Rust release · Dec 2, 2021

Rust 1.57: what's new, what was stabilized

Short answerRust 1.57 was released on Dec 2, 2021: 21 newly stable APIs ([T; N]::as_mut_slice, [T; N]::as_slice, collections::TryReserveError, HashMap::try_reserve, ...); 1 compatibility note that can affect existing code.

Dec 2, 2021release date

0language stabilizations

21APIs stabilized

-new edition

← Rust 1.56 All versions Rust 1.58 →

21 APIs stabilized

[T; N]::as_mut_slice, [T; N]::as_slice, collections::TryReserveError, HashMap::try_reserve, HashSet::try_reserve, String::try_reserve, String::try_reserve_exact, Vec::try_reserve, Vec::try_reserve_exact, VecDeque::try_reserve, VecDeque::try_reserve_exact, Iterator::map_while, iter::MapWhile, proc_macro::is_available, Command::get_program, Command::get_args, Command::get_envs, Command::get_current_dir, CommandArgs, CommandEnvs, hint::unreachable_unchecked

Can it break my build?

The release lists these compatibility notes. Most only affect rare code, but read them before you raise your toolchain in CI:

[Ignore derived Clone and Debug implementations during dead code analysis.][85200]

Use it

rustup update stable        # get the newest release
rustup install 1.57          # or exactly this one
rustup override set 1.57     # pin a project to it

To require at least this version from users of your crate, set rust-version = "1.57" under [package] in Cargo.toml. Cargo then refuses to build it on older compilers with a clear message, and with resolver 3 (the default on the 2024 edition, Rust 1.85+) it also prefers dependency versions that support it.

Full release notes

Language

[Macro attributes may follow #[derive] and will see the original (pre-cfg) input.][87220]
[Accept curly-brace macros in expressions, like m!{ .. }.method() and m!{ .. }?.][88690]
[Allow panicking in constant evaluation.][89508]
[Ignore derived Clone and Debug implementations during dead code analysis.][85200]

Compiler

[Create more accurate debuginfo for vtables.][89597]
[Add armv6k-nintendo-3ds at Tier 3\*.][88529]
[Add armv7-unknown-linux-uclibceabihf at Tier 3\*.][88952]
[Add m68k-unknown-linux-gnu at Tier 3\*.][88321]
[Add SOLID targets at Tier 3\*:][86191] aarch64-kmc-solid_asp3, armv7a-kmc-solid_asp3-eabi, armv7a-kmc-solid_asp3-eabihf

\* Refer to Rust's [platform support page][platform-support-doc] for more information on Rust's tiered platform support.

Libraries

[Avoid allocations and copying in Vec::leak][89337]
[Add #[repr(i8)] to Ordering][89507]
[Optimize File::read_to_end and read_to_string][89582]
[Update to Unicode 14.0][89614]
[Many more functions are marked #[must_use]][89692], producing a warning when ignoring their return value. This helps catch mistakes such as expecting a function to mutate a value in place rather than return a new value.

Stabilized APIs

[[T; N]::as_mut_slice][array::as_mut_slice]
[[T; N]::as_slice][array::as_slice]
[collections::TryReserveError]
[HashMap::try_reserve]
[HashSet::try_reserve]
[String::try_reserve]
[String::try_reserve_exact]
[Vec::try_reserve]
[Vec::try_reserve_exact]
[VecDeque::try_reserve]
[VecDeque::try_reserve_exact]
[Iterator::map_while]
[iter::MapWhile]
[proc_macro::is_available]
[Command::get_program]
[Command::get_args]
[Command::get_envs]
[Command::get_current_dir]
[CommandArgs]
[CommandEnvs]

These APIs are now usable in const contexts:

[hint::unreachable_unchecked]

Cargo

[Stabilize custom profiles][cargo/9943]

Compatibility Notes

[Ignore derived Clone and Debug implementations during dead code analysis.][85200] This will break some builds that set #![deny(dead_code)].

Internal Changes

These changes provide no direct user facing benefits, but represent significant improvements to the internals and overall performance of rustc and related tools.

Added an experimental backend for codegen with libgccjit.

Release notes from the Rust project's RELEASES.md (MIT or Apache-2.0); summaries above are ours.