- Added numerous language and library features, including explicit discriminants for repr(Int) enums, ..=X pattern support, stabilized numeric and collection methods, and opaque types no longer implying lifetime bounds.
- Introduced breaking changes such as const‑evaluation errors becoming hard errors and the opaque‑type lifetime behavior change, which may break existing code.
- Enhanced tooling with new cargo commands (remove, improved publish), Rustdoc HTML‑tag warnings, LLVM BOLT/LTO enabled, new tier‑3 targets and Unicode 15 support.
Rust 1.66.0
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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.