Language

• You can now use macros for values in some built-in attributes. This primarily allows you to call macros within the #[doc] attribute. For example, to include external documentation in your crate, you can now write the following:

  #![doc = include_str!("README.md")]
  

• You can now cast between unsized slice types (and types which contain unsized slices) in const fn.

• You can now use multiple generic lifetimes with impl Trait where the lifetimes don't explicitly outlive another. In code this means that you can now have impl Trait<'a, 'b> where as before you could only have impl Trait<'a, 'b> where 'b: 'a.

Compiler

• Rustc will now search for custom JSON targets in /lib/rustlib/<target-triple>/target.json where / is the "sysroot" directory. You can find your sysroot directory by running rustc --print sysroot.
• Added wasm as a target_family for WebAssembly platforms.
• You can now use #[target_feature] on safe functions when targeting WebAssembly platforms.
• Improved debugger output for enums on Windows MSVC platforms.
• Added tier 3* support for bpfel-unknown-none and bpfeb-unknown-none.
• -Zmutable-noalias=yes is enabled by default when using LLVM 12 or above.

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

Libraries

• panic::panic_any will now #[track_caller].
• Added OutOfMemory as a variant of io::ErrorKind.
• proc_macro::Literal now implements FromStr.
• The implementations of vendor intrinsics in core::arch have been significantly refactored. The main user-visible changes are a 50% reduction in the size of libcore.rlib and stricter validation of constant operands passed to intrinsics. The latter is technically a breaking change, but allows Rust to more closely match the C vendor intrinsics API.

Stabilized APIs

• BTreeMap::into_keys
• BTreeMap::into_values
• HashMap::into_keys
• HashMap::into_values
• arch::wasm32
• VecDeque::binary_search
• VecDeque::binary_search_by
• VecDeque::binary_search_by_key
• VecDeque::partition_point

Cargo

• Added the --prune <spec> option to cargo-tree to remove a package from the dependency graph.
• Added the --depth option to cargo-tree to print only to a certain depth in the tree
• Added the no-proc-macro value to cargo-tree --edges to hide procedural macro dependencies.
• A new environment variable named CARGO_TARGET_TMPDIR is available. This variable points to a directory that integration tests and benches can use as a "scratchpad" for testing filesystem operations.

Compatibility Notes

• Mixing Option and Result via ? is no longer permitted in closures for inferred types.
• Previously unsound code is no longer permitted where different constructors in branches could require different lifetimes.
• As previously mentioned the std::arch intrinsics now uses stricter const checking than before and may reject some previously accepted code.
• i128 multiplication on Cortex M0+ platforms currently unconditionally causes overflow when compiled with codegen-units = 1.