Version 1.51.0 (2021-03-25)

Language

• You can now parameterize items such as functions, traits, and structs by constant values in addition to by types and lifetimes. Also known as "const generics" E.g. you can now write the following. Note: Only values of primitive integers, bool, or char types are currently permitted.

  #![allow(unused)]
  fn main() {
  struct GenericArray<T, const LENGTH: usize> {
      inner: [T; LENGTH]
  }
  
  impl<T, const LENGTH: usize> GenericArray<T, LENGTH> {
      const fn last(&self) -> Option<&T> {
          if LENGTH == 0 {
              None
          } else {
              Some(&self.inner[LENGTH - 1])
          }
      }
  }
  }

Compiler

• Added the -Csplit-debuginfo codegen option for macOS platforms. This option controls whether debug information is split across multiple files or packed into a single file. Note This option is unstable on other platforms.
• Added tier 3* support for aarch64_be-unknown-linux-gnu, aarch64-unknown-linux-gnu_ilp32, and aarch64_be-unknown-linux-gnu_ilp32 targets.
• Added tier 3 support for i386-unknown-linux-gnu and i486-unknown-linux-gnu targets.
• The target-cpu=native option will now detect individual features of CPUs.

* Refer to Rust's [platform support page][platform-support-doc] for more information on Rust's tiered platform support.

Libraries

• Box::downcast is now also implemented for any dyn Any + Send + Sync object.
• str now implements AsMut<str>.
• u64 and u128 now implement From<char>.
• Error is now implemented for &T where T implements Error.
• Poll::{map_ok, map_err} are now implemented for Poll<Option<Result<T, E>>>.
• unsigned_abs is now implemented for all signed integer types.
• io::Empty now implements io::Seek.
• rc::Weak<T> and sync::Weak<T>'s methods such as as_ptr are now implemented for T: ?Sized types.
• Div and Rem by their NonZero variant is now implemented for all unsigned integers.

Stabilized APIs

• Arc::decrement_strong_count
• Arc::increment_strong_count
• Once::call_once_force
• Peekable::next_if_eq
• Peekable::next_if
• Seek::stream_position
• array::IntoIter
• panic::panic_any
• ptr::addr_of!
• ptr::addr_of_mut!
• slice::fill_with
• slice::split_inclusive_mut
• slice::split_inclusive
• slice::strip_prefix
• slice::strip_suffix
• str::split_inclusive
• sync::OnceState
• task::Wake
• VecDeque::range
• VecDeque::range_mut

Cargo

• Added the split-debuginfo profile option to control the -Csplit-debuginfo codegen option.
• Added the resolver field to Cargo.toml to enable the new feature resolver and CLI option behavior. Version 2 of the feature resolver will try to avoid unifying features of dependencies where that unification could be unwanted. Such as using the same dependency with a std feature in a build scripts and proc-macros, while using the no-std feature in the final binary. See the Cargo book documentation for more information on the feature.

Rustdoc

• Rustdoc will now include documentation for methods available from nested Deref traits.
• You can now provide a --default-theme flag which sets the default theme to use for documentation.

Various improvements to intra-doc links:

• You can link to non-path primitives such as slice.
• You can link to associated items.
• You can now include generic parameters when linking to items, like Vec<T>.

Misc

• You can now pass --include-ignored to tests (e.g. with cargo test -- --include-ignored) to include testing tests marked #[ignore].

Compatibility Notes

• WASI platforms no longer use the wasm-bindgen ABI, and instead use the wasm32 ABI.
• rustc no longer promotes division, modulo and indexing operations to const that could fail.
• The minimum version of glibc for the following platforms has been bumped to version 2.31 for the distributed artifacts.
  • armv5te-unknown-linux-gnueabi
  • sparc64-unknown-linux-gnu
  • thumbv7neon-unknown-linux-gnueabihf
  • armv7-unknown-linux-gnueabi
  • x86_64-unknown-linux-gnux32
• atomic::spin_loop_hint has been deprecated. It's recommended to use hint::spin_loop instead.

Internal Only

• Consistently avoid constructing optimized MIR when not doing codegen