Rust release · Dec 19, 2019

Rust 1.40: what's new, what was stabilized

Short answerRust 1.40 was released on Dec 19, 2019: 21 newly stable APIs (BTreeMap::get_key_value, HashMap::get_key_value, Option::as_deref_mut, Option::as_deref, ...); 3 compatibility notes that can affect existing code.

Dec 19, 2019release date

0language stabilizations

21APIs stabilized

-new edition

← Rust 1.39 All versions Rust 1.41 →

21 APIs stabilized

BTreeMap::get_key_value, HashMap::get_key_value, Option::as_deref_mut, Option::as_deref, Option::flatten, UdpSocket::peer_addr, f32::to_be_bytes, f32::to_le_bytes, f32::to_ne_bytes, f64::to_be_bytes, f64::to_le_bytes, f64::to_ne_bytes, f32::from_be_bytes, f32::from_le_bytes, f32::from_ne_bytes, f64::from_be_bytes, f64::from_le_bytes, f64::from_ne_bytes, mem::take, slice::repeat, todo!

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:

[As previously announced, any previous NLL warnings in the 2015 edition are
[The include! macro will now warn if it failed to include the
[Using #[inline] on function prototypes and consts now emits a warning under

Use it

rustup update stable        # get the newest release
rustup install 1.40          # or exactly this one
rustup override set 1.40     # pin a project to it

To require at least this version from users of your crate, set rust-version = "1.40" 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

[You can now use tuple structs and tuple enum variant's constructors in const contexts.][65188] e.g.

```rust pub struct Point(i32, i32);

const ORIGIN: Point = { let constructor = Point;

constructor(0, 0) }; ```

[You can now mark structs, enums, and enum variants with the #[non_exhaustive] attribute to indicate that there may be variants or fields added in the future.][64639] For example this requires adding a wild-card branch (_ => {}) to any match statements on a non-exhaustive enum. [(RFC 2008)]
[You can now use function-like procedural macros in extern blocks and in type positions.][63931] e.g. type Generated = macro!();
[Function-like and attribute procedural macros can now emit macro_rules! items, so you can now have your macros generate macros.][64035]
[The meta pattern matcher in macro_rules! now correctly matches the modern attribute syntax.][63674] For example (#[$m:meta]) now matches #[attr], #[attr{tokens}], #[attr[tokens]], and #[attr(tokens)].

Compiler

[Added tier 3 support\* for the thumbv7neon-unknown-linux-musleabihf target.][66103]
[Added tier 3 support for the aarch64-unknown-none-softfloat target.][64589]
[Added tier 3 support for the mips64-unknown-linux-muslabi64, and mips64el-unknown-linux-muslabi64 targets.][65843]

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

Libraries

[The is_power_of_two method on unsigned numeric types is now a const function.][65092]

Stabilized APIs

[BTreeMap::get_key_value]
[HashMap::get_key_value]
[Option::as_deref_mut]
[Option::as_deref]
[Option::flatten]
[UdpSocket::peer_addr]
[f32::to_be_bytes]
[f32::to_le_bytes]
[f32::to_ne_bytes]
[f64::to_be_bytes]
[f64::to_le_bytes]
[f64::to_ne_bytes]
[f32::from_be_bytes]
[f32::from_le_bytes]
[f32::from_ne_bytes]
[f64::from_be_bytes]
[f64::from_le_bytes]
[f64::from_ne_bytes]
[mem::take]
[slice::repeat]
[todo!]

Cargo

[Cargo will now always display warnings, rather than only on fresh builds.][cargo/7450]
[Feature flags (except --all-features) passed to a virtual workspace will now produce an error.][cargo/7507] Previously these flags were ignored.
[You can now publish dev-dependencies without including a version.][cargo/7333]

Misc

[You can now specify the #[cfg(doctest)] attribute to include an item only when running documentation tests with rustdoc.][63803]

Compatibility Notes

As previously announced, any previous NLL warnings in the 2015 edition are now hard errors.
The include! macro will now warn if it failed to include the entire file. The include! macro unintentionally only includes the first _expression_ in a file, and this can be unintuitive. This will become either a hard error in a future release, or the behavior may be fixed to include all expressions as expected.
[Using #[inline] on function prototypes and consts now emits a warning under unused_attribute lint.]65294 Using #[inline] anywhere else inside traits or extern blocks now correctly emits a hard error.

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