Version 1.9.0 (2016-05-26)

Language

The #[deprecated] attribute when applied to an API will generate warnings when used. The warnings may be suppressed with #[allow(deprecated)]. RFC 1270.
fn item types are zero sized, and each fn names a unique type. This will break code that transmutes fns, so calling transmute on a fn type will generate a warning for a few cycles, then will be converted to an error.
Field and method resolution understand visibility, so private fields and methods cannot prevent the proper use of public fields and methods.
The parser considers unicode codepoints in the PATTERN_WHITE_SPACE category to be whitespace.

Stabilized APIs

Libraries

Cargo

Cargo can now run concurrently.
Top-level overrides allow specific revisions of crates to be overridden through the entire crate graph. This is intended to make upgrades easier for large projects, by allowing crates to be forked temporarily until they've been upgraded and republished.
Cargo exports a CARGO_PKG_AUTHORS environment variable.
Cargo will pass the contents of the RUSTFLAGS variable to rustc on the commandline. rustc arguments can also be specified in the build.rustflags configuration key.

Performance

The time complexity of comparing variables for equivalence during type unification is reduced from O(n!) to O(n). This leads to major compilation time improvement in some scenarios.
ToString is specialized for str, giving it the same performance as to_owned.
Spawning processes with Command::output no longer creates extra threads.
#[derive(PartialEq)] and #[derive(PartialOrd)] emit less code for C-like enums.

Misc

Passing the --quiet flag to a test runner will produce much-abbreviated output.
The Rust Project now publishes std binaries for the mips-unknown-linux-musl, mipsel-unknown-linux-musl, and i586-pc-windows-msvc targets.

Compatibility Notes

std::sync::Once is poisoned if its initialization function fails.
It is illegal to define methods with the same name in overlapping inherent impl blocks.
fn item types are zero sized, and each fn names a unique type. This will break code that transmutes fns, so calling transmute on a fn type will generate a warning for a few cycles, then will be converted to an error.
Improvements to const evaluation may trigger new errors when integer literals are out of range.