Rust 1.28.0

Language

• The #[repr(transparent)] attribute is now stable. This attribute allows a Rust newtype wrapper (struct NewType<T>(T);) to be represented as the inner type across Foreign Function Interface (FFI) boundaries.
• The keywords pure, sizeof, alignof, and offsetof have been unreserved and can now be used as identifiers.
• The GlobalAlloc trait and #[global_allocator] attribute are now stable. This will allow users to specify a global allocator for their program.
• Unit test functions marked with the #[test] attribute can now return Result<(), E: Debug> in addition to ().
• The lifetime specifier for macro_rules! is now stable. This allows macros to easily target lifetimes.

Compiler

• The s and z optimisation levels are now stable. These optimisations prioritise making smaller binary sizes. z is the same as s with the exception that it does not vectorise loops, which typically results in an even smaller binary.
• The short error format is now stable. Specified with --error-format=short this option will provide a more compressed output of rust error messages.
• Added a lint warning when you have duplicated macro_exports.
• Reduced the number of allocations in the macro parser. This can improve compile times of macro heavy crates on average by 5%.

Libraries

• Implemented Default for &mut str.
• Implemented From<bool> for all integer and unsigned number types.
• Implemented Extend for ().
• The Debug implementation of time::Duration should now be more easily human readable. Previously a Duration of one second would printed as Duration { secs: 1, nanos: 0 } and will now be printed as 1s.
• Implemented From<&String> for Cow<str>, From<&Vec<T>> for Cow<[T]>, From<Cow<CStr>> for CString, From<CString>, From<CStr>, From<&CString> for Cow<CStr>, From<OsString>, From<OsStr>, From<&OsString> for Cow<OsStr>, From<&PathBuf> for Cow<Path>, and From<Cow<Path>> for PathBuf.
• Implemented Shl and Shr for Wrapping<u128> and Wrapping<i128>.
• DirEntry::metadata now uses fstatat instead of lstat when possible. This can provide up to a 40% speed increase.
• Improved error messages when using format!.

Stabilized APIs

• Iterator::step_by
• Path::ancestors
• SystemTime::UNIX_EPOCH
• alloc::GlobalAlloc
• alloc::Layout
• alloc::LayoutErr
• alloc::System
• alloc::alloc
• alloc::alloc_zeroed
• alloc::dealloc
• alloc::realloc
• alloc::handle_alloc_error
• btree_map::Entry::or_default
• fmt::Alignment
• hash_map::Entry::or_default
• iter::repeat_with
• num::NonZeroUsize
• num::NonZeroU128
• num::NonZeroU16
• num::NonZeroU32
• num::NonZeroU64
• num::NonZeroU8
• ops::RangeBounds
• slice::SliceIndex
• slice::from_mut
• slice::from_ref
• {Any + Send + Sync}::downcast_mut
• {Any + Send + Sync}::downcast_ref
• {Any + Send + Sync}::is

Cargo

• Cargo will now no longer allow you to publish crates with build scripts that modify the src directory. The src directory in a crate should be considered to be immutable.

Misc

• The suggestion_applicability field in rustc's json output is now stable. This will allow dev tools to check whether a code suggestion would apply to them.

Compatibility Notes

• Rust will consider trait objects with duplicated constraints to be the same type as without the duplicated constraint. For example the below code will now fail to compile.

  trait Trait {}
  
  impl Trait + Send {
      fn test(&self) { println!("one"); } //~ ERROR duplicate definitions with name `test`
  }
  
  impl Trait + Send + Send {
      fn test(&self) { println!("two"); }
  }