Rust 1.2: what's new, what was stabilized

Highlights

• [Dynamically-sized-type coercions][dst] 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][parcodegen] 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)][win]. 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][align], 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][packed]. 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][ad]. 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][extend-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][debugfmt] 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][strup] and [to_lowercase][strlow] 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.