. This is often mapped to GCC's personality function (see the libstd implementation for more information), but crates which do not trigger a panic can be assured that this function is never called. The language item's name is `eh_personality`.

The second function, `rust_begin_panic`, is also used by the failure mechanisms of the compiler. When a panic happens, this controls the message that's displayed on the screen. While the language item's name is `panic_fmt`, the symbol name is `rust_begin_panic`.

A third function, `rust_eh_unwind_resume`, is also needed if the `custom_unwind_resume` flag is set in the options of the compilation target. It allows customizing the process of resuming unwind at the end of the landing pads. The language item's name is `eh_unwind_resume`.

# `link_args`

The tracking issue for this feature is: #29596

You can tell `rustc` how to customize linking, and that is via the `link_args` attribute. This attribute is applied to `extern` blocks and specifies raw flags which need to get passed to the linker when producing an artifact. An example usage would be:

    #![feature(link_args)]

    #[link_args = "-foo -bar -baz"]
    extern {}
    # fn main() {}

Note that this feature is currently hidden behind the `feature(link_args)` gate because this is not a sanctioned way of performing linking. Right now `rustc` shells out to the system linker (`gcc` on most systems, `link.exe` on MSVC), so it makes sense to provide extra command line arguments, but this will not always be the case. In the future `rustc` may use LLVM directly to link native libraries, in which case `link_args` will have no meaning. You can achieve the same effect as the `link_args` attribute with the `-C link-args` argument to `rustc`.

It is highly recommended to not use this attribute, and rather use the more formal `#[link(...)]` attribute on `extern` blocks instead.

# `link_cfg`

The tracking issue for this feature is: #37406

# `link_llvm_intrinsics`

The tracking issue for this feature is: #29602

# `linkage`

The tracking issue for this feature is: #29603

# `log_syntax`

The tracking issue for this feature is: #29598

# `loop_break_value`

The tracking issue for this feature is: #37339

# `macro_reexport`

The tracking issue for this feature is: #29638

# `main`

The tracking issue for this feature is: #29634

# `naked_functions`

The tracking issue for this feature is: #32408

# `needs_allocator`

The tracking issue for this feature is: #27389

# `needs_panic_runtime`

The tracking issue for this feature is: #32837

# `never_type`

The tracking issue for this feature is: #35121

# `no_core`

The tracking issue for this feature is: #29639

# `no_debug`

The tracking issue for this feature is: #29721

# `non_ascii_idents`

The tracking issue for this feature is: #28979

# `omit_gdb_pretty_printer_section`

The tracking issue for this feature is: None.

# `on_unimplemented`

The tracking issue for this feature is: #29628

# `optin_builtin_traits`

The tracking issue for this feature is: #13231

# `panic_runtime`

The tracking issue for this feature is: #32837

# `placement_in_syntax`

The tracking issue for this feature is: #27779

# `platform_intrinsics`

The tracking issue for this feature is: #27731

# `plugin`

The tracking issue for this feature is: #29597

This feature is part of "compiler plugins." It will often be used with the `plugin_registrar` and `rustc_private` features.

`rustc` can load compiler plugins, which are user-provided libraries that extend the compiler's behavior with new syntax extensions, lint checks, etc.

A plugin is a dynamic library crate with a designated registrar function that registers extensions with `rustc`. Other crates can load these extensions using the crate attribute `#![plugin(...)]`. See the `rustc_plugin` documentation for more about the mechanics of defining and loading a plugin.

If present, arguments passed as `#![plugin(foo(... args ...))]` are not interpreted by rustc itself. They are provided to the plugin through the `Registry`'s `args` method.

In the vast majority of cases, a plugin should only be used through `#![plugin]` and not through an `extern crate` item. Linking a plugin would pull in all of libsyntax and librustc as dependencies of your crate. This is generally unwanted unless you are building another plugin. The `plugin_as_library` lint checks these guidelines.

The usual practice is to put compiler plugins in their own crate, separate from any `macro_rules!` macros or ordinary Rust code meant to be used by consumers of a library.

# Syntax extensions

Plugins can extend Rust's syntax in various ways. One kind of syntax extension is the procedural macro. These are invoked the same way as ordinary macros, but the expansion is performed by arbitrary Rust code that manipulates syntax trees at compile time.

Let's write a plugin `roman_numerals.rs` that implements Roman numeral integer literals.

