Language

• Stabilize #[cfg(panic = "...")] for either "unwind" or "abort".
• Stabilize #[cfg(target_has_atomic = "...")] for each integer size and "ptr".

Compiler

• Enable combining +crt-static and relocation-model=pic on x86_64-unknown-linux-gnu
• Fixes wrong unreachable_pub lints on nested and glob public reexport
• Stabilize -Z instrument-coverage as -C instrument-coverage
• Stabilize -Z print-link-args as --print link-args
• Add new Tier 3 target mips64-openwrt-linux-musl*
• Add new Tier 3 target armv7-unknown-linux-uclibceabi (softfloat)*
• Fix invalid removal of newlines from doc comments
• Add kernel target for RustyHermit
• Deny mixing bin crate type with lib crate types
• Make rustc use RUST_BACKTRACE=full by default
• Upgrade to LLVM 14

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

Libraries

• Guarantee call order for sort_by_cached_key
• Improve Duration::try_from_secs_f32/f64 accuracy by directly processing exponent and mantissa
• Make Instant::{duration_since, elapsed, sub} saturating
• Remove non-monotonic clocks workarounds in Instant::now
• Make BuildHasherDefault, iter::Empty and future::Pending covariant

Stabilized APIs

• Arc::new_cyclic
• Rc::new_cyclic
• slice::EscapeAscii
• <[u8]>::escape_ascii
• u8::escape_ascii
• Vec::spare_capacity_mut
• MaybeUninit::assume_init_drop
• MaybeUninit::assume_init_read
• i8::abs_diff
• i16::abs_diff
• i32::abs_diff
• i64::abs_diff
• i128::abs_diff
• isize::abs_diff
• u8::abs_diff
• u16::abs_diff
• u32::abs_diff
• u64::abs_diff
• u128::abs_diff
• usize::abs_diff
• Display for io::ErrorKind
• From<u8> for ExitCode
• Not for ! (the "never" type)
• _Op_Assign<$t> for Wrapping<$t>
• arch::is_aarch64_feature_detected!

Cargo

• Port cargo from toml-rs to toml_edit
• Stabilize -Ztimings as --timings
• Stabilize namespaced and weak dependency features.
• Accept more cargo:rustc-link-arg-* types from build script output.
• cargo-new should not add ignore rule on Cargo.lock inside subdirs

Misc

• Ship docs on Tier 2 platforms by reusing the closest Tier 1 platform docs
• Drop rustc-docs from complete profile
• bootstrap: tidy up flag handling for llvm build

Compatibility Notes

• Remove compiler-rt linking hack on Android
• Mitigations for platforms with non-monotonic clocks have been removed from Instant::now. On platforms that don't provide monotonic clocks, an instant is not guaranteed to be greater than an earlier instant anymore.
• Instant::{duration_since, elapsed, sub} do not panic anymore on underflow, saturating to 0 instead. In the real world the panic happened mostly on platforms with buggy monotonic clock implementations rather than catching programming errors like reversing the start and end times. Such programming errors will now results in 0 rather than a panic.
• In a future release we're planning to increase the baseline requirements for the Linux kernel to version 3.2, and for glibc to version 2.17. We'd love your feedback in PR #95026.

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.

• Switch all libraries to the 2021 edition