Language

• Macro attributes may follow #[derive] and will see the original (pre-cfg) input.
• Accept curly-brace macros in expressions, like m!{ .. }.method() and m!{ .. }?.
• Allow panicking in constant evaluation.
• Ignore derived Clone and Debug implementations during dead code analysis.

Compiler

• Create more accurate debuginfo for vtables.
• Add armv6k-nintendo-3ds at Tier 3*.
• Add armv7-unknown-linux-uclibceabihf at Tier 3*.
• Add m68k-unknown-linux-gnu at Tier 3*.
• Add SOLID targets at Tier 3*: aarch64-kmc-solid_asp3, armv7a-kmc-solid_asp3-eabi, armv7a-kmc-solid_asp3-eabihf

* Refer to Rust's platform support page for more information on Rust's tiered platform support.

Libraries

• Avoid allocations and copying in Vec::leak
• Add #[repr(i8)] to Ordering
• Optimize File::read_to_end and read_to_string
• Update to Unicode 14.0
• Many more functions are marked #[must_use], producing a warning when ignoring their return value. This helps catch mistakes such as expecting a function to mutate a value in place rather than return a new value.

Stabilised APIs

• [T; N]::as_mut_slice
• [T; N]::as_slice
• collections::TryReserveError
• HashMap::try_reserve
• HashSet::try_reserve
• String::try_reserve
• String::try_reserve_exact
• Vec::try_reserve
• Vec::try_reserve_exact
• VecDeque::try_reserve
• VecDeque::try_reserve_exact
• Iterator::map_while
• iter::MapWhile
• proc_macro::is_available
• Command::get_program
• Command::get_args
• Command::get_envs
• Command::get_current_dir
• CommandArgs
• CommandEnvs

These APIs are now usable in const contexts:

• hint::unreachable_unchecked

Cargo

• Stabilize custom profiles

Compatibility notes

• Ignore derived Clone and Debug implementations during dead code analysis. This will break some builds that set #![deny(dead_code)].

Internal changes

These changes provide no direct user facing benefits, but represent significant improvements to the internals and overall performance of rustc and related tools.

• Added an experimental backend for codegen with libgccjit.