Announcing Rust 1.67.0

Stabilization of integer logarithm methods and correction of #[must_use] behavior on async functions provide meaningful standard library and compiler consistency improvements.

If you’d like to help us out by testing future releases, you might consider updating locally to use the beta channel (rustup default beta) or the nightly channel (rustup default nightly). Please report any bugs you might come across!

#[must_use] effective on async fn

async functions annotated with #[must_use] now apply that attribute to the output of the returned impl Future. The Future trait itself is already annotated with #[must_use], so all types implementing Future are automatically #[must_use], which meant that previously there was no way to indicate that the output of the Future is itself significant and should be used in some way.

With 1.67, the compiler will now warn if the output isn’t used in some way.

#[must_use]
async fn bar() -> u32 { 0 }

async fn caller() {
    bar().await;
}

warning: unused output of future returned by `bar` that must be used
 --> src/lib.rs:5:5
  |
5 |     bar().await;
  |     ^^^^^^^^^^^
  |
  = note: `#[warn(unused_must_use)]` on by default

std::sync::mpsc implementation updated

Rust’s standard library has had a multi-producer, single-consumer channel since before 1.0, but in this release the implementation is switched out to be based on crossbeam-channel. This release contains no API changes, but the new implementation fixes a number of bugs and improves the performance and maintainability of the implementation.

Users should not notice any significant changes in behavior as of this release.