Version 1.2.0 (2015-08-07)

• ~1200 changes, numerous bugfixes

Highlights

• Dynamically-sized-type coercions allow smart pointer types like Rc to contain types without a fixed size, arrays and trait objects, finally enabling use of Rc<[T]> and completing the implementation of DST.
• Parallel codegen is now working again, which can substantially speed up large builds in debug mode; It also gets another ~33% speedup when bootstrapping on a 4 core machine (using 8 jobs). It's not enabled by default, but will be "in the near future". It can be activated with the -C codegen-units=N flag to rustc.
• This is the first release with experimental support for linking with the MSVC linker and lib C on Windows (instead of using the GNU variants via MinGW). It is yet recommended only for the most intrepid Rustaceans.
• Benchmark compilations are showing a 30% improvement in bootstrapping over 1.1.

Breaking Changes

• The to_uppercase and to_lowercase methods on char now do unicode case mapping, which is a previously-planned change in behavior and considered a bugfix.
• mem::align_of now specifies the minimum alignment for T, which is usually the alignment programs are interested in, and the same value reported by clang's alignof. mem::min_align_of is deprecated. This is not known to break real code.
• The #[packed] attribute is no longer silently accepted by the compiler. This attribute did nothing and code that mentioned it likely did not work as intended.
• Associated type defaults are now behind the associated_type_defaults feature gate. In 1.1 associated type defaults did not work, but could be mentioned syntactically. As such this breakage has minimal impact.

Language

• Patterns with ref mut now correctly invoke DerefMut when matching against dereferenceable values.

Libraries

• The Extend trait, which grows a collection from an iterator, is implemented over iterators of references, for String, Vec, LinkedList, VecDeque, EnumSet, BinaryHeap, VecMap, BTreeSet and BTreeMap. RFC.
• The iter::once function returns an iterator that yields a single element, and iter::empty returns an iterator that yields no elements.
• The matches and rmatches methods on str return iterators over substring matches.
• Cell and RefCell both implement Eq.
• A number of methods for wrapping arithmetic are added to the integral types, wrapping_div, wrapping_rem, wrapping_neg, wrapping_shl, wrapping_shr. These are in addition to the existing wrapping_add, wrapping_sub, and wrapping_mul methods, and alternatives to the Wrapping type.. It is illegal for the default arithmetic operations in Rust to overflow; the desire to wrap must be explicit.
• The {:#?} formatting specifier displays the alternate, pretty-printed form of the Debug formatter. This feature was actually introduced prior to 1.0 with little fanfare.
• fmt::Formatter implements fmt::Write, a fmt-specific trait for writing data to formatted strings, similar to io::Write.
• fmt::Formatter adds 'debug builder' methods, debug_struct, debug_tuple, debug_list, debug_set, debug_map. These are used by code generators to emit implementations of Debug.
• str has new to_uppercase and to_lowercase methods that convert case, following Unicode case mapping.
• It is now easier to handle poisoned locks. The PoisonError type, returned by failing lock operations, exposes into_inner, get_ref, and get_mut, which all give access to the inner lock guard, and allow the poisoned lock to continue to operate. The is_poisoned method of RwLock and Mutex can poll for a poisoned lock without attempting to take the lock.
• On Unix the FromRawFd trait is implemented for Stdio, and AsRawFd for ChildStdin, ChildStdout, ChildStderr. On Windows the FromRawHandle trait is implemented for Stdio, and AsRawHandle for ChildStdin, ChildStdout, ChildStderr.
• io::ErrorKind has a new variant, InvalidData, which indicates malformed input.

Misc

• rustc employs smarter heuristics for guessing at typos.
• rustc emits more efficient code for no-op conversions between unsafe pointers.
• Fat pointers are now passed in pairs of immediate arguments, resulting in faster compile times and smaller code.