    #![crate_type="dylib"]
    #![feature(plugin_registrar, rustc_private)]

    extern crate syntax;
    extern crate rustc;
    extern crate rustc_plugin;

    use syntax::parse::token;
    use syntax::tokenstream::TokenTree;
    use syntax::ext::base::{ExtCtxt, MacResult, DummyResult, MacEager};
    use syntax::ext::build::AstBuilder;  // A trait for expr_usize.
    use syntax::ext::quote::rt::Span;
    use rustc_plugin::Registry;

    fn expand_rn(cx: &mut ExtCtxt, sp: Span, args: &[TokenTree])
            -> Box<MacResult + 'static> {

        static NUMERALS: &'static [(&'static str, usize)] = &[
            ("M", 1000), ("CM", 900), ("D", 500), ("CD", 400),
            ("C",  100), ("XC",  90), ("L",  50), ("XL",  40),
            ("X",   10), ("IX",   9), ("V",   5), ("IV",   4),
            ("I",    1)];

        if args.len() != 1 {
            cx.span_err(
                sp,
                &format!("argument should be a single identifier, but got {} arguments", args.len()));
            return DummyResult::any(sp);
        }

        let text = match args[0] {
            TokenTree::Token(_, token::Ident(s)) => s.to_string(),
            _ => {
                cx.span_err(sp, "argument should be a single identifier");
                return DummyResult::any(sp);
            }
        };

        let mut text = &*text;
        let mut total = 0;
        while !text.is_empty() {
            match NUMERALS.iter().find(|&&(rn, _)| text.starts_with(rn)) {
                Some(&(rn, val)) => {
                    total += val;
                    text = &text[rn.len()..];
                }
                None => {
                    cx.span_err(sp, "invalid Roman numeral");
                    return DummyResult::any(sp);
                }
            }
        }

        MacEager::expr(cx.expr_usize(sp, total))
    }

    #[plugin_registrar]
    pub fn plugin_registrar(reg: &mut Registry) {
        reg.register_macro("rn", expand_rn);
    }

Then we can use `rn!()` like any other macro:

    #![feature(plugin)]
    #![plugin(roman_numerals)]

    fn main() {
        assert_eq!(rn!(MMXV), 2015);
    }

The advantages over a simple `fn(&str) -> u32` are:

- The (arbitrarily complex) conversion is done at compile time.
- Input validation is also performed at compile time.
- It can be extended to allow use in patterns, which effectively gives a way to define new literal syntax for any data type.

In addition to procedural macros, you can define new `derive`-like attributes and other kinds of extensions. See `Registry::register_syntax_extension` and the `SyntaxExtension` enum. For a more involved macro example, see `regex_macros`.

## Tips and tricks

Some of the macro debugging tips are applicable.

You can use `syntax::parse` to turn token trees into higher-level syntax elements like expressions:

    fn expand_foo(cx: &mut ExtCtxt, sp: Span, args: &[TokenTree])
            -> Box<MacResult+'static> {

        let mut parser = cx.new_parser_from_tts(args);

        let expr: P<Expr> = parser.parse_expr();

Looking through `libsyntax` parser code will give you a feel for how the parsing infrastructure works.

Keep the `Span`s of everything you parse, for better error reporting. You can wrap `Spanned` around your custom data structures.

Calling `ExtCtxt::span_fatal` will immediately abort compilation. It's better to instead call `ExtCtxt::span_err` and return `DummyResult` so that the compiler can continue and find further errors.

To print syntax fragments for debugging, you can use `span_note` together with `syntax::print::pprust::*_to_string`.

The example above produced an integer literal using `AstBuilder::expr_usize`. As an alternative to the `AstBuilder` trait, `libsyntax` provides a set of quasiquote macros. They are undocumented and very rough around the edges. However, the implementation may be a good starting point for an improved quasiquote as an ordinary plugin library.

# Lint plugins

Plugins can extend Rust's lint infrastructure with additional checks for code style, safety, etc. Now let's write a plugin `lint_plugin_test.rs` that warns about any item named `lintme`.

