Language

• const fn signatures can now include generic trait bounds
• const fn signatures can now use impl Trait in argument and return position
• Function pointers can now be created, cast, and passed around in a const fn
• Recursive calls can now set the value of a function's opaque impl Trait return type

Compiler

• Linking modifier syntax in #[link] attributes and on the command line, as well as the whole-archive modifier specifically, are now supported
• The char type is now described as UTF-32 in debuginfo
• The #[target_feature] attribute can now be used with aarch64 features
• X86 #[target_feature = "adx"] is now stable

Libraries

• ManuallyDrop<T> is now documented to have the same layout as T
• #[ignore = "…"] messages are printed when running tests
• Consistently show absent stdio handles on Windows as NULL handles
• Make std::io::stdio::lock() return 'static handles. Previously, the creation of locked handles to stdin/stdout/stderr would borrow the handles being locked, which prevented writing let out = std::io::stdout().lock(); because out would outlive the return value of stdout(). Such code now works, eliminating a common pitfall that affected many Rust users.
• Vec::from_raw_parts is now less restrictive about its inputs
• std::thread::available_parallelism now takes cgroup quotas into account. Since available_parallelism is often used to create a thread pool for parallel computation, which may be CPU-bound for performance, available_parallelism will return a value consistent with the ability to use that many threads continuously, if possible. For instance, in a container with 8 virtual CPUs but quotas only allowing for 50% usage, available_parallelism will return 4.

Stabilized APIs

• Pin::static_mut
• Pin::static_ref
• Vec::retain_mut
• VecDeque::retain_mut
• Write for Cursor<[u8; N]>
• std::os::unix::net::SocketAddr::from_pathname
• std::process::ExitCode and std::process::Termination. The stabilization of these two APIs now makes it possible for programs to return errors from main with custom exit codes.
• std::thread::JoinHandle::is_finished

These APIs are now usable in const contexts:

• <*const T>::offset and <*mut T>::offset
• <*const T>::wrapping_offset and <*mut T>::wrapping_offset
• <*const T>::add and <*mut T>::add
• <*const T>::sub and <*mut T>::sub
• <*const T>::wrapping_add and <*mut T>::wrapping_add
• <*const T>::wrapping_sub and <*mut T>::wrapping_sub
• <[T]>::as_mut_ptr
• <[T]>::as_ptr_range
• <[T]>::as_mut_ptr_range

Cargo

No feature changes, but see compatibility notes.

Compatibility Notes

• Previously native static libraries were linked as whole-archive in some cases, but now rustc tries not to use whole-archive unless explicitly requested. This change may result in linking errors in some cases. To fix such errors, native libraries linked from the command line, build scripts, or #[link] attributes need to
  • (more common) either be reordered to respect dependencies between them (if a depends on b then a should go first and b second)
  • (less common) or be updated to use the +whole-archive modifier.
• Catching a second unwind from FFI code while cleaning up from a Rust panic now causes the process to abort
• Proc macros no longer see ident matchers wrapped in groups
• The number of # in r# raw string literals is now required to be less than 256
• When checking that a dyn type satisfies a trait bound, supertrait bounds are now enforced
• cargo vendor now only accepts one value for each --sync flag
• cfg predicates in all() and any() are always evaluated to detect errors, instead of short-circuiting. The compatibility considerations here arise in nightly-only code that used the short-circuiting behavior of all to write something like cfg(all(feature = "nightly", syntax-requiring-nightly)), which will now fail to compile. Instead, use either cfg_attr(feature = "nightly", ...) or nested uses of cfg.
• bootstrap: static-libstdcpp is now enabled by default, and can now be disabled when llvm-tools is enabled

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.

• debuginfo: Refactor debuginfo generation for types
• Remove the everybody loops pass