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.

The tracking issue for this feature is: None

Every rustc target defaults to some linker. For example, Linux targets default to gcc. In some cases, you may want to override the default; you can do that with the unstable CLI argument: -Z linker-flavor.

Here how you would use this flag to link a Rust binary for the thumbv7m-none-eabi using LLD instead of GCC.

$ xargo rustc --target thumbv7m-none-eabi -- \
    -C linker=ld.lld \
    -Z linker-flavor=ld \
    -Z print-link-args | tr ' ' '\n'
"ld.lld"
"-L"
"$SYSROOT/lib/rustlib/thumbv7m-none-eabi/lib"
"$PWD/target/thumbv7m-none-eabi/debug/deps/app-512e9dbf385f233c.0.o"
"-o"
"$PWD/target/thumbv7m-none-eabi/debug/deps/app-512e9dbf385f233c"
"--gc-sections"
"-L"
"$PWD/target/thumbv7m-none-eabi/debug/deps"
"-L"
"$PWD/target/debug/deps"
"-L"
"$SYSROOT/lib/rustlib/thumbv7m-none-eabi/lib"
"-Bstatic"
"$SYSROOT/lib/rustlib/thumbv7m-none-eabi/lib/libcore-e1ccb7dfb1cb9ebb.rlib"
"-Bdynamic"

Whereas the default is:

$ xargo rustc --target thumbv7m-none-eabi -- \
    -C link-arg=-nostartfiles \
    -Z print-link-args | tr ' ' '\n'
"arm-none-eabi-gcc"
"-L"
"$SYSROOT/lib/rustlib/thumbv7m-none-eabi/lib"
"$PWD/target/thumbv7m-none-eabi/debug/deps/app-961e39416baa38d9.0.o"
"-o"
"$PWD/target/thumbv7m-none-eabi/debug/deps/app-961e39416baa38d9"
"-Wl,--gc-sections"
"-nodefaultlibs"
"-L"
"$PWD/target/thumbv7m-none-eabi/debug/deps"
"-L"
"$PWD/target/debug/deps"
"-L"
"$SYSROOT/lib/rustlib/thumbv7m-none-eabi/lib"
"-Wl,-Bstatic"
"$SYSROOT/lib/rustlib/thumbv7m-none-eabi/lib/libcore-e1ccb7dfb1cb9ebb.rlib"
"-nostartfiles"
"-Wl,-Bdynamic"

The tracking issue for this feature is: #42524.

This feature allows the generation of code coverage reports.

Set the -Zprofile compiler flag in order to enable gcov profiling.

For example:

cargo new testgcov --bin
cd testgcov
export RUSTFLAGS="-Zprofile"
cargo build
cargo run

Once you've built and run your program, files with the gcno (after build) and gcda (after execution) extensions will be created. You can parse them with llvm-cov gcov or grcov.

The tracking issue for this feature is: #41555

The -Z remap-path-prefix-from, -Z remap-path-prefix-to commandline option pair allows to replace prefixes of any file paths the compiler emits in various places. This is useful for bringing debuginfo paths into a well-known form and for achieving reproducible builds independent of the directory the compiler was executed in. All paths emitted by the compiler are affected, including those in error messages.

In order to map all paths starting with /home/foo/my-project/src to /sources/my-project, one would invoke the compiler as follows:

rustc -Zremap-path-prefix-from="/home/foo/my-project/src" -Zremap-path-prefix-to="/sources/my-project"

Debuginfo for code from the file /home/foo/my-project/src/foo/mod.rs, for example, would then point debuggers to /sources/my-project/foo/mod.rs instead of the original file.

The options can be specified multiple times when multiple prefixes should be mapped:

rustc -Zremap-path-prefix-from="/home/foo/my-project/src" \
      -Zremap-path-prefix-to="/sources/my-project" \
      -Zremap-path-prefix-from="/home/foo/my-project/build-dir" \
      -Zremap-path-prefix-to="/stable-build-dir"

When the options are given multiple times, the nth -from will be matched up with the nth -to and they can appear anywhere on the commandline. Mappings specified later on the line will take precedence over earlier ones.

The tracking issue for this feature is: #38487

In the MSP430 architecture, interrupt handlers have a special calling convention. You can use the "msp430-interrupt" ABI to make the compiler apply the right calling convention to the interrupt handlers you define.

#![feature(abi_msp430_interrupt)]
#![no_std]

// Place the interrupt handler at the appropriate memory address
// (Alternatively, you can use `#[used]` and remove `pub` and `#[no_mangle]`)
#[link_section = "__interrupt_vector_10"]
#[no_mangle]
pub static TIM0_VECTOR: extern "msp430-interrupt" fn() = tim0;

// The interrupt handler
extern "msp430-interrupt" fn tim0() {
    // ..
}

$ msp430-elf-objdump -CD ./target/msp430/release/app
Disassembly of section __interrupt_vector_10:

0000fff2 <TIM0_VECTOR>:
    fff2:       00 c0           interrupt service routine at 0xc000

Disassembly of section .text:

0000c000 <int::tim0>:
    c000:       00 13           reti

The tracking issue for this feature is: #38788

When emitting PTX code, all vanilla Rust functions (fn) get translated to "device" functions. These functions are not callable from the host via the CUDA API so a crate with only device functions is not too useful!

OTOH, "global" functions can be called by the host; you can think of them as the real public API of your crate. To produce a global function use the "ptx-kernel" ABI.

#![feature(abi_ptx)]
#![no_std]

pub unsafe extern "ptx-kernel" fn global_function() {
    device_function();
}

pub fn device_function() {
    // ..
}

$ xargo rustc --target nvptx64-nvidia-cuda --release -- --emit=asm

$ cat $(find -name '*.s')
//
// Generated by LLVM NVPTX Back-End
//

.version 3.2
.target sm_20
.address_size 64

        // .globl       _ZN6kernel15global_function17h46111ebe6516b382E

.visible .entry _ZN6kernel15global_function17h46111ebe6516b382E()
{


        ret;
}

        // .globl       _ZN6kernel15device_function17hd6a0e4993bbf3f78E
.visible .func _ZN6kernel15device_function17hd6a0e4993bbf3f78E()
{


        ret;
}

The tracking issue for this feature is: #42202

The MSVC ABI on x86 Windows uses the thiscall calling convention for C++ instance methods by default; it is identical to the usual (C) calling convention on x86 Windows except that the first parameter of the method, the this pointer, is passed in the ECX register.

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

The tracking issue for this feature is: #40180

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));
}

This feature does not have a tracking issue, it is an unstable implementation detail of the global_allocator feature not intended for use outside the compiler.

The tracking issue for this feature is: #42219

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

The tracking issue for this feature is: #44874

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.

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() {}

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
# }
#}

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.

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() {}

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.

If you need more power and don't mind losing some of the niceties of asm!, check out global_asm.

The tracking issue for this feature is: #29661

The tracking issue for this feature is: #34981

At present, literals are only accepted as the value of a key-value pair in attributes. What's more, only string literals are accepted. This means that literals can only appear in forms of #[attr(name = "value")] or #[attr = "value"].

The attr_literals unstable feature allows other types of literals to be used in attributes. Here are some examples of attributes that can now be used with this feature enabled:

#[attr]
#[attr(true)]
#[attr(ident)]
#[attr(ident, 100, true, "true", ident = 100, ident = "hello", ident(100))]
#[attr(100)]
#[attr(enabled = true)]
#[enabled(true)]
#[attr("hello")]
#[repr(C, align = 4)]
#[repr(C, align(4))]

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!()
    }
}

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;
}

The tracking issue for this feature is: #31436

The catch_expr feature adds support for a catch expression. The catch expression creates a new scope one can use the ? operator in.

# #![allow(unused_variables)]
#![feature(catch_expr)]

#fn main() {
use std::num::ParseIntError;

let result: Result<i32, ParseIntError> = do catch {
    Ok("1".parse::<i32>()?
        + "2".parse::<i32>()?
        + "3".parse::<i32>()?)
};
assert_eq!(result, Ok(6));

let result: Result<i32, ParseIntError> = do catch {
    Ok("1".parse::<i32>()?
        + "foo".parse::<i32>()?
        + "3".parse::<i32>()?)
};
assert!(result.is_err());
#}

The tracking issue for this feature is: #29717

The tracking issue for this feature is: #32976

The tracking issue for this feature is: #29594

The tracking issue for this feature is: #29718

The tracking issue for this feature is: #44490

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is: #29599

The concat_idents feature adds a macro for concatenating multiple identifiers into one identifier.

#![feature(concat_idents)]

fn main() {
    fn foobar() -> u32 { 23 }
    let f = concat_idents!(foo, bar);
    assert_eq!(f(), 23);
}

The tracking issue for this feature is: #34511

The conservative_impl_trait feature allows a conservative form of abstract return types.

