Rust release · Aug 15, 2019

Rust 1.37: what's new, what was stabilized

Short answerRust 1.37 was released on Aug 15, 2019: 20 newly stable APIs (BufReader::buffer, BufWriter::buffer, Cell::from_mut, Cell<[T]>::as_slice_of_cells, ...); 2 compatibility notes that can affect existing code.

Aug 15, 2019release date

0language stabilizations

20APIs stabilized

-new edition

← Rust 1.36 All versions Rust 1.38 →

20 APIs stabilized

BufReader::buffer, BufWriter::buffer, Cell::from_mut, Cell<[T]>::as_slice_of_cells, DoubleEndedIterator::nth_back, Option::xor, Wrapping::reverse_bits, i128::reverse_bits, i16::reverse_bits, i32::reverse_bits, i64::reverse_bits, i8::reverse_bits, isize::reverse_bits, slice::copy_within, u128::reverse_bits, u16::reverse_bits, u32::reverse_bits, u64::reverse_bits, u8::reverse_bits, usize::reverse_bits

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:

[Using ... for inclusive range patterns will now warn by default.][61342]
[Using a trait object without the dyn will now warn by default.][61203]

Use it

rustup update stable        # get the newest release
rustup install 1.37          # or exactly this one
rustup override set 1.37     # pin a project to it

To require at least this version from users of your crate, set rust-version = "1.37" 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

#[must_use] will now warn if the type is contained in a [tuple][61100], [Box][62228], or an [array][62235] and unused.
[You can now use the cfg and cfg_attr attributes on generic parameters.][61547]
[You can now use enum variants through type alias.][61682] e.g. You can write the following: ```rust type MyOption = Option<u8>;

fn increment_or_zero(x: MyOption) -> u8 { match x { MyOption::Some(y) => y + 1, MyOption::None => 0, } } ```

[You can now use _ as an identifier for consts.][61347] e.g. You can write const _: u32 = 5;.
[You can now use #[repr(align(X)] on enums.][61229]
[The ? Kleene macro operator is now available in the 2015 edition.][60932]

Compiler

[You can now enable Profile-Guided Optimization with the -C profile-generate and -C profile-use flags.][61268] For more information on how to use profile guided optimization, please refer to the [rustc book][rustc-book-pgo].
[The rust-lldb wrapper script should now work again.][61827]

Libraries

[mem::MaybeUninit<T> is now ABI-compatible with T.][61802]

Stabilized APIs

[BufReader::buffer]
[BufWriter::buffer]
[Cell::from_mut]
[Cell<[T]>::as_slice_of_cells][Cell<slice>::as_slice_of_cells]
[DoubleEndedIterator::nth_back]
[Option::xor]
[Wrapping::reverse_bits]
[i128::reverse_bits]
[i16::reverse_bits]
[i32::reverse_bits]
[i64::reverse_bits]
[i8::reverse_bits]
[isize::reverse_bits]
[slice::copy_within]
[u128::reverse_bits]
[u16::reverse_bits]
[u32::reverse_bits]
[u64::reverse_bits]
[u8::reverse_bits]
[usize::reverse_bits]

Cargo

[Cargo.lock files are now included by default when publishing executable crates with executables.][cargo/7026]
[You can now specify default-run="foo" in [package] to specify the default executable to use for cargo run.][cargo/7056]

Compatibility Notes

Using ... for inclusive range patterns will now warn by default. Please transition your code to using the ..= syntax for inclusive ranges instead.
Using a trait object without the dyn will now warn by default. Please transition your code to use dyn Trait for trait objects instead.

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