Language

• if, match, and loop expressions can now be used in const functions.
• Additionally you are now also able to coerce and cast to slices (&[T]) in const functions.
• The #[track_caller] attribute can now be added to functions to use the function's caller's location information for panic messages.
• Recursively indexing into tuples no longer needs parentheses. E.g. x.0.0 over (x.0).0.
• mem::transmute can now be used in statics and constants. Note You currently can't use mem::transmute in constant functions.

Compiler

• You can now use the cdylib target on Apple iOS and tvOS platforms.
• Enabled static "Position Independent Executables" by default for x86_64-unknown-linux-musl.

Libraries

• mem::forget is now a const fn.
• String now implements From<char>.
• The leading_ones, and trailing_ones methods have been stabilised for all integer types.
• vec::IntoIter<T> now implements AsRef<[T]>.
• All non-zero integer types (NonZeroU8) now implement TryFrom for their zero-able equivalent (e.g. TryFrom<u8>).
• &[T] and &mut [T] now implement PartialEq<Vec<T>>.
• (String, u16) now implements ToSocketAddrs.
• vec::Drain<'_, T> now implements AsRef<[T]>.

Stabilized APIs

• Option::zip
• vec::Drain::as_slice

Cargo

Added a number of new environment variables that are now available when compiling your crate.

• CARGO_BIN_NAME and CARGO_CRATE_NAME Providing the name of the specific binary being compiled and the name of the crate.
• CARGO_PKG_LICENSE The license from the manifest of the package.
• CARGO_PKG_LICENSE_FILE The path to the license file.

Compatibility Notes

• The target configuration option abi_blacklist has been renamed to unsupported_abis. The old name will still continue to work.
• Rustc will now warn if you cast a C-like enum that implements Drop. This was previously accepted but will become a hard error in a future release.
• Rustc will fail to compile if you have a struct with #[repr(i128)] or #[repr(u128)]. This representation is currently only allowed on enums.
• Tokens passed to macro_rules! are now always captured. This helps ensure that spans have the correct information, and may cause breakage if you were relying on receiving spans with dummy information.
• The InnoSetup installer for Windows is no longer available. This was a legacy installer that was replaced by a MSI installer a few years ago but was still being built.
• {f32, f64}::asinh now returns the correct values for negative numbers.
• Rustc will no longer accept overlapping trait implementations that only differ in how the lifetime was bound.
• Rustc now correctly relates the lifetime of an existential associated type. This fixes some edge cases where rustc would erroneously allow you to pass a shorter lifetime than expected.
• Rustc now dynamically links to libz (also called zlib) on Linux. The library will need to be installed for rustc to work, even though we expect it to be already available on most systems.
• Tests annotated with #[should_panic] are broken on ARMv7 while running under QEMU.
• Pretty printing of some tokens in procedural macros changed. The exact output returned by rustc's pretty printing is an unstable implementation detail: we recommend any macro relying on it to switch to a more robust parsing system.