Abstract return types allow a function to hide a concrete return type behind a trait interface similar to trait objects, while still generating the same statically dispatched code as with concrete types.

#![feature(conservative_impl_trait)]

fn even_iter() -> impl Iterator<Item=u32> {
    (0..).map(|n| n * 2)
}

fn main() {
    let first_four_even_numbers = even_iter().take(4).collect::<Vec<_>>();
    assert_eq!(first_four_even_numbers, vec![0, 2, 4, 6]);
}

In today's Rust, you can write function signatures like:

fn consume_iter_static<I: Iterator<Item=u8>>(iter: I) { }

fn consume_iter_dynamic(iter: Box<Iterator<Item=u8>>) { }

In both cases, the function does not depend on the exact type of the argument. The type held is "abstract", and is assumed only to satisfy a trait bound.

• In the _static version using generics, each use of the function is specialized to a concrete, statically-known type, giving static dispatch, inline layout, and other performance wins.
• In the _dynamic version using trait objects, the concrete argument type is only known at runtime using a vtable.

On the other hand, while you can write:

fn produce_iter_dynamic() -> Box<Iterator<Item=u8>> { }

...but you cannot write something like:

fn produce_iter_static() -> Iterator<Item=u8> { }

That is, in today's Rust, abstract return types can only be written using trait objects, which can be a significant performance penalty. This RFC proposes "unboxed abstract types" as a way of achieving signatures like produce_iter_static. Like generics, unboxed abstract types guarantee static dispatch and inline data layout.

The tracking issue for this feature is: #24111

The const_fn feature allows marking free functions and inherent methods as const, enabling them to be called in constants contexts, with constant arguments.

#![feature(const_fn)]

const fn double(x: i32) -> i32 {
    x * 2
}

const FIVE: i32 = 5;
const TEN: i32 = double(FIVE);

fn main() {
    assert_eq!(5, FIVE);
    assert_eq!(10, TEN);
}

The tracking issue for this feature is: #29947

The const_indexing feature allows the constant evaluation of index operations on constant arrays and repeat expressions.

# #![allow(unused_variables)]
#![feature(const_indexing)]

#fn main() {
const ARR: [usize; 5] = [1, 2, 3, 4, 5];
const ARR2: [usize; ARR[1]] = [42, 99];
#}

The tracking issue for this feature is: #44490

The tracking issue for this feature is: #44660

The crate_in_paths feature allows to explicitly refer to the crate root in absolute paths using keyword crate.

crate can be used only in absolute paths, i.e. either in ::crate::a::b::c form or in use items where the starting :: is added implicitly.
Paths like crate::a::b::c are not accepted currently.

This feature is required in feature(extern_absolute_paths) mode to refer to any absolute path in the local crate (absolute paths refer to extern crates by default in that mode), but can be used without feature(extern_absolute_paths) as well.

#![feature(crate_in_paths)]

// Imports, `::` is added implicitly
use crate::m::f;
use crate as root;

mod m {
    pub fn f() -> u8 { 1 }
    pub fn g() -> u8 { 2 }
    pub fn h() -> u8 { 3 }

    // OK, visibilities implicitly add starting `::` as well, like imports
    pub(in crate::m) struct S;
}

mod n
{
    use crate::m::f;
    use crate as root;
    pub fn check() {
        assert_eq!(f(), 1);
        // `::` is required in non-import paths
        assert_eq!(::crate::m::g(), 2);
        assert_eq!(root::m::h(), 3);
    }
}

fn main() {
    assert_eq!(f(), 1);
    assert_eq!(::crate::m::g(), 2);
    assert_eq!(root::m::h(), 3);
    n::check();
}

The tracking issue for this feature is: #45388

The crate_visibility_modifier feature allows the crate keyword to be used as a visibility modifier synonymous to pub(crate), indicating that a type (function, &c.) is to be visible to the entire enclosing crate, but not to other crates.

# #![allow(unused_variables)]
#![feature(crate_visibility_modifier)]

#fn main() {
crate struct Foo {
    bar: usize,
}
#}

The tracking issue for this feature is: #29642

The tracking issue for this feature is: #29644

The tracking issue for this feature is: #39412

The tracking issue for this feature is: #27336

The tracking issue for this feature is: #43781

The doc_cfg feature allows an API be documented as only available in some specific platforms. This attribute has two effects:

1. In the annotated item's documentation, there will be a message saying "This is supported on (platform) only".

2. The item's doc-tests will only run on the specific platform.

This feature was introduced as part of PR #43348 to allow the platform-specific parts of the standard library be documented.

# #![allow(unused_variables)]
#![feature(doc_cfg)]

#fn main() {
#[cfg(any(windows, feature = "documentation"))]
#[doc(cfg(windows))]
/// The application's icon in the notification area (a.k.a. system tray).
///
/// # Examples
///
/// ```no_run
/// extern crate my_awesome_ui_library;
/// use my_awesome_ui_library::current_app;
/// use my_awesome_ui_library::windows::notification;
///
/// let icon = current_app().get::<notification::Icon>();
/// icon.show();
/// icon.show_message("Hello");
/// ```
pub struct Icon {
    // ...
}
#}

The tracking issue for this feature is: #44027

The doc_masked feature allows a crate to exclude types from a given crate from appearing in lists of trait implementations. The specifics of the feature are as follows:

1. When rustdoc encounters an extern crate statement annotated with a #[doc(masked)] attribute, it marks the crate as being masked.

2. When listing traits a given type implements, rustdoc ensures that traits from masked crates are not emitted into the documentation.

3. When listing types that implement a given trait, rustdoc ensures that types from masked crates are not emitted into the documentation.

This feature was introduced in PR #44026 to ensure that compiler-internal and implementation-specific types and traits were not included in the standard library's documentation. Such types would introduce broken links into the documentation.

The tracking issue for this feature is: #45040

The doc_spotlight feature allows the use of the spotlight parameter to the #[doc] attribute, to "spotlight" a specific trait on the return values of functions. Adding a #[doc(spotlight)] attribute to a trait definition will make rustdoc print extra information for functions which return a type that implements that trait. This attribute is applied to the Iterator, io::Read, and io::Write traits in the standard library.

You can do this on your own traits, like this:

#![feature(doc_spotlight)]

#[doc(spotlight)]
pub trait MyTrait {}

pub struct MyStruct;
impl MyTrait for MyStruct {}

/// The docs for this function will have an extra line about `MyStruct` implementing `MyTrait`,
/// without having to write that yourself!
pub fn my_fn() -> MyStruct { MyStruct }

This feature was originally implemented in PR #45039.

The tracking issue for this feature is: #28237

The tracking issue for this feature is: #34761

The tracking issue for this feature is: #28498

The tracking issue for this feature is: #44662

The tracking issue for this feature is: #37854

The tracking issue for this feature is: #44660

The extern_absolute_paths feature enables mode allowing to refer to names from other crates "inline", without introducing extern crate items, using absolute paths like ::my_crate::a::b.

::my_crate::a::b will resolve to path a::b in crate my_crate.

feature(crate_in_paths) can be used in feature(extern_absolute_paths) mode for referring to absolute paths in the local crate (::crate::a::b).

feature(extern_in_paths) provides the same effect by using keyword extern to refer to paths from other crates (extern::my_crate::a::b).

#![feature(extern_absolute_paths)]

// Suppose we have a dependency crate `xcrate` available through `Cargo.toml`, or `--extern`
// options, or standard Rust distribution, or some other means.

use xcrate::Z;

fn f() {
    use xcrate;
    use xcrate as ycrate;
    let s = xcrate::S;
    assert_eq!(format!("{:?}", s), "S");
    let z = ycrate::Z;
    assert_eq!(format!("{:?}", z), "Z");
}

fn main() {
    let s = ::xcrate::S;
    assert_eq!(format!("{:?}", s), "S");
    let z = Z;
    assert_eq!(format!("{:?}", z), "Z");
}

The tracking issue for this feature is: #44660

The extern_in_paths feature allows to refer to names from other crates "inline", without introducing extern crate items, using keyword extern.

For example, extern::my_crat::a::b will resolve to path a::b in crate my_crate.

feature(extern_absolute_paths) mode provides the same effect by resolving absolute paths like ::my_crate::a::b to paths from extern crates by default.

