The 1.40.0 pre-release is ready for testing. Rust 1.40.0 is scheduled to be released on Thursday December 19th. Release notes can be found here: https://github.com/rust-lang/rust/blob/master/RELEASES.md ; note that we forgot to backport release notes to stable this time around, and barring other bugs (that make us reproduce the artifacts) it probably won't be in the tarballs.
You can try it out locally with:
RUSTUP_DIST_SERVER=https://dev-static.rust-lang.org rustup update stable