Rust release · Nov 7, 2019

Rust 1.39: what's new, what was stabilized

Short answerRust 1.39 was released on Nov 7, 2019: 3 newly stable APIs (Pin::into_inner, Instant::checked_duration_since, Instant::saturating_duration_since); 4 compatibility notes that can affect existing code.

Nov 7, 2019release date

0language stabilizations

3APIs stabilized

-new edition

← Rust 1.38 All versions Rust 1.40 →

3 APIs stabilized

Pin::into_inner, Instant::checked_duration_since, Instant::saturating_duration_since

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:

[Code that was previously accepted by the old borrow checker, but rejected by
[rustdoc now requires rustc to be installed and in the same directory to
[The try! macro will now issue a deprecation warning.][62672] It is
[asinh(-0.0) now correctly returns -0.0.][63698] Previously this

Use it

rustup update stable        # get the newest release
rustup install 1.39          # or exactly this one
rustup override set 1.39     # pin a project to it

To require at least this version from users of your crate, set rust-version = "1.39" 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 create async functions and blocks with async fn, async move {}, and async {} respectively, and you can now call .await on async expressions.][63209]
[You can now use certain attributes on function, closure, and function pointer parameters.][64010] These attributes include cfg, cfg_attr, allow, warn, deny, forbid as well as inert helper attributes used by procedural macro attributes applied to items. e.g. ``rust fn len( #[cfg(windows)] slice: &[u16], #[cfg(not(windows))] slice: &[u8], ) -> usize { slice.len() } ``
[You can now take shared references to bind-by-move patterns in the if guards of match arms.][63118] e.g. ```rust fn main() { let array: Box<[u8; 4]> = Box::new([1, 2, 3, 4]);

match array { nums // ---- nums is bound by move. if nums.iter().sum::<u8>() == 10 // ^------ .iter() implicitly takes a reference to nums. => { drop(nums); // ----------- Legal as nums was bound by move and so we have ownership. } _ => unreachable!(), } } ```

Compiler

[Added tier 3\* support for the i686-unknown-uefi target.][64334]
[Added tier 3 support for the sparc64-unknown-openbsd target.][63595]
[rustc will now trim code snippets in diagnostics to fit in your terminal.][63402] **Note** Cargo currently doesn't use this feature. Refer to [cargo#7315][cargo/7315] to track this feature's progress.
[You can now pass --show-output argument to test binaries to print the output of successful tests.][62600]

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

Libraries

[Vec::new and String::new are now const functions.][64028]
[LinkedList::new is now a const function.][63684]
[str::len, [T]::len and str::as_bytes are now const functions.][63770]
[The abs, wrapping_abs, and overflowing_abs numeric functions are now const.][63786]

Stabilized APIs

[Pin::into_inner]
[Instant::checked_duration_since]
[Instant::saturating_duration_since]

Cargo

[You can now publish git dependencies if supplied with a version.][cargo/7237]
[The --all flag has been renamed to --workspace.][cargo/7241] Using --all is now deprecated.

Misc

[You can now pass -Clinker to rustdoc to control the linker used for compiling doctests.][63834]

Compatibility Notes

Code that was previously accepted by the old borrow checker, but rejected by the NLL borrow checker is now a hard error in Rust 2018. This was previously a warning, and will also become a hard error in the Rust 2015 edition in the 1.40.0 release.
rustdoc now requires rustc to be installed and in the same directory to run tests. This should improve performance when running a large amount of doctests.
The try! macro will now issue a deprecation warning. It is recommended to use the ? operator instead.
asinh(-0.0) now correctly returns -0.0. Previously this returned 0.0.

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