#![feature(extern_in_paths)]

// Suppose we have a dependency crate `xcrate` available through `Cargo.toml`, or `--extern`
// options, or standard Rust distribution, or some other means.

use extern::xcrate::Z;

fn f() {
    use extern::xcrate;
    use extern::xcrate as ycrate;
    let s = xcrate::S;
    assert_eq!(format!("{:?}", s), "S");
    let z = ycrate::Z;
    assert_eq!(format!("{:?}", z), "Z");
}

fn main() {
    let s = extern::xcrate::S;
    assert_eq!(format!("{:?}", s), "S");
    let z = Z;
    assert_eq!(format!("{:?}", z), "Z");
}

The tracking issue for this feature is: #43467

The tracking issue for this feature is: #44732

The external_doc feature allows the use of the include parameter to the #[doc] attribute, to include external files in documentation. Use the attribute in place of, or in addition to, regular doc comments and #[doc] attributes, and rustdoc will load the given file when it renders documentation for your crate.

With the following files in the same directory:

external-doc.md:

# My Awesome Type

This is the documentation for this spectacular type.

lib.rs:

#![feature(external_doc)]

#[doc(include = "external-doc.md")]
pub struct MyAwesomeType;

rustdoc will load the file external-doc.md and use it as the documentation for the MyAwesomeType struct.

When locating files, rustdoc will base paths in the src/ directory, as if they were alongside the lib.rs for your crate. So if you want a docs/ folder to live alongside the src/ directory, start your paths with ../docs/ for rustdoc to properly find the file.

This feature was proposed in RFC #1990 and initially implemented in PR #44781.

The tracking issue for this feature is #43302.

The fn_must_use feature allows functions and methods to be annotated with #[must_use], indicating that the unused_must_use lint should require their return values to be used (similarly to how types annotated with must_use, most notably Result, are linted if not used).

#![feature(fn_must_use)]

#[must_use]
fn double(x: i32) -> i32 {
    2 * x
}

fn main() {
    double(4); // warning: unused return value of `double` which must be used

    let _ = double(4); // (no warning)
}

The tracking issue for this feature is: #29635

The tracking issue for this feature is: #43122

The generators feature gate in Rust allows you to define generator or coroutine literals. A generator is a "resumable function" that syntactically resembles a closure but compiles to much different semantics in the compiler itself. The primary feature of a generator is that it can be suspended during execution to be resumed at a later date. Generators use the yield keyword to "return", and then the caller can resume a generator to resume execution just after the yield keyword.

Generators are an extra-unstable feature in the compiler right now. Added in RFC 2033 they're mostly intended right now as a information/constraint gathering phase. The intent is that experimentation can happen on the nightly compiler before actual stabilization. A further RFC will be required to stabilize generators/coroutines and will likely contain at least a few small tweaks to the overall design.

A syntactical example of a generator is:

#![feature(generators, generator_trait)]

use std::ops::{Generator, GeneratorState};

fn main() {
    let mut generator = || {
        yield 1;
        return "foo"
    };

    match generator.resume() {
        GeneratorState::Yielded(1) => {}
        _ => panic!("unexpected value from resume"),
    }
    match generator.resume() {
        GeneratorState::Complete("foo") => {}
        _ => panic!("unexpected value from resume"),
    }
}

Generators are closure-like literals which can contain a yield statement. The yield statement takes an optional expression of a value to yield out of the generator. All generator literals implement the Generator trait in the std::ops module. The Generator trait has one main method, resume, which resumes execution of the generator at the previous suspension point.

An example of the control flow of generators is that the following example prints all numbers in order:

#![feature(generators, generator_trait)]

use std::ops::Generator;

fn main() {
    let mut generator = || {
        println!("2");
        yield;
        println!("4");
    };

    println!("1");
    generator.resume();
    println!("3");
    generator.resume();
    println!("5");
}

At this time the main intended use case of generators is an implementation primitive for async/await syntax, but generators will likely be extended to ergonomic implementations of iterators and other primitives in the future. Feedback on the design and usage is always appreciated!

The Generator trait in std::ops currently looks like:

# #![feature(generator_trait)]
# use std::ops::GeneratorState;

pub trait Generator {
    type Yield;
    type Return;
    fn resume(&mut self) -> GeneratorState<Self::Yield, Self::Return>;
}

The Generator::Yield type is the type of values that can be yielded with the yield statement. The Generator::Return type is the returned type of the generator. This is typically the last expression in a generator's definition or any value passed to return in a generator. The resume function is the entry point for executing the Generator itself.

The return value of resume, GeneratorState, looks like:

pub enum GeneratorState<Y, R> {
    Yielded(Y),
    Complete(R),
}

The Yielded variant indicates that the generator can later be resumed. This corresponds to a yield point in a generator. The Complete variant indicates that the generator is complete and cannot be resumed again. Calling resume after a generator has returned Complete will likely result in a panic of the program.

The closure-like syntax for generators alludes to the fact that they also have closure-like semantics. Namely:

• When created, a generator executes no code. A closure literal does not actually execute any of the closure's code on construction, and similarly a generator literal does not execute any code inside the generator when constructed.

• Generators can capture outer variables by reference or by move, and this can be tweaked with the move keyword at the beginning of the closure. Like closures all generators will have an implicit environment which is inferred by the compiler. Outer variables can be moved into a generator for use as the generator progresses.

• Generator literals produce a value with a unique type which implements the std::ops::Generator trait. This allows actual execution of the generator through the Generator::resume method as well as also naming it in return types and such.

• Traits like Send and Sync are automatically implemented for a Generator depending on the captured variables of the environment. Unlike closures, generators also depend on variables live across suspension points. This means that although the ambient environment may be Send or Sync, the generator itself may not be due to internal variables live across yield points being not-Send or not-Sync. Note that generators, like closures, do not implement traits like Copy or Clone automatically.

• Whenever a generator is dropped it will drop all captured environment variables.

Note that unlike closures generators at this time cannot take any arguments. That is, generators must always look like || { ... }. This restriction may be lifted at a future date, the design is ongoing!

In the compiler, generators are currently compiled as state machines. Each yield expression will correspond to a different state that stores all live variables over that suspension point. Resumption of a generator will dispatch on the current state and then execute internally until a yield is reached, at which point all state is saved off in the generator and a value is returned.

Let's take a look at an example to see what's going on here:

#![feature(generators, generator_trait)]

use std::ops::Generator;

fn main() {
    let ret = "foo";
    let mut generator = move || {
        yield 1;
        return ret
    };

    generator.resume();
    generator.resume();
}

This generator literal will compile down to something similar to:

#![feature(generators, generator_trait)]

use std::ops::{Generator, GeneratorState};

fn main() {
    let ret = "foo";
    let mut generator = {
        enum __Generator {
            Start(&'static str),
            Yield1(&'static str),
            Done,
        }

        impl Generator for __Generator {
            type Yield = i32;
            type Return = &'static str;

            fn resume(&mut self) -> GeneratorState<i32, &'static str> {
                use std::mem;
                match mem::replace(self, __Generator::Done) {
                    __Generator::Start(s) => {
                        *self = __Generator::Yield1(s);
                        GeneratorState::Yielded(1)
                    }

                    __Generator::Yield1(s) => {
                        *self = __Generator::Done;
                        GeneratorState::Complete(s)
                    }

                    __Generator::Done => {
                        panic!("generator resumed after completion")
                    }
                }
            }
        }

        __Generator::Start(ret)
    };

    generator.resume();
    generator.resume();
}

Notably here we can see that the compiler is generating a fresh type, __Generator in this case. This type has a number of states (represented here as an enum) corresponding to each of the conceptual states of the generator. At the beginning we're closing over our outer variable foo and then that variable is also live over the yield point, so it's stored in both states.

When the generator starts it'll immediately yield 1, but it saves off its state just before it does so indicating that it has reached the yield point. Upon resuming again we'll execute the return ret which returns the Complete state.

Here we can also note that the Done state, if resumed, panics immediately as it's invalid to resume a completed generator. It's also worth noting that this is just a rough desugaring, not a normative specification for what the compiler does.

The tracking issue for this feature is: #44265

The tracking issue for this feature is: #34761

The tracking issue for this feature is: #27389

Rust programs may need to change the allocator that they're running with from time to time. This use case is distinct from an allocator-per-collection (e.g. a Vec with a custom allocator) and instead is more related to changing the global default allocator, e.g. what Vec<T> uses by default.

Currently Rust programs don't have a specified global allocator. The compiler may link to a version of jemalloc on some platforms, but this is not guaranteed. Libraries, however, like cdylibs and staticlibs are guaranteed to use the "system allocator" which means something like malloc on Unixes and HeapAlloc on Windows.

