Rust 1.39 ships on Thursday!

Backport nominations

  • “save-analysis: Account for async desugaring in async fn return types” #65936

    • Accepted for beta backport
  • “resolve: Turn the “non-empty glob must import something” error into a lint” #65539

    • Accepted for beta backport

Working group sync


  • PGO is available in the stable compiler. Docs are in the rustc-dev-guide and the rustc-book

  • Unfortunately we don’t observe significant performance gains from applying it (except for tiny synthetic test cases).

  • @michaelwoerister asked for people to try PGO on irlo but haven’t gotten a lot of feedback.

  • There is one interesting theory that Rust doesn’t profit as much as C++ because it defaults to having fewer compilation units and thus can make better inlining decisions in the non-PGO case.

  • The working group will probably wind down as @michaelwoerister don’t have time to pursue further and there are no actual known issues in the implementation.

Link to full discussion


  • Pipelining support has shipped in Rust 1.38.

  • The working group is winding down.

Link to full discussion