1.57.0 initial draft

posts/2021-12-02-Rust-1.57.0.md

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+---
+layout: post
+title: "Announcing Rust 1.57.0"
+author: The Rust Release Team
+release: true
+---
+
+The Rust team is happy to announce a new version of Rust, 1.57.0.
+Rust is a programming language empowering everyone to build reliable and efficient software.
+
+If you have a previous version of Rust installed via rustup, getting Rust 1.57.0 is as easy as:
+
+```console
+rustup update stable
+```
+
+If you don't have it already, you can [get `rustup`][install]
+from the appropriate page on our website, and check out the
+[detailed release notes for 1.57.0][notes] on GitHub.
+
+[install]: https://www.rust-lang.org/install.html
+[notes]: https://github.com/rust-lang/rust/blob/master/RELEASES.md#version-1570-2021-12-02
+
+## What's in 1.57.0 stable
+
+Rust 1.57 brings `panic!` to const contexts, adds support for custom profiles to Cargo, and stabilizes fallible reservation APIs.
+
+### `panic!` in const contexts
+
+With previous versions of Rust, the `panic!` macro was not usable in `const fn` and other compile-time contexts. Now, this has been stabilized. Together with the stabilization of `panic!`, several other standard library APIs are now usable in const, such as `assert!`.
+
+This stabilization does not yet include the full formatting infrastructure, so the `panic!` macro must be called with either a static string (`panic!("...")`), or with a single `&str` interpolated value (`panic!("{}", a)`) which must be used with `{}` (no format specifiers or other traits).
+
+It is expected that in the future this support will expand, but this minimal stabilization already enables straightforward compile-time assertions, for example to verify the size of a type:
+
+```rust
+const _: () = assert!(std::mem::size_of::<u64>() == 8);
+const _: () = assert!(std::mem::size_of::<u8>() == 1);
+```
+
+### Cargo support for custom profiles
+
+Cargo has long supported four profiles: `dev`, `release`, `test`, and `bench`. With Rust 1.57, support has been added for arbitrarily named profiles.
+
+For example, if you want to enable link time optimizations ([LTO]) only when making the final production build, adding the following snippet to Cargo.toml enables the `lto` flag when this profile is selected, but avoids enabling it for regular release builds.
+
+```toml
+[profile.production]
+inherits = "release"
+lto = true
+```
+
+Note that custom profiles must specify a profile from which they inherit default settings. Once the profile has been defined, Cargo commands which build code can be asked to use it with `--profile production`. Currently, this will build artifacts in a separate directory (`target/production` in this case), which means that artifacts are not shared between directories.
+
+[LTO]: https://doc.rust-lang.org/nightly/cargo/reference/profiles.html#lto
+
+### Fallible allocation
+
+Rust 1.57 stabilizes `try_reserve` for `Vec`, `String`, `HashMap`, `HashSet`, and `VecDeque`. This API enables callers to fallibly allocate the backing storage for these types.
+
+Rust will usually abort the process if the global allocator fails, which is not always desirable. This API provides a method for avoiding that abort when working with the standard library collections. However, Rust does not guarantee that the returned memory is actually allocated by the kernel: for example, if overcommit is enabled on Linux, the memory may not be available when its use is attempted.
+
+### Stabilized APIs
+
+The following methods and trait implementations were stabilized.
+
+- [`[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`]
+
+The following previously stable functions are now `const`.
+
+- [`hint::unreachable_unchecked`]
+
+### Other changes
+
+There are other changes in the Rust 1.57.0 release: check out what changed in
+[Rust](https://github.com/rust-lang/rust/blob/master/RELEASES.md#version-1570-2021-12-02),
+[Cargo](https://github.com/rust-lang/cargo/blob/master/CHANGELOG.md#cargo-157-2021-12-02),
+and [Clippy](https://github.com/rust-lang/rust-clippy/blob/master/CHANGELOG.md#rust-157).
+
+### Contributors to 1.57.0
+
+Many people came together to create Rust 1.57.0.
+We couldn't have done it without all of you.
+[Thanks!](https://thanks.rust-lang.org/rust/1.57.0/)
+
+[`array::as_mut_slice`]: https://doc.rust-lang.org/std/primitive.array.html#method.as_mut_slice
+[`array::as_slice`]: https://doc.rust-lang.org/std/primitive.array.html#method.as_slice
+[`collections::TryReserveError`]: https://doc.rust-lang.org/std/collections/struct.TryReserveError.html
+[`HashMap::try_reserve`]: https://doc.rust-lang.org/std/collections/hash_map/struct.HashMap.html#method.try_reserve
+[`HashSet::try_reserve`]: https://doc.rust-lang.org/std/collections/hash_set/struct.HashSet.html#method.try_reserve
+[`String::try_reserve`]: https://doc.rust-lang.org/alloc/string/struct.String.html#method.try_reserve
+[`String::try_reserve_exact`]: https://doc.rust-lang.org/alloc/string/struct.String.html#method.try_reserve_exact
+[`Vec::try_reserve`]: https://doc.rust-lang.org/std/vec/struct.Vec.html#method.try_reserve
+[`Vec::try_reserve_exact`]: https://doc.rust-lang.org/std/vec/struct.Vec.html#method.try_reserve_exact
+[`VecDeque::try_reserve`]: https://doc.rust-lang.org/std/collections/struct.VecDeque.html#method.try_reserve
+[`VecDeque::try_reserve_exact`]: https://doc.rust-lang.org/std/collections/struct.VecDeque.html#method.try_reserve_exact
+[`Iterator::map_while`]: https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.map_while
+[`iter::MapWhile`]: https://doc.rust-lang.org/std/iter/struct.MapWhile.html
+[`proc_macro::is_available`]: https://doc.rust-lang.org/proc_macro/fn.is_available.html
+[`Command::get_program`]: https://doc.rust-lang.org/std/process/struct.Command.html#method.get_program
+[`Command::get_args`]: https://doc.rust-lang.org/std/process/struct.Command.html#method.get_args
+[`Command::get_envs`]: https://doc.rust-lang.org/std/process/struct.Command.html#method.get_envs
+[`Command::get_current_dir`]: https://doc.rust-lang.org/std/process/struct.Command.html#method.get_current_dir
+[`CommandArgs`]: https://doc.rust-lang.org/std/process/struct.CommandArgs.html
+[`CommandEnvs`]: https://doc.rust-lang.org/std/process/struct.CommandEnvs.html
+[`hint::unreachable_unchecked`]: https://doc.rust-lang.org/std/hint/fn.unreachable_unchecked.html