The #[global_allocator] attribute, however, allows configuring this choice. You can use this to implement a completely custom global allocator to route all default allocation requests to a custom object. Defined in RFC 1974 usage looks like:

#![feature(global_allocator, allocator_api, heap_api)]

use std::heap::{Alloc, System, Layout, AllocErr};

struct MyAllocator;

unsafe impl<'a> Alloc for &'a MyAllocator {
    unsafe fn alloc(&mut self, layout: Layout) -> Result<*mut u8, AllocErr> {
        System.alloc(layout)
    }

    unsafe fn dealloc(&mut self, ptr: *mut u8, layout: Layout) {
        System.dealloc(ptr, layout)
    }
}

#[global_allocator]
static GLOBAL: MyAllocator = MyAllocator;

fn main() {
    // This `Vec` will allocate memory through `GLOBAL` above
    let mut v = Vec::new();
    v.push(1);
}

And that's it! The #[global_allocator] attribute is applied to a static which implements the Alloc trait in the std::heap module. Note, though, that the implementation is defined for &MyAllocator, not just MyAllocator. You may wish, however, to also provide Alloc for MyAllocator for other use cases.

A crate can only have one instance of #[global_allocator] and this instance may be loaded through a dependency. For example #[global_allocator] above could have been placed in one of the dependencies loaded through extern crate.

Note that Alloc itself is an unsafe trait, with much documentation on the trait itself about usage and for implementors. Extra care should be taken when implementing a global allocator as well as the allocator may be called from many portions of the standard library, such as the panicking routine. As a result it is highly recommended to not panic during allocation and work in as many situations with as few dependencies as possible as well.

The tracking issue for this feature is: #35119

The global_asm! macro allows the programmer to write arbitrary assembly outside the scope of a function body, passing it through rustc and llvm to the assembler. The macro is a no-frills interface to LLVM's concept of module-level inline assembly. That is, all caveats applicable to LLVM's module-level inline assembly apply to global_asm!.

global_asm! fills a role not currently satisfied by either asm! or #[naked] functions. The programmer has all features of the assembler at their disposal. The linker will expect to resolve any symbols defined in the inline assembly, modulo any symbols marked as external. It also means syntax for directives and assembly follow the conventions of the assembler in your toolchain.

A simple usage looks like this:

# #![feature(global_asm)]
# you also need relevant target_arch cfgs
global_asm!(include_str!("something_neato.s"));

And a more complicated usage looks like this:

# #![feature(global_asm)]
# #![cfg(any(target_arch = "x86", target_arch = "x86_64"))]

pub mod sally {
    global_asm!(r#"
        .global foo
      foo:
        jmp baz
    "#);

    #[no_mangle]
    pub unsafe extern "C" fn baz() {}
}

// the symbols `foo` and `bar` are global, no matter where
// `global_asm!` was used.
extern "C" {
    fn foo();
    fn bar();
}

pub mod harry {
    global_asm!(r#"
        .global bar
      bar:
        jmp quux
    "#);

    #[no_mangle]
    pub unsafe extern "C" fn quux() {}
}

You may use global_asm! multiple times, anywhere in your crate, in whatever way suits you. The effect is as if you concatenated all usages and placed the larger, single usage in the crate root.

If you don't need quite as much power and flexibility as global_asm! provides, and you don't mind restricting your inline assembly to fn bodies only, you might try the asm feature instead.

The tracking issue for this feature is: #35118

The i128_type feature adds support for 128 bit signed and unsigned integer types.

#![feature(i128_type)]

fn main() {
    assert_eq!(1u128 + 1u128, 2u128);
    assert_eq!(u128::min_value(), 0);
    assert_eq!(u128::max_value(), 340282366920938463463374607431768211455);

    assert_eq!(1i128 - 2i128, -1i128);
    assert_eq!(i128::min_value(), -170141183460469231731687303715884105728);
    assert_eq!(i128::max_value(), 170141183460469231731687303715884105727);
}

The tracking issue for this feature is: #44524

The tracking issue for this feature is: #28237

To get a range that goes from 0 to 10 and includes the value 10, you can write 0..=10:

#![feature(inclusive_range_syntax)]

fn main() {
    for i in 0..=10 {
        println!("{}", i);
    }
}

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.

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 = "box_free"]
unsafe fn box_free<T: ?Sized>(ptr: *mut T) {
    libc::free(ptr as *mut libc::c_void)
}

#[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 box_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.

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.

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,
                               _column: 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,
                               _column: u32) -> ! {
    unsafe { intrinsics::abort() }
}

In many cases, you may need to manually link to the compiler_builtins crate when building a no_std binary. You may observe this via linker error messages such as "undefined reference to `__rust_probestack'". Using this crate also requires enabling the library feature compiler_builtins_lib. You can read more about this here.

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.

This is a list of all language items in Rust along with where they are located in the source code.

• Primitives
  • i8: libcore/num/mod.rs
  • i16: libcore/num/mod.rs
  • i32: libcore/num/mod.rs
  • i64: libcore/num/mod.rs
  • i128: libcore/num/mod.rs
  • isize: libcore/num/mod.rs
  • u8: libcore/num/mod.rs
  • u16: libcore/num/mod.rs
  • u32: libcore/num/mod.rs
  • u64: libcore/num/mod.rs
  • u128: libcore/num/mod.rs
  • usize: libcore/num/mod.rs
  • f32: libstd/f32.rs
  • f64: libstd/f64.rs
  • char: libstd_unicode/char.rs
  • slice: liballoc/slice.rs
  • str: liballoc/str.rs
  • const_ptr: libcore/ptr.rs
  • mut_ptr: libcore/ptr.rs
  • unsafe_cell: libcore/cell.rs
• Runtime
  • start: libstd/rt.rs
  • eh_personality: libpanic_unwind/emcc.rs (EMCC)
  • eh_personality: libpanic_unwind/seh64_gnu.rs (SEH64 GNU)
  • eh_personality: libpanic_unwind/seh.rs (SEH)
  • eh_unwind_resume: libpanic_unwind/seh64_gnu.rs (SEH64 GNU)
  • eh_unwind_resume: libpanic_unwind/gcc.rs (GCC)
  • msvc_try_filter: libpanic_unwind/seh.rs (SEH)
  • panic: libcore/panicking.rs
  • panic_bounds_check: libcore/panicking.rs
  • panic_fmt: libcore/panicking.rs
  • panic_fmt: libstd/panicking.rs
• Allocations
  • owned_box: liballoc/boxed.rs
  • exchange_malloc: liballoc/heap.rs
  • box_free: liballoc/heap.rs
• Operands
  • not: libcore/ops/bit.rs
  • bitand: libcore/ops/bit.rs
  • bitor: libcore/ops/bit.rs
  • bitxor: libcore/ops/bit.rs
  • shl: libcore/ops/bit.rs
  • shr: libcore/ops/bit.rs
  • bitand_assign: libcore/ops/bit.rs
  • bitor_assign: libcore/ops/bit.rs
  • bitxor_assign: libcore/ops/bit.rs
  • shl_assign: libcore/ops/bit.rs
  • shr_assign: libcore/ops/bit.rs
  • deref: libcore/ops/deref.rs
  • deref_mut: libcore/ops/deref.rs
  • index: libcore/ops/index.rs
  • index_mut: libcore/ops/index.rs
  • add: libcore/ops/arith.rs
  • sub: libcore/ops/arith.rs
  • mul: libcore/ops/arith.rs
  • div: libcore/ops/arith.rs
  • rem: libcore/ops/arith.rs
  • neg: libcore/ops/arith.rs
  • add_assign: libcore/ops/arith.rs
  • sub_assign: libcore/ops/arith.rs
  • mul_assign: libcore/ops/arith.rs
  • div_assign: libcore/ops/arith.rs
  • rem_assign: libcore/ops/arith.rs
  • eq: libcore/cmp.rs
  • ord: libcore/cmp.rs
• Functions
  • fn: libcore/ops/function.rs
  • fn_mut: libcore/ops/function.rs
  • fn_once: libcore/ops/function.rs
  • generator_state: libcore/ops/generator.rs
  • generator: libcore/ops/generator.rs
• Other
  • coerce_unsized: libcore/ops/unsize.rs
  • drop: libcore/ops/drop.rs
  • drop_in_place: libcore/ptr.rs
  • clone: libcore/clone.rs
  • copy: libcore/marker.rs
  • send: libcore/marker.rs
  • sized: libcore/marker.rs
  • unsize: libcore/marker.rs
  • sync: libcore/marker.rs
  • phantom_data: libcore/marker.rs
  • freeze: libcore/marker.rs
  • debug_trait: libcore/fmt/mod.rs
  • non_zero: libcore/nonzero.rs

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.

The tracking issue for this feature is: #37406

The tracking issue for this feature is: #29602

The tracking issue for this feature is: #29603

The tracking issue for this feature is: #29598

The tracking issue for this feature is: TODO(mark-i-m)

With this feature gate enabled, one can use ? as a Kleene operator meaning "0 or 1 repetitions" in a macro definition. Previously only + and * were allowed.

