Rust release · Dec 31, 2020

Rust 1.49: what's new, what was stabilized

Short answerRust 1.49 was released on Dec 31, 2020: 5 newly stable APIs (slice::select_nth_unstable, slice::select_nth_unstable_by, slice::select_nth_unstable_by_key, Poll::is_ready, ...); 5 compatibility notes that can affect existing code.

Dec 31, 2020release date

0language stabilizations

5APIs stabilized

-new edition

← Rust 1.48 All versions Rust 1.50 →

5 APIs stabilized

slice::select_nth_unstable, slice::select_nth_unstable_by, slice::select_nth_unstable_by_key, Poll::is_ready, Poll::is_pending

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:

[Demoted i686-unknown-freebsd from host tier 2 to target tier 2 support.][78746]
[Macros that end with a semi-colon are now treated as statements even if they expand to nothing.][78376]
[Rustc will now check for the validity of some built-in attributes on enum variants.][77015]
Leading whitespace is stripped more uniformly in documentation comments, which may change behavior. You
[Trait bounds are no longer inferred for associated types.][79904]

Use it

rustup update stable        # get the newest release
rustup install 1.49          # or exactly this one
rustup override set 1.49     # pin a project to it

To require at least this version from users of your crate, set rust-version = "1.49" 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

[Unions can now implement Drop, and you can now have a field in a union with ManuallyDrop<T>.][77547]
[You can now cast uninhabited enums to integers.][76199]
[You can now bind by reference and by move in patterns.][76119] This allows you to selectively borrow individual components of a type. E.g. ```rust #[derive(Debug)] struct Person { name: String, age: u8, }

let person = Person { name: String::from("Alice"), age: 20, };

// name is moved out of person, but age is referenced. let Person { name, ref age } = person; println!("{} {}", name, age); ```

Compiler

[Added tier 1\* support for aarch64-unknown-linux-gnu.][78228]
[Added tier 2 support for aarch64-apple-darwin.][75991]
[Added tier 2 support for aarch64-pc-windows-msvc.][75914]
[Added tier 3 support for mipsel-unknown-none.][78676]
[Raised the minimum supported LLVM version to LLVM 9.][78848]
[Output from threads spawned in tests is now captured.][78227]
[Change os and vendor values to "none" and "unknown" for some targets][78951]

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

Libraries

[RangeInclusive now checks for exhaustion when calling contains and indexing.][78109]
[ToString::to_string now no longer shrinks the internal buffer in the default implementation.][77997]

Stabilized APIs

[slice::select_nth_unstable]
[slice::select_nth_unstable_by]
[slice::select_nth_unstable_by_key]

The following previously stable methods are now const.

[Poll::is_ready]
[Poll::is_pending]

Cargo

[Building a crate with cargo-package should now be independently reproducible.][cargo/8864]
[cargo-tree now marks proc-macro crates.][cargo/8765]
[Added CARGO_PRIMARY_PACKAGE build-time environment variable.][cargo/8758] This variable will be set if the crate being built is one the user selected to build, either with -p or through defaults.
[You can now use glob patterns when specifying packages & targets.][cargo/8752]

Compatibility Notes

[Demoted i686-unknown-freebsd from host tier 2 to target tier 2 support.][78746]
[Macros that end with a semi-colon are now treated as statements even if they expand to nothing.][78376]
[Rustc will now check for the validity of some built-in attributes on enum variants.][77015] Previously such invalid or unused attributes could be ignored.
Leading whitespace is stripped more uniformly in documentation comments, which may change behavior. You read [this post about the changes][rustdoc-ws-post] for more details.
[Trait bounds are no longer inferred for associated types.][79904]

Internal Only

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

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