--- title: Rust 1.99.0 stabilizes C-variadic function definitions... description: Rust 1.99.0 is scheduled for release on October 1, with a pre-release build already available for testing. The release stabilizes C-variadic function... canonical: https://daily.dev/posts/rust-1-99-0-stabilizes-c-variadic-function-definitions-and-adds-new-standard-library-apis-v6elwytnz twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Rust 1.99.0 stabilizes C-variadic function definitions and adds new standard library APIs | daily.dev og:description: Rust 1.99.0 is scheduled for release on October 1, with a pre-release build already available for testing. The release stabilizes C-variadic function... og:url: https://daily.dev/posts/rust-1-99-0-stabilizes-c-variadic-function-definitions-and-adds-new-standard-library-apis-v6elwytnz og:image: https://api.daily.dev/og/posts/V6ELWytnZ.png og:image:alt: Rust 1.99.0 stabilizes C-variadic function definitions and adds new standard library APIs og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # Rust 1.99.0 stabilizes C-variadic function definitions and adds new standard library APIs **[Collections](https://daily.dev/sources/collections)** · 1 min read · 11 upvotes · 0 comments ## Summary Rust 1.99.0 is scheduled for release on October 1, with a pre-release build already available for testing. The release stabilizes C-variadic function definitions using extern "C" and extern "C-unwind", introducing a new VaList type and VaArgSafe trait. It also settles safety requirements for reading size and alignment from raw pointers to unsized types, updates Box::leak documentation to recommend Box::into_non_null over round-trip unleaking, and stabilizes several new standard library APIs including IntoIterator for Box<[T; N]>, VecDeque::retain_back, Vec::into_parts/from_parts, and std::fs::set_times. Testers can install the pre-release via rustup pointed at a custom dist server and report issues on the internals forum or a tracking GitHub issue. ## Content Rust 1.99.0 is out. It was scheduled for October 1 and went through the usual pre-release testing first. Anyone who wants to try an upcoming release can point rustup at a custom dist server to get the pre-release build locally, then report problems in the internals forum thread or on the GitHub issue that tracks the pre-release process. ## Variadic functions in Rust You can now define C-ABI variadic functions directly in Rust, using `extern "C"` or `extern "C-unwind"`. Two new items support this: the `VaList` type and the `VaArgSafe` trait. ## Size and alignment from raw pointers The release adds functions that read the size and alignment of the value behind a raw pointer. The safety requirements for doing this with pointers to unsized types are now settled. ## Box::leak documentation The docs now recommend against round-tripping to "unleak" after `Box::leak`. They point to `Box::into_non_null` instead. ## Newly stabilized APIs - `IntoIterator` for `Box<[T; N]>` - `VecDeque::retain_back` - `Vec::into_parts` and `Vec::from_parts` - `std::fs::set_times` The Cargo and Clippy changelogs have their own lists of changes for this release. ## Questions this post answers ### How do I define a C-ABI variadic function in Rust? As of Rust 1.99.0, you can define C-ABI variadic functions directly using extern "C" and extern "C-unwind" function signatures. This stabilization comes with a new VaList type for accessing the variadic arguments and a VaArgSafe trait that marks which types are safe to pass through a variadic interface. _Rust developers tracking FFI changes like this can follow release updates on daily.dev._ ### Should I round-trip unleak memory after calling Box::leak in Rust? No, the updated Box::leak documentation in Rust 1.99.0 recommends against round-trip unleaking a value after leaking it. Instead, it points developers to Box::into_non_null as the preferred way to handle this pattern, since re-boxing a leaked reference carries safety pitfalls the docs now call out explicitly. _Developers refining memory safety patterns in Rust can keep up with API guidance via daily.dev._ ### What new standard library APIs were stabilized in Rust 1.99.0? Rust 1.99.0 stabilizes IntoIterator for Box<[T; N]>, VecDeque::retain_back, Vec::into_parts and Vec::from_parts, and std::fs::set_times. These additions give more direct ways to iterate over boxed arrays, selectively retain elements from the back of a VecDeque, decompose or reconstruct a Vec from its raw parts, and set file timestamps. _Those upgrading Rust toolchains can scan new stdlib APIs like these through daily.dev._ ## Similar posts on daily.dev - [Announcing Rust 1.97.0](https://daily.dev/posts/announcing-rust-1-97-0-fqxhq3pfj) · Rust · 22 upvotes · 0 comments - [Rust introduces new Range types](https://daily.dev/posts/rust-introduces-new-range-types-txhclns35) · InfoWorld · 25 upvotes · 0 comments --- Tags: [#rust](https://daily.dev/tags/rust) [View this post on daily.dev](https://daily.dev/posts/rust-1-99-0-stabilizes-c-variadic-function-definitions-and-adds-new-standard-library-apis-v6elwytnz) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"Rust 1.99.0 stabilizes C-variadic function definitions and adds new standard library APIs","url":"https://daily.dev/posts/rust-1-99-0-stabilizes-c-variadic-function-definitions-and-adds-new-standard-library-apis-v6elwytnz","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/rust-1-99-0-stabilizes-c-variadic-function-definitions-and-adds-new-standard-library-apis-v6elwytnz"},"datePublished":"2026-10-01T12:47:10.714Z","dateModified":"2026-10-01T13:30:54.698Z","description":"Rust 1.99.0 is scheduled for release on October 1, with a pre-release build already available for testing. 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