For example:

# #![allow(unused_variables)]
#fn main() {
macro_rules! foo {
    (something $(,)?) // `?` indicates `,` is "optional"...
        => {}
}
#}

The tracking issue for this feature is: #46895

With this feature gate enabled, the list of fragment specifiers gains one more entry:

• lifetime: a lifetime. Examples: 'static, 'a.

A lifetime variable may be followed by anything.

The tracking issue for this feature is: #29638

The tracking issue for this feature is: #41022

With this feature gate enabled, the list of fragment specifiers gains one more entry:

• vis: a visibility qualifier. Examples: nothing (default visibility); pub; pub(crate).

A vis variable may be followed by a comma, ident, type, or path.

The tracking issue for this feature is: #29634

The tracking issue for this feature is: #42640

Match default bindings (also called "default binding modes in match") improves ergonomics for pattern-matching on references by introducing automatic dereferencing (and a corresponding shift in binding modes) for large classes of patterns that would otherwise not compile.

For example, under match default bindings,

#![feature(match_default_bindings)]

fn main() {
    let x: &Option<_> = &Some(0);

    match x {
        Some(y) => {
            println!("y={}", *y);
        },
        None => {},
    }
}

compiles and is equivalent to either of the below:

fn main() {
    let x: &Option<_> = &Some(0);

    match *x {
        Some(ref y) => {
            println!("y={}", *y);
        },
        None => {},
    }
}

or

fn main() {
    let x: &Option<_> = &Some(0);

    match x {
        &Some(ref y) => {
            println!("y={}", *y);
        },
        &None => {},
    }
}

The tracking issue for this feature is: #32408

The tracking issue for this feature is: #27389

The tracking issue for this feature is: #32837

The tracking issue for this feature is: #34511

The tracking issue for this feature is: #35121

The tracking issue for this feature is: #44928

The tracking issue for this feature is: #29639

The tracking issue for this feature is: #29721

The tracking issue for this feature is: #28979

The non_ascii_idents feature adds support for non-ASCII identifiers.

# #![allow(unused_variables)]
#![feature(non_ascii_idents)]

#fn main() {
const ε: f64 = 0.00001f64;
const Π: f64 = 3.14f64;
#}

^Lexer:
IDENTIFIER :
      XID_start XID_continue^*
   | _ XID_continue⁺

An identifier is any nonempty Unicode string of the following form:

Either

• The first character has property XID_start
• The remaining characters have property XID_continue

Or

• The first character is _
• The identifier is more than one character, _ alone is not an identifier
• The remaining characters have property XID_continue

that does not occur in the set of strict keywords.

Note: XID_start and XID_continue as character properties cover the character ranges used to form the more familiar C and Java language-family identifiers.

The tracking issue for this feature is: #44109

The non_exhaustive gate allows you to use the #[non_exhaustive] attribute on structs and enums. When applied within a crate, users of the crate will need to use the _ pattern when matching enums and use the .. pattern when matching structs. Structs marked as non_exhaustive will not be able to be created normally outside of the defining crate. This is demonstrated below:

use std::error::Error as StdError;

#[non_exhaustive]
pub enum Error {
    Message(String),
    Other,
}
impl StdError for Error {
    fn description(&self) -> &str {
        // This will not error, despite being marked as non_exhaustive, as this
        // enum is defined within the current crate, it can be matched
        // exhaustively.
        match *self {
            Message(ref s) => s,
            Other => "other or unknown error",
        }
    }
}

use mycrate::Error;

// This will not error as the non_exhaustive Error enum has been matched with
// a wildcard.
match error {
    Message(ref s) => ...,
    Other => ...,
    _ => ...,
}

#[non_exhaustive]
pub struct Config {
    pub window_width: u16,
    pub window_height: u16,
}

// We can create structs as normal within the defining crate when marked as
// non_exhaustive.
let config = Config { window_width: 640, window_height: 480 };

// We can match structs exhaustively when within the defining crate.
if let Ok(Config { window_width, window_height }) = load_config() {
    // ...
}

use mycrate::Config;

// We cannot create a struct like normal if it has been marked as
// non_exhaustive.
let config = Config { window_width: 640, window_height: 480 };
// By adding the `..` we can match the config as below outside of the crate
// when marked non_exhaustive.
let &Config { window_width, window_height, .. } = config;

The tracking issue for this feature is: #44660

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

The tracking issue for this feature is: #29628

The on_unimplemented feature provides the #[rustc_on_unimplemented] attribute, which allows trait definitions to add specialized notes to error messages when an implementation was expected but not found.

For example:

#![feature(on_unimplemented)]

#[rustc_on_unimplemented="an iterator over elements of type `{A}` \
    cannot be built from a collection of type `{Self}`"]
trait MyIterator<A> {
    fn next(&mut self) -> A;
}

fn iterate_chars<I: MyIterator<char>>(i: I) {
    // ...
}

fn main() {
    iterate_chars(&[1, 2, 3][..]);
}

When the user compiles this, they will see the following;

error[E0277]: the trait bound `&[{integer}]: MyIterator<char>` is not satisfied
  --> <anon>:14:5
   |
14 |     iterate_chars(&[1, 2, 3][..]);
   |     ^^^^^^^^^^^^^ an iterator over elements of type `char` cannot be built from a collection of type `&[{integer}]`
   |
   = help: the trait `MyIterator<char>` is not implemented for `&[{integer}]`
   = note: required by `iterate_chars`

error: aborting due to previous error

The tracking issue for this feature is #13231

The optin_builtin_traits feature gate allows you to define auto traits.

Auto traits, like Send or Sync in the standard library, are marker traits that are automatically implemented for every type, unless the type, or a type it contains, has explicitly opted out via a negative impl.

impl !Type for Trait

Example:

#![feature(optin_builtin_traits)]

auto trait Valid {}

struct True;
struct False;

impl !Valid for False {}

struct MaybeValid<T>(T);

fn must_be_valid<T: Valid>(_t: T) { }

fn main() {
    // works
    must_be_valid( MaybeValid(True) );
                
    // compiler error - trait bound not satisfied
    // must_be_valid( MaybeValid(False) );
}

The tracking issue for this feature is: #29864

The tracking issue for this feature is: #32837

The tracking issue for this feature is: #27779

The tracking issue for this feature is: #27731

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.

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.

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 Spans 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.

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 Lints, 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.

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.

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

The tracking issue for this feature is: #38356

This feature flag guards the new procedural macro features as laid out by RFC 1566, which alongside the now-stable custom derives, provide stabilizable alternatives to the compiler plugin API (which requires the use of perma-unstable internal APIs) for programmatically modifying Rust code at compile-time.

The two new procedural macro kinds are:

• Function-like procedural macros which are invoked like regular declarative macros, and:

• Attribute-like procedural macros which can be applied to any item which built-in attributes can be applied to, and which can take arguments in their invocation as well.

Additionally, this feature flag implicitly enables the use_extern_macros feature, which allows macros to be imported like any other item with use statements, as compared to applying #[macro_use] to an extern crate declaration. It is important to note that procedural macros may only be imported in this manner, and will throw an error otherwise.

You must declare the proc_macro feature in both the crate declaring these new procedural macro kinds as well as in any crates that use them.

As with custom derives, procedural macros may only be declared in crates of the proc-macro type, and must be public functions. No other public items may be declared in proc-macro crates, but private items are fine.

To declare your crate as a proc-macro crate, simply add:

[lib]
proc-macro = true

to your Cargo.toml.

Unlike custom derives, however, the name of the function implementing the procedural macro is used directly as the procedural macro's name, so choose carefully.

Additionally, both new kinds of procedural macros return a TokenStream which wholly replaces the original invocation and its input.

As referenced above, the new procedural macros are not meant to be imported via #[macro_use] and will throw an error if they are. Instead, they are meant to be imported like any other item in Rust, with use statements:

#![feature(proc_macro)]

// Where `my_proc_macros` is some crate of type `proc_macro`
extern crate my_proc_macros;

// And declares a `#[proc_macro] pub fn my_bang_macro()` at its root.
use my_proc_macros::my_bang_macro;

fn main() {
    println!("{}", my_bang_macro!());
}

Any panics in a procedural macro implementation will be caught by the compiler and turned into an error message pointing to the problematic invocation. Thus, it is important to make your panic messages as informative as possible: use Option::expect instead of Option::unwrap and Result::expect instead of Result::unwrap, and inform the user of the error condition as unambiguously as you can.

The proc_macro::TokenStream type is hardcoded into the signatures of procedural macro functions for both input and output. It is a wrapper around the compiler's internal representation for a given chunk of Rust code.

