An important bugfix in 1.39.0 that wasn't mentioned in the release announcement is probably: https://github.com/rust-lang/rust/pull/63402

It's important because unless rustfmt is available, bindgen would put everything onto one line: https://github.com/rust-lang/rust-bindgen/issues/1600

Rustc would generate (unrelated) warnings for the generated code but before the bugfix it'd print the entire line, hitting the maximum log size limit on CI and causing the build to fail because of that.