Rust release · May 6, 2021

Rust 1.52: what's new, what was stabilized

Short answerRust 1.52 was released on May 6, 2021: 19 newly stable APIs (Arguments::as_str, char::MAX, char::REPLACEMENT_CHARACTER, char::UNICODE_VERSION, ...); 7 compatibility notes that can affect existing code. Point release: 1.52.1 (May 10, 2021).

May 6, 2021release date

0language stabilizations

19APIs stabilized

-new edition

← Rust 1.51 All versions Rust 1.53 →

19 APIs stabilized

Arguments::as_str, char::MAX, char::REPLACEMENT_CHARACTER, char::UNICODE_VERSION, char::decode_utf16, char::from_digit, char::from_u32_unchecked, char::from_u32, slice::partition_point, str::rsplit_once, str::split_once, char::len_utf8, char::len_utf16, char::to_ascii_uppercase, char::to_ascii_lowercase, char::eq_ignore_ascii_case, u8::to_ascii_uppercase, u8::to_ascii_lowercase, u8::eq_ignore_ascii_case

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:

[Cargo build scripts are now forbidden from setting RUSTC_BOOTSTRAP.][cargo/9181]
[Removed support for the x86_64-rumprun-netbsd target.][82594]
[Deprecated the x86_64-sun-solaris target in favor of x86_64-pc-solaris.][82216]
[Rustdoc now only accepts ,, , and \t as delimiters for specifying
[Rustc now catches more cases of pub_use_of_private_extern_crate][80763]
[Changes in how proc macros handle whitespace may lead to panics when used
[Turn #[derive] into a regular macro attribute][79078]

Use it

rustup update stable        # get the newest release
rustup install 1.52          # or exactly this one
rustup override set 1.52     # pin a project to it

To require at least this version from users of your crate, set rust-version = "1.52" 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

[Added the unsafe_op_in_unsafe_fn lint, which checks whether the unsafe code in an unsafe fn is wrapped in a unsafe block.][79208] This lint is allowed by default, and may become a warning or hard error in a future edition.
[You can now cast mutable references to arrays to a pointer of the same type as the element.][81479]

Compiler

[Upgraded the default LLVM to LLVM 12.][81451]

Added tier 3\* support for the following targets.

[s390x-unknown-linux-musl][82166]
[riscv32gc-unknown-linux-musl & riscv64gc-unknown-linux-musl][82202]
[powerpc-unknown-openbsd][82733]

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

Libraries

[OsString now implements Extend and FromIterator.][82121]
[cmp::Reverse now has #[repr(transparent)] representation.][81879]
[Arc<impl Error> now implements error::Error.][80553]
[All integer division and remainder operations are now const.][80962]

Stabilized APIs

[Arguments::as_str]
[char::MAX]
[char::REPLACEMENT_CHARACTER]
[char::UNICODE_VERSION]
[char::decode_utf16]
[char::from_digit]
[char::from_u32_unchecked]
[char::from_u32]
[slice::partition_point]
[str::rsplit_once]
[str::split_once]

The following previously stable APIs are now const.

[char::len_utf8]
[char::len_utf16]
[char::to_ascii_uppercase]
[char::to_ascii_lowercase]
[char::eq_ignore_ascii_case]
[u8::to_ascii_uppercase]
[u8::to_ascii_lowercase]
[u8::eq_ignore_ascii_case]

Rustdoc

[Rustdoc lints are now treated as a tool lint, meaning that lints are now prefixed with rustdoc:: (e.g. #[warn(rustdoc::broken_intra_doc_links)]).][80527] Using the old style is still allowed, and will become a warning in a future release.
[Rustdoc now supports argument files.][82261]
[Rustdoc now generates smart punctuation for documentation.][79423]
[You can now use "task lists" in Rustdoc Markdown.][81766] E.g. ```markdown
[x] Complete
[ ] Todo ```

Misc

[You can now pass multiple filters to tests.][81356] E.g. cargo test -- foo bar will run all tests that match foo and bar.
[Rustup now distributes PDB symbols for the std library on Windows, allowing you to see std symbols when debugging.][82218]

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.

[Check the result cache before the DepGraph when ensuring queries][81855]
[Try fast_reject::simplify_type in coherence before doing full check][81744]
[Only store a LocalDefId in some HIR nodes][81611]
[Store HIR attributes in a side table][79519]

Compatibility Notes

1.52.1 (May 10, 2021)

This release disables incremental compilation, unless the user has explicitly opted in via the newly added RUSTC_FORCE_INCREMENTAL=1 environment variable.

This is due to the widespread, and frequently occurring, breakage encountered by Rust users due to newly enabled incremental verification in 1.52.0. Notably, Rust users **should** upgrade to 1.52.0 or 1.52.1: the bugs that are detected by newly added incremental verification are still present in past stable versions, and are not yet fixed on any channel. These bugs can lead to miscompilation of Rust binaries.

These problems only affect incremental builds, so release builds with Cargo should not be affected unless the user has explicitly opted into incremental. Debug and check builds are affected.

See 84970 for more details.

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