These are procedural macros that are invoked like regular declarative macros. They are declared as public functions in crates of the proc_macro type and using the #[proc_macro] attribute. The name of the declared function becomes the name of the macro as it is to be imported and used. The function must be of the kind fn(TokenStream) -> TokenStream where the sole argument is the input to the macro and the return type is the macro's output.

This kind of macro can expand to anything that is valid for the context it is invoked in, including expressions and statements, as well as items.

Note: invocations of this kind of macro require a wrapping [], {} or () like regular macros, but these do not appear in the input, only the tokens between them. The tokens between the braces do not need to be valid Rust syntax.

my_macro_crate/src/lib.rs

#![feature(proc_macro)]

// This is always necessary to get the `TokenStream` typedef.
extern crate proc_macro;

use proc_macro::TokenStream;

#[proc_macro]
pub fn say_hello(_input: TokenStream) -> TokenStream {
    // This macro will accept any input because it ignores it. 
    // To enforce correctness in macros which don't take input,
    // you may want to add `assert!(_input.to_string().is_empty());`.
    "println!(\"Hello, world!\")".parse().unwrap()
}

my_macro_user/Cargo.toml

[dependencies]
my_macro_crate = { path = "<relative path to my_macro_crate>" }

my_macro_user/src/lib.rs

#![feature(proc_macro)]

extern crate my_macro_crate;

use my_macro_crate::say_hello;

fn main() {
    say_hello!();
}

As expected, this prints Hello, world!.

These are arguably the most powerful flavor of procedural macro as they can be applied anywhere attributes are allowed.

They are declared as public functions in crates of the proc-macro type, using the #[proc_macro_attribute] attribute. The name of the function becomes the name of the attribute as it is to be imported and used. The function must be of the kind fn(TokenStream, TokenStream) -> TokenStream where:

The first argument represents any metadata for the attribute (see the reference chapter on attributes). Only the metadata itself will appear in this argument, for example:

• #[my_macro] will get an empty string.
• #[my_macro = "string"] will get = "string".
• #[my_macro(ident)] will get (ident).
• etc.

The second argument is the item that the attribute is applied to. It can be a function, a type definition, an impl block, an extern block, or a module—attribute invocations can take the inner form (#![my_attr]) or outer form (#[my_attr]).

The return type is the output of the macro which wholly replaces the item it was applied to. Thus, if your intention is to merely modify an item, it must be copied to the output. The output must be an item; expressions, statements and bare blocks are not allowed.

There is no restriction on how many items an attribute-like procedural macro can emit as long as they are valid in the given context.

my_macro_crate/src/lib.rs

#![feature(proc_macro)]

extern crate proc_macro;

use proc_macro::TokenStream;

/// Adds a `/// ### Panics` docstring to the end of the input's documentation
///
/// Does not assert that its receiver is a function or method.
#[proc_macro_attribute]
pub fn panics_note(args: TokenStream, input: TokenStream) -> TokenStream {
    let args = args.to_string();
    let mut input = input.to_string();

    assert!(args.starts_with("= \""), "`#[panics_note]` requires an argument of the form \
                                       `#[panics_note = \"panic note here\"]`");

    // Get just the bare note string
    let panics_note = args.trim_matches(&['=', ' ', '"'][..]);

    // The input will include all docstrings regardless of where the attribute is placed,
    // so we need to find the last index before the start of the item
    let insert_idx = idx_after_last_docstring(&input);

    // And insert our `### Panics` note there so it always appears at the end of an item's docs
    input.insert_str(insert_idx, &format!("/// # Panics \n/// {}\n", panics_note));

    input.parse().unwrap()
}

// `proc-macro` crates can contain any kind of private item still
fn idx_after_last_docstring(input: &str) -> usize {
    // Skip docstring lines to find the start of the item proper
    input.lines().skip_while(|line| line.trim_left().starts_with("///")).next()
        // Find the index of the first non-docstring line in the input
        // Note: assumes this exact line is unique in the input
        .and_then(|line_after| input.find(line_after))
        // No docstrings in the input
        .unwrap_or(0)
}

my_macro_user/Cargo.toml

[dependencies]
my_macro_crate = { path = "<relative path to my_macro_crate>" }

my_macro_user/src/lib.rs

#![feature(proc_macro)]

extern crate my_macro_crate;

use my_macro_crate::panics_note;

/// Do the `foo` thing.
#[panics_note = "Always."]
pub fn foo() {
    panic!()
}

Then the rendered documentation for pub fn foo will look like this:

pub fn foo()

---

Do the foo thing.

Panics

Always.

The tracking issue for this feature is: #42524.

The tracking issue for this feature is: #29601

The tracking issue for this feature is: #27731

The tracking issue for this feature is: #43036

This feature enables the repr(transparent) attribute on structs, which enables the use of newtypes without the usual ABI implications of wrapping the value in a struct.

It's sometimes useful to add additional type safety by introducing newtypes. For example, code that handles numeric quantities in different units such as millimeters, centimeters, grams, kilograms, etc. may want to use the type system to rule out mistakes such as adding millimeters to grams:

# #![allow(unused_variables)]
#fn main() {
use std::ops::Add;

struct Millimeters(f64);
struct Grams(f64);

impl Add<Millimeters> for Millimeters {
    type Output = Millimeters;

    fn add(self, other: Millimeters) -> Millimeters {
        Millimeters(self.0 + other.0)
    }
}

// Likewise: impl Add<Grams> for Grams {}
#}

Other uses of newtypes include using PhantomData to add lifetimes to raw pointers or to implement the "phantom types" pattern. See the PhantomData documentation and the Nomicon for more details.

The added type safety is especially useful when interacting with C or other languages. However, in those cases we need to ensure the newtypes we add do not introduce incompatibilities with the C ABI.

Luckily, repr(C) newtypes are laid out just like the type they wrap on all platforms which Rust currently supports, and likely on many more. For example, consider this C declaration:

struct Object {
    double weight; //< in grams
    double height; //< in millimeters
    // ...
}

void frobnicate(struct Object *);

While using this C code from Rust, we could add repr(C) to the Grams and Millimeters newtypes introduced above and use them to add some type safety while staying compatible with the memory layout of Object:

# #![allow(unused_variables)]
#fn main() {
#[repr(C)]
struct Grams(f64);

#[repr(C)]
struct Millimeters(f64);

#[repr(C)]
struct Object {
    weight: Grams,
    height: Millimeters,
    // ...
}

extern {
    fn frobnicate(_: *mut Object);
}
#}

This works even when adding some PhantomData fields, because they are zero-sized and therefore don't have to affect the memory layout.

However, there's more to the ABI than just memory layout: there's also the question of how function call arguments and return values are passed. Many common ABI treat a struct containing a single field differently from that field itself, at least when the field is a scalar (e.g., integer or float or pointer).

To continue the above example, suppose the C library also exposes a function like this:

double calculate_weight(double height);

Using our newtypes on the Rust side like this will cause an ABI mismatch on many platforms:

extern {
    fn calculate_weight(height: Millimeters) -> Grams;
}

For example, on x86_64 Linux, Rust will pass the argument in an integer register, while the C function expects the argument to be in a floating-point register. Likewise, the C function will return the result in a floating-point register while Rust will expect it in an integer register.

Note that this problem is not specific to floats: To give another example, 32-bit x86 linux will pass and return struct Foo(i32); on the stack while i32 is placed in registers.

So while repr(C) happens to do the right thing with respect to memory layout, it's not quite the right tool for newtypes in FFI. Instead of declaring a C struct, we need to communicate to the Rust compiler that our newtype is just for type safety on the Rust side. This is what repr(transparent) does.

The attribute can be applied to a newtype-like structs that contains a single field. It indicates that the newtype should be represented exactly like that field's type, i.e., the newtype should be ignored for ABI purpopses: not only is it laid out the same in memory, it is also passed identically in function calls.

In the above example, the ABI mismatches can be prevented by making the newtypes Grams and Millimeters transparent like this:

# #![allow(unused_variables)]
#![feature(repr_transparent)]

#fn main() {
#[repr(transparent)]
struct Grams(f64);

#[repr(transparent)]
struct Millimeters(f64);
#}

In addition to that single field, any number of zero-sized fields are permitted, including but not limited to PhantomData:

# #![allow(unused_variables)]
#![feature(repr_transparent)]

#fn main() {
use std::marker::PhantomData;

struct Foo { /* ... */ }

#[repr(transparent)]
struct FooPtrWithLifetime<'a>(*const Foo, PhantomData<&'a Foo>);

#[repr(transparent)]
struct NumberWithUnit<T, U>(T, PhantomData<U>);

struct CustomZst;

#[repr(transparent)]
struct PtrWithCustomZst<'a> {
    ptr: FooPtrWithLifetime<'a>,
    some_marker: CustomZst,
}
#}

Transparent structs can be nested: PtrWithCustomZst is also represented exactly like *const Foo.

