Rust 1.99: C variadics, Vec::into_parts, and a Cargo debug profile

Rust 1.99.0 shipped on 1 October 2026 with stabilized C variadics, a set of allocation conveniences in the standard library, a new Cargo profile, and a handful of compatibility notes worth a scan before you bump your toolchain. This review covers the language, the standard library, Cargo, and the upgrade notes, based on the official release notes in RELEASES.md.

Language

The headline change is the stabilization of C-variadic function definitions. Rust can now declare functions that take a C-style variadic argument list, and #[unsafe(naked)] functions can define them too. This helps FFI code that wraps C libraries which accept variable arguments.

Outlined modules (mod foo;) are now allowed anywhere in the body of a custom attribute or derive macro, which removes a real limitation for proc-macro and derive authors.

A new allow-by-default lint, raw_borrows_via_references, flags references that decay immediately into raw borrows. It is off by default, so it will not break your build.

Rust now guarantees that the contents of an UnsafeCell can be accessed without going through get, and the invalid_reference_casting lint was adjusted to match.

Standard library

The most useful additions are allocation-adjacent. Vec::into_parts and Vec::from_parts decompose and rebuild a Vec from its raw pointer, length, and capacity. Box::into_non_null and Box::from_non_null do the same for boxes. String::from_utf8_lossy_owned returns an owned String from a lossy UTF-8 conversion.

IntoIterator is now implemented for Box<[T; N]> and its references. VecDeque::retain_back fills a gap in the deque API. std::fs::set_times and std::fs::set_times_nofollow let you set file timestamps.

Cargo

Cargo gains a new built-in debug profile. It is currently identical to dev; the point is to prepare for a future where dev is reworked for faster iteration and debug becomes the place for actual debugging.

On edition 2024 and later, a workspace member can override an inherited dependency’s default-features. So serde = { workspace = true, default-features = false } now actually turns off default features even when the workspace root enables them. On earlier editions the override is ignored with a warning.

Performance and platforms

rustdoc’s trait-impl filtering got smarter, with performance improvements of about 20 percent on average and up to 40 percent on some real-world crates. Documentation builds should get a little faster.

The riscv64-unknown-linux-musl target is promoted to Tier 2 with host tools.

RangeInclusive (a..=b) iteration is now optimized better in some circumstances. A side effect: the behavior of a RangeInclusive that has already been exhausted as an iterator changed. The start() and end() methods on such a range may return different values, and using such a range as a slice index may behave differently. These behaviors were never guaranteed stable, so the change is not treated as breaking.

What to check before upgrading

The legacy integral modules are now fully deprecated. std::i32::MAX should be i32::MAX.

no_mangle_generic_items is now a hard error.

Pin::new_unchecked had its safety invariants changed slightly, so unsafe code that calls it deserves a re-read.

In Cargo, incremental compilation is now disabled by default when running in CI, which is detected via the CI environment variable.

The Rust Reference no longer recommends matching a union alongside another value in a single pattern, the pattern used in a manually-written tagged union. The compiler can read the union contents before checking the rest of the pattern, which can cause undefined behavior. This is guidance in the Reference, not a compiler change.

Internally, the compiler moved to LLVM 23. It is not visible in your code, but it can surface as a subtle codegen difference, so keep it in mind if a benchmark moves after upgrading.