Version 1.39.0 (2019-11-07)

Language

• You can now create async functions and blocks with async fn, async move {}, and async {} respectively, and you can now call .await on async expressions.
• You can now use certain attributes on function, closure, and function pointer parameters. These attributes include cfg, cfg_attr, allow, warn, deny, forbid as well as inert helper attributes used by procedural macro attributes applied to items. e.g.

  #![allow(unused)]
  fn main() {
  fn len(
      #[cfg(windows)] slice: &[u16],
      #[cfg(not(windows))] slice: &[u8],
  ) -> usize {
      slice.len()
  }
  }

• You can now take shared references to bind-by-move patterns in the if guards of match arms. e.g.

  fn main() {
      let array: Box<[u8; 4]> = Box::new([1, 2, 3, 4]);
  
      match array {
          nums
  //      ---- `nums` is bound by move.
              if nums.iter().sum::<u8>() == 10
  //                 ^------ `.iter()` implicitly takes a reference to `nums`.
          => {
              drop(nums);
  //          ----------- Legal as `nums` was bound by move and so we have ownership.
          }
          _ => unreachable!(),
      }
  }

Compiler

• Added tier 3* support for the i686-unknown-uefi target.
• Added tier 3 support for the sparc64-unknown-openbsd target.
• rustc will now trim code snippets in diagnostics to fit in your terminal. Note Cargo currently doesn't use this feature. Refer to cargo#7315 to track this feature's progress.
• You can now pass --show-output argument to test binaries to print the output of successful tests.

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

Libraries

• Vec::new and String::new are now const functions.
• LinkedList::new is now a const function.
• str::len, [T]::len and str::as_bytes are now const functions.
• The abs, wrapping_abs, and overflowing_abs numeric functions are now const.

Stabilized APIs

• Pin::into_inner
• Instant::checked_duration_since
• Instant::saturating_duration_since

Cargo

• You can now publish git dependencies if supplied with a version.
• The --all flag has been renamed to --workspace. Using --all is now deprecated.

Misc

• You can now pass -Clinker to rustdoc to control the linker used for compiling doctests.

Compatibility Notes

• Code that was previously accepted by the old borrow checker, but rejected by the NLL borrow checker is now a hard error in Rust 2018. This was previously a warning, and will also become a hard error in the Rust 2015 edition in the 1.40.0 release.
• rustdoc now requires rustc to be installed and in the same directory to run tests. This should improve performance when running a large amount of doctests.
• The try! macro will now issue a deprecation warning. It is recommended to use the ? operator instead.
• asinh(-0.0) now correctly returns -0.0. Previously this returned 0.0.