Because repr(transparent) delegates all representation concerns to another type, it is incompatible with all other repr(..) attributes. It also cannot be applied to enums, unions, empty structs, structs whose fields are all zero-sized, or structs with multiple non-zero-sized fields.

The tracking issue for this feature is: #35118

The tracking issue for this feature is: #29642

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

The tracking issue for this feature is: #27731

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),
        _ => {},
    }
}

The tracking issue for this feature is: #31844

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

The tracking issue for this feature is: #29633

The tracking issue for this feature is: #37403

The tracking issue for this feature is: #15701

The tracking issue for this feature is: #31434

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

The tracking issue for this feature is: #43301

The tracking issue for this feature is: #29594

The tracking issue for this feature is #29598.

With trace_macros you can trace the expansion of macros in your code.

#![feature(trace_macros)]

fn main() {
    trace_macros!(true);
    println!("Hello, Rust!");
    trace_macros!(false);
}

The cargo build output:

note: trace_macro
 --> src/main.rs:5:5
  |
5 |     println!("Hello, Rust!");
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: expanding `println! { "Hello, Rust!" }`
  = note: to `print ! ( concat ! ( "Hello, Rust!" , "\n" ) )`
  = note: expanding `print! { concat ! ( "Hello, Rust!" , "\n" ) }`
  = note: to `$crate :: io :: _print ( format_args ! ( concat ! ( "Hello, Rust!" , "\n" ) )
          )`

    Finished dev [unoptimized + debuginfo] target(s) in 0.60 secs

The tracking issue for this feature is: #41517

The tracking issue for this feature is: #23416

The tracking issue for this feature is #29625

See Also: fn_traits

The unboxed_closures feature allows you to write functions using the "rust-call" ABI, required for implementing the Fn* family of traits. "rust-call" functions must have exactly one (non self) argument, a tuple representing the argument list.

#![feature(unboxed_closures)]

extern "rust-call" fn add_args(args: (u32, u32)) -> u32 {
    args.0 + args.1
}

fn main() {}

The tracking issue for this feature is: #44524

The tracking issue for this feature is: #34511.

The universal_impl_trait feature extends the conservative_impl_trait feature allowing the impl Trait syntax in arguments (universal quantification).

#![feature(universal_impl_trait)]
use std::ops::Not;

fn any_zero(values: impl IntoIterator<Item = i32>) -> bool {
    for val in values { if val == 0 { return true; } }
    false
}

fn main() {
    let test1 = -5..;
    let test2 = vec![1, 8, 42, -87, 60];
    assert!(any_zero(test1));
    assert!(bool::not(any_zero(test2)));
}

The tracking issue for this feature is: #42877

This is a part of RFC0401. According to the RFC, there should be an implementation like this:

impl<..., T, U: ?Sized> Unsized<(..., U)> for (..., T) where T: Unsized<U> {}

This implementation is currently gated behind #[feature(unsized_tuple_coercion)] to avoid insta-stability. Therefore you can use it like this:

#![feature(unsized_tuple_coercion)]

fn main() {
    let x : ([i32; 3], [i32; 3]) = ([1, 2, 3], [4, 5, 6]);
    let y : &([i32; 3], [i32]) = &x;
    assert_eq!(y.1[0], 4);
}

The tracking issue for this feature is: #32836

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

The tracking issue for this feature is: #35896

The tracking issue for this feature is: 40289.

The #[used] attribute can be applied to static variables to prevent the Rust compiler from optimizing them away even if they appear to be unused by the crate (appear to be "dead code").

#![feature(used)]

#[used]
static FOO: i32 = 1;

static BAR: i32 = 2;

fn main() {}

If you compile this program into an object file, you'll see that FOO makes it to the object file but BAR doesn't. Neither static variable is used by the program.

$ rustc -C opt-level=3 --emit=obj used.rs

$ nm -C used.o
0000000000000000 T main
                 U std::rt::lang_start
0000000000000000 r used::FOO
0000000000000000 t used::main

Note that the linker knows nothing about the #[used] attribute and will remove #[used] symbols if they are not referenced by other parts of the program:

$ rustc -C opt-level=3 used.rs

$ nm -C used | grep FOO

"This doesn't sound too useful then!" you may think but keep reading.

To preserve the symbols all the way to the final binary, you'll need the cooperation of the linker. Here's one example:

The ELF standard defines two special sections, .init_array and .pre_init_array, that may contain function pointers which will be executed before the main function is invoked. The linker will preserve symbols placed in these sections (at least when linking programs that target the *-*-linux-* targets).

#![feature(used)]

extern "C" fn before_main() {
    println!("Hello, world!");
}

#[link_section = ".init_array"]
#[used]
static INIT_ARRAY: [extern "C" fn(); 1] = [before_main];

fn main() {}

So, #[used] and #[link_section] can be combined to obtain "life before main".

$ rustc -C opt-level=3 before-main.rs

$ ./before-main
Hello, world!

Another example: ARM Cortex-M microcontrollers need their reset handler, a pointer to the function that will executed right after the microcontroller is turned on, to be placed near the start of their FLASH memory to boot properly.

This condition can be met using #[used] and #[link_section] plus a linker script.

#![feature(lang_items)]
#![feature(used)]
#![no_main]
#![no_std]

extern "C" fn reset_handler() -> ! {
    loop {}
}

#[link_section = ".reset_handler"]
#[used]
static RESET_HANDLER: extern "C" fn() -> ! = reset_handler;

#[lang = "panic_fmt"]
fn panic_fmt() {}

MEMORY
{
  FLASH : ORIGIN = 0x08000000, LENGTH = 128K
  RAM : ORIGIN = 0x20000000, LENGTH = 20K
}

SECTIONS
{
  .text ORIGIN(FLASH) :
  {
    /* Vector table */
    LONG(ORIGIN(RAM) + LENGTH(RAM)); /* initial SP value */
    KEEP(*(.reset_handler));

    /* Omitted: The rest of the vector table */

    *(.text.*);
  } > FLASH

  /DISCARD/ :
  {
    /* Unused unwinding stuff */
    *(.ARM.exidx.*)
  }
}

$ xargo rustc --target thumbv7m-none-eabi --release -- \
    -C link-arg=-Tlink.x -C link-arg=-nostartfiles

$ arm-none-eabi-objdump -Cd target/thumbv7m-none-eabi/release/app
./target/thumbv7m-none-eabi/release/app:     file format elf32-littlearm


Disassembly of section .text:

08000000 <app::RESET_HANDLER-0x4>:
 8000000:       20005000        .word   0x20005000

08000004 <app::RESET_HANDLER>:
 8000004:       08000009                                ....

08000008 <app::reset_handler>:
 8000008:       e7fe            b.n     8000008 <app::reset_handler>

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

The tracking issue for this feature is: #44488

The tracking issue for this feature is: #27783

The tracking issue for this feature is: #33082

See also alloc_system.

This feature has been replaced by the jemallocator crate on crates.io..

The tracking issue for this feature is: #32838

See also global_allocator.

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.

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 through the #[global_allocator] attribute:

#![feature(alloc_system, global_allocator, allocator_api)]

extern crate alloc_system;

use alloc_system::System;

#[global_allocator]
static A: System = 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:

(The alloc_jemalloc crate cannot be used to control the global allocator, crate.io’s jemallocator crate provides equivalent functionality.)

# Cargo.toml
[dependencies]
jemallocator = "0.1"

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

extern crate jemallocator;

#[global_allocator]
static ALLOC: jemallocator::Jemalloc = jemallocator::Jemalloc;

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

The tracking issue for this feature is #32838

Sometimes you want the memory for one collection to use a different allocator than the memory for another collection. In this case, replacing the global allocator is not a workable option. Instead, you need to pass in an instance of an Alloc to each collection for which you want a custom allocator.

TBD

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

The tracking issue for this feature is: #39658

The tracking issue for this feature is: #27779

The tracking issue for this feature is: #47336

The tracking issue for this feature is: #46179

The tracking issue for this feature is: #45323

The tracking issue for this feature is: #31100

This feature is internal to the Rust compiler and is not intended for general use.

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is: #27732

The tracking issue for this feature is: #30172

The tracking issue for this feature is: #30877

The tracking issue for this feature is: None.

This feature is required to link to the compiler_builtins crate which contains "compiler intrinsics". Compiler intrinsics are software implementations of basic operations like multiplication of u64s. These intrinsics are only required on platforms where these operations don't directly map to a hardware instruction.

