Rust 1.99 adds C-variadic functions, new APIs and LLVM 23 support

Rust 1.99.0 is out, with changes for developers across the language, standard library and tooling. The practical mix includes stable C-ABI variadic function definitions, new standard-library APIs and support for LLVM 23, alongside a compiler error that may catch some existing code.

Watch Desk analysis

What happened

The release was published on 1 October. Its official release notes cover updates to the language, compiler, libraries, Cargo, Clippy and Rustdoc.

Our top picks

• C-ABI variadic functions are stable
  Developers can define variadic functions using the C and C-unwind ABIs.
• A stricter check for nomangle
  Applying [nomangle] to a generic function now produces a hard build error instead of a warning, so affected projects may need changes.
• More ways to work with collections and ownership
  New APIs include Vec::intoparts, Box::intononnull, VecDeque::retainback and IntoIterator for boxed arrays.
• File timestamps get a standard-library API
  std::fs::settimes lets code set file times through the standard library.
• LLVM 23 support arrives
  The release adds support for the new LLVM version, alongside other platform and tooling improvements.

Why it matters

There is useful day-to-day material here, but also a compatibility detail worth noticing before an upgrade: generic functions marked [nomangle] now fail the build rather than merely prompting a warning. The new APIs and C-variadic support offer more options for library and systems developers; LLVM 23 support matters to toolchain users.

Our read

This is a substantive language-and-toolchain release, not just a version number with a fresh coat of paint. Rust users should check the release notes and test projects that use [nomangle] on generic functions before updating. The compiler has made its opinion rather harder to ignore.

What to watch

• Whether projects need changes for the new [nomangle] error.
• Which new stable APIs prove useful in libraries and applications.
• How LLVM 23 support affects toolchain and platform compatibility.

Discussion spark: Should Rust make questionable code a hard error as soon as possible, or is keeping it as a warning the better way to protect upgrade paths?

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