- Added coinductive sized predicates, async fn future output annotations, and varargs support for non‑C call conventions; stabilized many const APIs (integer ilog, char conversions) and merged crossbeam channel into std::sync::mpsc.
- Optimized MIR constant propagation, field‑ordering layout, and LLVM archive writer; expanded platform support with new tier‑3 targets and promoted UEFI to tier‑2.
- Breaking changes: repr(Rust) layout may increase type sizes, underscore‑suffixed string literals are now hard errors, and Cargo now errors on duplicate registry indices; 0.5 rounding now follows tie‑to‑even.
Rust 1.67.0
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Language: allow shortening lifetimes of &mut during unsize coercions and introduce new lints for invalid runtime symbols and c void return types.