You should never need to explicitly link to the compiler_builtins crate when building "std" programs as compiler_builtins is already in the dependency graph of std. But you may need it when building no_std binary crates. If you get a linker error like:

$PWD/src/main.rs:11: undefined reference to `__aeabi_lmul'
$PWD/src/main.rs:11: undefined reference to `__aeabi_uldivmod'

That means that you need to link to this crate.

When you link to this crate, make sure it only appears once in your crate dependency graph. Also, it doesn't matter where in the dependency graph you place the compiler_builtins crate.

#![feature(compiler_builtins_lib)]
#![no_std]

extern crate compiler_builtins;

The tracking issue for this feature is: #29599

The tracking issue for this feature is: #24111

The tracking issue for this feature is: #32110

The tracking issue for this feature is: #32110

This feature is internal to the Rust compiler and is not intended for general use.

This feature is internal to the Rust compiler and is not intended for general use.

This feature is internal to the Rust compiler and is not intended for general use.

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is: #32110

The tracking issue for this feature is: #32110

The tracking issue for this feature is: #30132

The tracking issue for this feature is: #46316

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is: #33906

This feature is internal to the Rust compiler and is not intended for general use.

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is: #43244

The tracking issue for this feature is: #46507

The tracking issue for this feature is: #44400

The tracking issue for this feature is: #44733

This introduces a new method for the Entry API of maps (std::collections::HashMap and std::collections::BTreeMap), so that occupied entries can be modified before any potential inserts into the map.

For example:

#![feature(entry_and_modify)]
# fn main() {
use std::collections::HashMap;

struct Foo {
    new: bool,
}

let mut map: HashMap<&str, Foo> = HashMap::new();

map.entry("quux")
   .and_modify(|e| e.new = false)
   .or_insert(Foo { new: true });
# }

This is not possible with the stable API alone since inserting a default before modifying the new field would mean we would lose the default state:

# fn main() {
use std::collections::HashMap;

struct Foo {
    new: bool,
}

let mut map: HashMap<&str, Foo> = HashMap::new();

map.entry("quux").or_insert(Foo { new: true }).new = false;
# }

In the above code the new field will never be true, even though we only intended to update that field to false for previously extant entries.

To achieve the same effect as and_modify we would have to manually match against the Occupied and Vacant variants of the Entry enum, which is a little less user-friendly, and much more verbose:

# fn main() {
use std::collections::HashMap;
use std::collections::hash_map::Entry;

struct Foo {
    new: bool,
}

let mut map: HashMap<&str, Foo> = HashMap::new();

match map.entry("quux") {
    Entry::Occupied(entry) => {
        entry.into_mut().new = false;
    },
    Entry::Vacant(entry) => {
        entry.insert(Foo { new: true });
    },
};
# }

The tracking issue for this feature is: #44324

The entry_or_default feature adds a new method to hash_map::Entry and btree_map::Entry, or_default, when V: Default. This method is semantically identical to or_insert_with(Default::default), and will insert the default value for the type if no entry exists for the current key.

The tracking issue for this feature is: #27745

The tracking issue for this feature is: #47115

The tracking issue for this feature is: #35428

This feature is internal to the Rust compiler and is not intended for general use.

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is: #27778

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is: #27726

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is #29625

See Also: unboxed_closures

The fn_traits feature allows for implementation of the Fn* traits for creating custom closure-like types.

#![feature(unboxed_closures)]
#![feature(fn_traits)]

struct Adder {
    a: u32
}

impl FnOnce<(u32, )> for Adder {
    type Output = u32;
    extern "rust-call" fn call_once(self, b: (u32, )) -> Self::Output {
        self.a + b.0
    }
}

fn main() {
    let adder = Adder { a: 3 };
    assert_eq!(adder(2), 5);
}

The tracking issue for this feature is: #28796

The tracking issue for this feature is: #45703

The tracking issue for this feature is: #40895

The tracking issue for this feature is: #46588

The tracking issue for this feature is: #35602

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is: #43122

The tracking issue for this feature is: #27745

The tracking issue for this feature is: #44971

The tracking issue for this feature is: #46344

The tracking issue for this feature is: #27700

The tracking issue for this feature is: #35118

The tracking issue for this feature is: #28237

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is: #32976

The tracking issue for this feature is: #27802

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is: #27709

The tracking issue for this feature is: #44582

The tracking issue for this feature is: #39480

The tracking issue for this feature is: #44705

The tracking issue for this feature is: #27741

The tracking issue for this feature is: #45594

This feature is internal to the Rust compiler and is not intended for general use.

This feature is internal to the Rust compiler and is not intended for general use.

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is: #27794

The tracking issue for this feature is: #27705

The tracking issue for this feature is: #44286

The tracking issue for this feature is: #27800

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is: #47653

The tracking issue for this feature is: #27730

The tracking issue for this feature is: #41079

The tracking issue for this feature is: #33577

The tracking issue for this feature is: #45860

The tracking issue for this feature is: #43738

The tracking issue for this feature is: #32837

The tracking issue for this feature is: #32837

The tracking issue for this feature is: #27721

The tracking issue for this feature is: #27779

The tracking issue for this feature is: #27779

The tracking issue for this feature is: #43941

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is: #27812

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

This feature is internal to the Rust compiler and is not intended for general use.

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

The tracking issue for this feature is: #32311

The tracking issue for this feature is: #27751

The tracking issue for this feature is: #44608

The tracking issue for this feature is: [#42788]

The tracking issue for this feature is: #43570

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is: #27812

This feature is internal to the Rust compiler and is not intended for general use.

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is: #27730

The tracking issue for this feature is: #34767

The tracking issue for this feature is: #27747

The tracking issue for this feature is: #35729

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

The tracking issue for this feature is: #41891

The tracking issue for this feature is: #41020

The slice_rsplit feature enables two methods on slices: slice.rsplit(predicate) and slice.rsplit_mut(predicate).

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is: #44643

The splice() method on String allows you to replace a range of values in a string with another range of values.

A simple example:

# #![allow(unused_variables)]
#![feature(splice)]
#fn main() {
let mut s = String::from("α is alpha, β is beta");
let beta_offset = s.find('β').unwrap_or(s.len());

// Replace the range up until the β from the string
s.splice(..beta_offset, "Α is capital alpha; ");
assert_eq!(s, "Α is capital alpha; β is beta");
#}

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

The tracking issue for this feature is: #42168

The tracking issue for this feature is: #27791

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is: #43874

Retains only the characters specified by the predicate.

In other words, remove all characters c such that f(c) returns false. This method operates in place and preserves the order of the retained characters.

# #![allow(unused_variables)]
#![feature(string_retain)]

#fn main() {
let mut s = String::from("f_o_ob_ar");

s.retain(|c| c != '_');

assert_eq!(s, "foobar");
#}

The tracking issue for this feature is: #42818

The tracking issue for this feature is: #44030

The tracking issue for this feature is: #42781

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)

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.

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is: #27716

The tracking issue for this feature is: #41263

The tracking issue for this feature is: #47338

The tracking issue for this feature is: #37572

The tracking issue for this feature is: #33417

The tracking issue for this feature is: #42327

This introduces a new trait Try for extending the ? operator to types other than Result (a part of RFC 1859). The trait provides the canonical way to view a type in terms of a success/failure dichotomy. This will allow ? to supplant the try_opt! macro on Option and the try_ready! macro on Poll, among other things.

Here's an example implementation of the trait:

/// A distinct type to represent the `None` value of an `Option`.
///
/// This enables using the `?` operator on `Option`; it's rarely useful alone.
#[derive(Debug)]
#[unstable(feature = "try_trait", issue = "42327")]
pub struct None { _priv: () }

#[unstable(feature = "try_trait", issue = "42327")]
impl<T> ops::Try for Option<T>  {
    type Ok = T;
    type Error = None;

    fn into_result(self) -> Result<T, None> {
        self.ok_or(None { _priv: () })
    }

    fn from_ok(v: T) -> Self {
        Some(v)
    }

    fn from_error(_: None) -> Self {
        None
    }
}

Note the Error associated type here is a new marker. The ? operator allows interconversion between different Try implementers only when the error type can be converted Into the error type of the enclosing function (or catch block). Having a distinct error type (as opposed to just (), or similar) restricts this to where it's semantically meaningful.

The tracking issue for this feature is: #27783

The tracking issue for this feature is: #46104

The tracking issue for this feature is: #43751

The tracking issue for this feature is: #27732

This feature is internal to the Rust compiler and is not intended for general use.

The tracking issue for this feature is: #40062

The tracking issue for this feature is: #41758

This feature is internal to the Rust compiler and is not intended for general use.

This feature is internal to the Rust compiler and is not intended for general use.

This feature is internal to the Rust compiler and is not intended for general use.

This feature is internal to the Rust compiler and is not intended for general use.