Language

• Unions with mutable references or tuples of allowed types are now allowed
• It is now considered valid to deallocate memory pointed to by a shared reference &T if every byte in T is inside an UnsafeCell
• Unused tuple struct fields are now warned against in an allow-by-default lint, unused_tuple_struct_fields, similar to the existing warning for unused struct fields. This lint will become warn-by-default in the future.

Compiler

• Add Nintendo Switch as tier 3 target
  • Refer to Rust's platform support page for more information on Rust's tiered platform support.
• Only compile #[used] as llvm.compiler.used for ELF targets
• Add the --diagnostic-width compiler flag to define the terminal width.
• Add support for link-flavor rust-lld for iOS, tvOS and watchOS

Libraries

• Remove restrictions on compare-exchange memory ordering.
• You can now write! or writeln! into an OsString: Implement fmt::Write for OsString
• Make RwLockReadGuard covariant
• Implement FusedIterator for std::net::[Into]Incoming
• impl<T: AsRawFd> AsRawFd for {Arc,Box}<T>
• ptr::copy and ptr::swap are doing untyped copies
• Add cgroupv1 support to available_parallelism
• Mitigate many incorrect uses of mem::uninitialized

Stabilized APIs

• future::IntoFuture
• future::poll_fn
• task::ready!
• num::NonZero*::checked_mul
• num::NonZero*::checked_pow
• num::NonZero*::saturating_mul
• num::NonZero*::saturating_pow
• num::NonZeroI*::abs
• num::NonZeroI*::checked_abs
• num::NonZeroI*::overflowing_abs
• num::NonZeroI*::saturating_abs
• num::NonZeroI*::unsigned_abs
• num::NonZeroI*::wrapping_abs
• num::NonZeroU*::checked_add
• num::NonZeroU*::checked_next_power_of_two
• num::NonZeroU*::saturating_add
• os::unix::process::CommandExt::process_group
• os::windows::fs::FileTypeExt::is_symlink_dir
• os::windows::fs::FileTypeExt::is_symlink_file

These types were previously stable in std::ffi, but are now also available in core and alloc:

• core::ffi::CStr
• core::ffi::FromBytesWithNulError
• alloc::ffi::CString
• alloc::ffi::FromVecWithNulError
• alloc::ffi::IntoStringError
• alloc::ffi::NulError

These types were previously stable in std::os::raw, but are now also available in core::ffi and std::ffi:

• ffi::c_char
• ffi::c_double
• ffi::c_float
• ffi::c_int
• ffi::c_long
• ffi::c_longlong
• ffi::c_schar
• ffi::c_short
• ffi::c_uchar
• ffi::c_uint
• ffi::c_ulong
• ffi::c_ulonglong
• ffi::c_ushort

These APIs are now usable in const contexts:

• slice::from_raw_parts

Cargo

• Packages can now inherit settings from the workspace so that the settings can be centralized in one place. See workspace.package and workspace.dependencies for more details on how to define these common settings.
• Cargo commands can now accept multiple --target flags to build for multiple targets at once, and the build.target config option may now take an array of multiple targets.
• The --jobs argument can now take a negative number to count backwards from the max CPUs.
• cargo add will now update Cargo.lock.
• Added the --crate-type flag to cargo rustc to override the crate type.
• Significantly improved the performance fetching git dependencies from GitHub when using a hash in the rev field.

Misc

• The rust-analyzer rustup component is now available on the stable channel.

Compatibility Notes

• The minimum required versions for all -linux-gnu targets are now at least kernel 3.2 and glibc 2.17, for targets that previously supported older versions: Increase the minimum linux-gnu versions
• Network primitives are now implemented with the ideal Rust layout, not the C system layout. This can cause problems when transmuting the types.
• Add assertion that transmute_copy's U is not larger than T
• A soundness bug in BTreeMap was fixed that allowed data it was borrowing to be dropped before the container.
• The Drop behavior of C-like enums cast to ints has changed. These are already discouraged by a compiler warning.
• Relate late-bound closure lifetimes to parent fn in NLL
• Errors at const-eval time are now in future incompatibility reports
• On the thumbv6m-none-eabi target, some incorrect asm! statements were erroneously accepted if they used the high registers (r8 to r14) as an input/output operand. This is no longer accepted.
• impl Trait was accidentally accepted as the associated type value of return-position impl Trait, without fulfilling all the trait bounds of that associated type, as long as the hidden type satisfies said bounds. This has been fixed.

Internal Changes

These changes do not affect any public interfaces of Rust, but they represent significant improvements to the performance or internals of rustc and related tools.

• Windows builds now use profile-guided optimization, providing 10-20% improvements to compiler performance: Utilize PGO for windows x64 rustc dist builds
• Stop keeping metadata in memory before writing it to disk
• compiletest: strip debuginfo by default for mode=ui
• Many improvements to generated code for derives, including performance improvements:
  • Don't use match-destructuring for derived ops on structs.
  • Many small deriving cleanups
  • More derive output improvements
  • Clarify deriving code
  • Final derive output improvements
  • Stop injecting #[allow(unused_qualifications)] in generated derive implementations
  • Improve derive(Debug)
• Bump to clap 3
• fully move dropck to mir
• Optimize Vec::insert for the case where index == len.
• Convert rust-analyzer to an in-tree tool