    #![feature(plugin_registrar)]
    #![feature(box_syntax, rustc_private)]

    extern crate syntax;

    // Load rustc as a plugin to get macros
    #[macro_use]
    extern crate rustc;
    extern crate rustc_plugin;

    use rustc::lint::{EarlyContext, LintContext, LintPass, EarlyLintPass,
                      EarlyLintPassObject, LintArray};
    use rustc_plugin::Registry;
    use syntax::ast;

    declare_lint!(TEST_LINT, Warn, "Warn about items named 'lintme'");

    struct Pass;

    impl LintPass for Pass {
        fn get_lints(&self) -> LintArray {
            lint_array!(TEST_LINT)
        }
    }

    impl EarlyLintPass for Pass {
        fn check_item(&mut self, cx: &EarlyContext, it: &ast::Item) {
            if it.ident.name.as_str() == "lintme" {
                cx.span_lint(TEST_LINT, it.span, "item is named 'lintme'");
            }
        }
    }

    #[plugin_registrar]
    pub fn plugin_registrar(reg: &mut Registry) {
        reg.register_early_lint_pass(box Pass as EarlyLintPassObject);
    }

Then code like

    #![plugin(lint_plugin_test)]

    fn lintme() { }

will produce a compiler warning:

    foo.rs:4:1: 4:16 warning: item is named 'lintme', #[warn(test_lint)] on by default
    foo.rs:4 fn lintme() { }
             ^~~~~~~~~~~~~~~

The components of a lint plugin are:

- one or more `declare_lint!` invocations, which define static `Lint` structs;

- a struct holding any state needed by the lint pass (here, none);

- a `LintPass` implementation defining how to check each syntax element. A single `LintPass` may call `span_lint` for several different `Lint`s, but should register them all through the `get_lints` method.

Lint passes are syntax traversals, but they run at a late stage of compilation where type information is available. `rustc`'s built-in lints mostly use the same infrastructure as lint plugins, and provide examples of how to access type information.

Lints defined by plugins are controlled by the usual attributes and compiler flags, e.g. `#[allow(test_lint)]` or `-A test-lint`. These identifiers are derived from the first argument to `declare_lint!`, with appropriate case and punctuation conversion.

You can run `rustc -W help foo.rs` to see a list of lints known to `rustc`, including those provided by plugins loaded by `foo.rs`.

# `plugin_registrar`

The tracking issue for this feature is: #29597

This feature is part of "compiler plugins." It will often be used with the `plugin` and `rustc_private` features as well. For more details, see their docs.

# `prelude_import`

The tracking issue for this feature is: None.

# `proc_macro`

The tracking issue for this feature is: #38356

# `pub_restricted`

The tracking issue for this feature is: #32409

# `quote`

The tracking issue for this feature is: #29601

# `relaxed_adts`

The tracking issue for this feature is: #35626

# `repr_simd`

The tracking issue for this feature is: #27731

# `rustc_attrs`

The tracking issue for this feature is: #29642

# `rustc_diagnostic_macros`

The tracking issue for this feature is: None.

# `sanitizer_runtime`

The tracking issue for this feature is: None.

# `simd`

The tracking issue for this feature is: #27731

# `simd_ffi`

The tracking issue for this feature is: #27731

# `slice_patterns`

The tracking issue for this feature is: #23121

See also `advanced_slice_patterns`.

If you want to match against a slice or array, you can use `&` with the `slice_patterns` feature:

    #![feature(slice_patterns)]

    fn main() {
        let v = vec!["match_this", "1"];

        match &v[..] {
            &["match_this", second] => println!("The second element is {}", second),
            _ => {},
        }
    }

# `specialization`

The tracking issue for this feature is: [#31844]

# `staged_api`

The tracking issue for this feature is: None.

# `start`

The tracking issue for this feature is: #29633

# `static_nobundle`

The tracking issue for this feature is: #37403

# `static_recursion`

The tracking issue for this feature is: #29719

# `stmt_expr_attributes`

The tracking issue for this feature is: #15701

# `struct_field_attributes`

The tracking issue for this feature is: #38814

# `structural_match`

The tracking issue for this feature is: #31434

# `target_feature`

The tracking issue for this feature is: None.

# `test`

The tracking issue for this feature is: None.

The internals of the `test` crate are unstable, behind the `test` flag. The most widely used part of the `test` crate are benchmark tests, which can test the performance of your code. Let's make our `src/lib.rs` look like this (comments elided):

    #![feature(test)]

    extern crate test;

    pub fn add_two(a: i32) -> i32 {
        a + 2
    }

    #[cfg(test)]
    mod tests {
        use super::*;
        use test::Bencher;

        #[test]
        fn it_works() {
            assert_eq!(4, add_two(2));
        }

        #[bench]
        fn bench_add_two(b: &mut Bencher) {
            b.iter(|| add_two(2));
        }
    }

Note the `test` feature gate, which enables this unstable feature.

