Rust 1.21: what's new, what was stabilized

Short answerRust 1.21 was released on Oct 12, 2017: 3 compatibility notes that can affect existing code.

Oct 12, 2017release date

0language stabilizations

0APIs stabilized

-new edition

← Rust 1.20 All versions Rust 1.22 →

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:

[Changes in method matching against higher-ranked types][43880] This may cause
[rustc's JSON error output's byte position start at top of file.][42973]
[unused_results lint no longer ignores booleans][43728]

Use it

rustup update stable        # get the newest release
rustup install 1.21          # or exactly this one
rustup override set 1.21     # pin a project to it

To require at least this version from users of your crate, set rust-version = "1.21" 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 static references for literals.][43838] Example: ``rust fn main() { let x: &'static u32 = &0; } ``
[Relaxed path syntax. Optional :: before < is now allowed in all contexts.][43540] Example: ``rust my_macro!(Vec<i32>::new); // Always worked my_macro!(Vec::<i32>::new); // Now works ``

Compiler

[Upgraded jemalloc to 4.5.0][43911]
[Enabled unwinding panics on Redox][43917]
[Now runs LLVM in parallel during translation phase.][43506] This should reduce peak memory usage.

Libraries

[Generate builtin impls for Clone for all arrays and tuples that are T: Clone][43690]
[Stdin, Stdout, and Stderr now implement AsRawFd.][43459]
[Rc and Arc now implement From<&[T]> where T: Clone, From<str>, From<String>, From<Box<T>> where T: ?Sized, and From<Vec<T>>.][42565]

Stabilized APIs

[std::mem::discriminant]

Cargo

[You can now call cargo install with multiple package names][cargo/4216]
[Cargo commands inside a virtual workspace will now implicitly pass --all][cargo/4335]
[Added a [patch] section to Cargo.toml to handle prepublication dependencies][cargo/4123] [RFC 1969]
[include & exclude fields in Cargo.toml now accept gitignore like patterns][cargo/4270]
[Added the --all-targets option][cargo/4400]
[Using required dependencies as a feature is now deprecated and emits a warning][cargo/4364]

Misc

[Cargo docs are moving][43916] to doc.rust-lang.org/cargo
[The rustdoc book is now available][43863] at doc.rust-lang.org/rustdoc
[Added a preview of RLS has been made available through rustup][44204] Install with rustup component add rls-preview
[std::os documentation for Unix, Linux, and Windows now appears on doc.rust-lang.org][43348] Previously only showed std::os::unix.

Compatibility Notes

Changes in method matching against higher-ranked types This may cause breakage in subtyping corner cases. A more in-depth explanation is available.
rustc's JSON error output's byte position start at top of file. Was previously relative to the rustc's internal CodeMap struct which required the unstable library libsyntax to correctly use.
unused_results lint no longer ignores booleans

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