Language

• The unsafe keyword is now syntactically permitted on modules. This is still rejected semantically, but can now be parsed by procedural macros.

Compiler

• Stabilised the -C link-self-contained=<yes|no> compiler flag. This tells rustc whether to link its own C runtime and libraries or to rely on a external linker to find them. (Supported only on windows-gnu, linux-musl, and wasi platforms.)
• You can now use -C target-feature=+crt-static on linux-gnu targets. Note: If you're using cargo you must explicitly pass the --target flag.
• Added tier 2* support for aarch64-unknown-linux-musl.

* Refer to Rust's platform support page for more information on Rust's tiered platform support.

Libraries

• io::Write is now implemented for &ChildStdin &Sink, &Stdout, and &Stderr.
• All arrays of any length now implement TryFrom<Vec<T>>.
• The matches! macro now supports having a trailing comma.
• Vec<A> now implements PartialEq<[B]> where A: PartialEq<B>.
• The RefCell::{replace, replace_with, clone} methods now all use #[track_caller].

Stabilized APIs

• slice::as_ptr_range
• slice::as_mut_ptr_range
• VecDeque::make_contiguous
• future::pending
• future::ready

The following previously stable methods are now const fn's:

• Option::is_some
• Option::is_none
• Option::as_ref
• Result::is_ok
• Result::is_err
• Result::as_ref
• Ordering::reverse
• Ordering::then

Cargo

Rustdoc

• You can now link to items in rustdoc using the intra-doc link syntax. E.g. /// Uses [`std::future`] will automatically generate a link to std::future's documentation. See "Linking to items by name" for more information.
• You can now specify #[doc(alias = "<alias>")] on items to add search aliases when searching through rustdoc's UI.

Compatibility Notes

• Promotion of references to 'static lifetime inside const fn now follows the same rules as inside a fn body. In particular, &foo() will not be promoted to 'static lifetime any more inside const fns.
• Associated type bindings on trait objects are now verified to meet the bounds declared on the trait when checking that they implement the trait.
• When trait bounds on associated types or opaque types are ambiguous, the compiler no longer makes an arbitrary choice on which bound to use.
• Fixed recursive nonterminals not being expanded in macros during pretty-print/reparse check. This may cause errors if your macro wasn't correctly handling recursive nonterminal tokens.
• &mut references to non zero-sized types are no longer promoted.
• rustc will now warn if you use attributes like #[link_name] or #[cold] in places where they have no effect.
• Updated _mm256_extract_epi8 and _mm256_extract_epi16 signatures in arch::{x86, x86_64} to return i32 to match the vendor signatures.
• mem::uninitialized will now panic if any inner types inside a struct or enum disallow zero-initialization.
• #[target_feature] will now error if used in a place where it has no effect.
• Foreign exceptions are now caught by catch_unwind and will cause an abort. Note: This behaviour is not guaranteed and is still considered undefined behaviour, see the catch_unwind documentation for further information.

Internal Only

These changes provide no direct user facing benefits, but represent significant improvements to the internals and overall performance of rustc and related tools.

• Building rustc from source now uses ninja by default over make. You can continue building with make by setting ninja=false in your bootstrap.toml.
• cg_llvm: fewer_names in uncached_llvm_type
• Made ensure_sufficient_stack() non-generic