We've imported the `test` crate, which contains our benchmarking support. We have a new function as well, with the `bench` attribute. Unlike regular tests, which take no arguments, benchmark tests take a `&mut Bencher`. This `Bencher` provides an `iter` method, which takes a closure. This closure contains the code we'd like to benchmark.

We can run benchmark tests with `cargo bench`:

    $ cargo bench
       Compiling adder v0.0.1 (file:///home/steve/tmp/adder)
         Running target/release/adder-91b3e234d4ed382a

    running 2 tests
    test tests::it_works ... ignored
    test tests::bench_add_two ... bench:         1 ns/iter (+/- 0)

    test result: ok. 0 passed; 0 failed; 1 ignored; 1 measured

Our non-benchmark test was ignored. You may have noticed that `cargo bench` takes a bit longer than `cargo test`. This is because Rust runs our benchmark a number of times, and then takes the average. Because we're doing so little work in this example, we have a `1 ns/iter (+/- 0)`, but this would show the variance if there was one.

Advice on writing benchmarks:

- Move setup code outside the `iter` loop; only put the part you want to measure inside
- Make the code do "the same thing" on each iteration; do not accumulate or change state
- Make the outer function idempotent too; the benchmark runner is likely to run it many times
- Make the inner `iter` loop short and fast so benchmark runs are fast and the calibrator can adjust the run-length at fine resolution
- Make the code in the `iter` loop do something simple, to assist in pinpointing performance improvements (or regressions)

## Gotcha: optimizations

There's another tricky part to writing benchmarks: benchmarks compiled with optimizations activated can be dramatically changed by the optimizer so that the benchmark is no longer benchmarking what one expects. For example, the compiler might recognize that some calculation has no external effects and remove it entirely.

    #![feature(test)]

    extern crate test;
    use test::Bencher;

    #[bench]
    fn bench_xor_1000_ints(b: &mut Bencher) {
        b.iter(|| {
            (0..1000).fold(0, |old, new| old ^ new);
        });
    }

gives the following results

    running 1 test
    test bench_xor_1000_ints ... bench:         0 ns/iter (+/- 0)

    test result: ok. 0 passed; 0 failed; 0 ignored; 1 measured

The benchmarking runner offers two ways to avoid this. Either, the closure that the `iter` method receives can return an arbitrary value which forces the optimizer to consider the result used and ensures it cannot remove the computation entirely. This could be done for the example above by adjusting the `b.iter` call to

    # #![allow(unused_variables)]
    #
    #fn main() {
    # struct X;
    # impl X { fn iter<T, F>(&self, _: F) where F: FnMut() -> T {} } let b = X;
    b.iter(|| {
        // Note lack of `;` (could also use an explicit `return`).
        (0..1000).fold(0, |old, new| old ^ new)
    });

    #}

Or, the other option is to call the generic `test::black_box` function, which is an opaque "black box" to the optimizer and so forces it to consider any argument as used.

    #![feature(test)]

    extern crate test;

    # fn main() {
    # struct X;
    # impl X { fn iter<T, F>(&self, _: F) where F: FnMut() -> T {} } let b = X;
    b.iter(|| {
        let n = test::black_box(1000);

        (0..n).fold(0, |a, b| a ^ b)
    })
    # }

Neither of these read or modify the value, and are very cheap for small values. Larger values can be passed indirectly to reduce overhead (e.g. `black_box(&huge_struct)`).

Performing either of the above changes gives the following benchmarking results

    running 1 test
    test bench_xor_1000_ints ... bench:       131 ns/iter (+/- 3)

    test result: ok. 0 passed; 0 failed; 0 ignored; 1 measured

However, the optimizer can still modify a testcase in an undesirable manner even when using either of the above.

# `thread_local`

The tracking issue for this feature is: #29594

# `trace_macros`

The tracking issue for this feature is: #29598

# `type_ascription`

The tracking issue for this feature is: #23416

# `unboxed_closures`

The tracking issue for this feature is: #29625

# `untagged_unions`

The tracking issue for this feature is: #32836

# `unwind_attributes`

The tracking issue for this feature is: None.

# `use_extern_macros`

The tracking issue for this feature is: #35896

# `windows_subsystem`

The tracking issue for this feature is: #37499