Rust release · Sep 9, 2021

Rust 1.55: what's new, what was stabilized

Short answerRust 1.55 was released on Sep 9, 2021: 18 newly stable APIs (Bound::cloned, Drain::as_str, IntoInnerError::into_error, IntoInnerError::into_parts, ...); 4 compatibility notes that can affect existing code.

Sep 9, 2021release date

0language stabilizations

18APIs stabilized

-new edition

← Rust 1.54 All versions Rust 1.56 →

18 APIs stabilized

Bound::cloned, Drain::as_str, IntoInnerError::into_error, IntoInnerError::into_parts, MaybeUninit::assume_init_mut, MaybeUninit::assume_init_ref, MaybeUninit::write, array::map, ops::ControlFlow, x86::_bittest, x86::_bittestandcomplement, x86::_bittestandreset, x86::_bittestandset, x86_64::_bittest64, x86_64::_bittestandcomplement64, x86_64::_bittestandreset64, x86_64::_bittestandset64, str::from_utf8_unchecked

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:

[std functions that return an io::Error will no longer use the
[Using environment variable names with process::Command on Windows now
[Rustdoc will now warn on using rustdoc lints that aren't prefixed
RUSTFLAGS is no longer set for build scripts. Build scripts

Use it

rustup update stable        # get the newest release
rustup install 1.55          # or exactly this one
rustup override set 1.55     # pin a project to it

To require at least this version from users of your crate, set rust-version = "1.55" 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 write open "from" range patterns (X..), which will start at X and will end at the maximum value of the integer.][83918]
[You can now explicitly import the prelude of different editions through std::prelude (e.g. use std::prelude::rust_2021::*;).][86294]

Compiler

[Added tier 3\* support for powerpc64le-unknown-freebsd.][83572]

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

Libraries

[Updated std's float parsing to use the Eisel-Lemire algorithm.][86761] These improvements should in general provide faster string parsing of floats, no longer reject certain valid floating point values, and reduce the produced code size for non-stripped artifacts.
[string::Drain now implements AsRef<str> and AsRef<[u8]>.][86858]

Stabilized APIs

[Bound::cloned]
[Drain::as_str]
[IntoInnerError::into_error]
[IntoInnerError::into_parts]
[MaybeUninit::assume_init_mut]
[MaybeUninit::assume_init_ref]
[MaybeUninit::write]
[array::map]
[ops::ControlFlow]
[x86::_bittest]
[x86::_bittestandcomplement]
[x86::_bittestandreset]
[x86::_bittestandset]
[x86_64::_bittest64]
[x86_64::_bittestandcomplement64]
[x86_64::_bittestandreset64]
[x86_64::_bittestandset64]

The following previously stable functions are now const.

[str::from_utf8_unchecked]

Cargo

[Cargo will now deduplicate compiler diagnostics to the terminal when invoking rustc in parallel such as when using cargo test.][cargo/9675]
[The package definition in cargo metadata now includes the "default_run" field from the manifest.][cargo/9550]
[Added cargo d as an alias for cargo doc.][cargo/9680]
[Added {lib} as formatting option for cargo tree to print the "lib_name" of packages.][cargo/9663]

Rustdoc

[Added "Go to item on exact match" search option.][85876]
[The "Implementors" section on traits no longer shows redundant method definitions.][85970]
[Trait implementations are toggled open by default.][86260] This should make the implementations more searchable by tools like CTRL+F in your browser.
[Intra-doc links should now correctly resolve associated items (e.g. methods) through type aliases.][86334]
[Traits which are marked with #[doc(hidden)] will no longer appear in the "Trait Implementations" section.][86513]

Compatibility Notes

std functions that return an io::Error will no longer use the ErrorKind::Other variant. This is to better reflect that these kinds of errors could be categorised into newer more specific ErrorKind variants, and that they do not represent a user error.
Using environment variable names with process::Command on Windows now behaves as expected. Previously using environment variables with Command would cause them to be ASCII-uppercased.
Rustdoc will now warn on using rustdoc lints that aren't prefixed with rustdoc::
RUSTFLAGS is no longer set for build scripts. Build scripts should use CARGO_ENCODED_RUSTFLAGS instead. See the documentation for more details.

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