This is a version of Rust that's closer to what Mozilla is recommending for compiling Firefox, Seamonkey, and Thunderbird. To see the recommended minimum and maximum versions of Rust that upstream recommends for Firefox (or for Seamonkey/Thunderbird using the backend from the same version of Firefox), see this link: https://firefox-source-docs.mozilla.org/writing-rust-code/update-policy.html This version of Rust has been compiled with the bundled version of LLVM to avoid depending on any particular version of LLVM. This Rust package will be updated in place to follow the requirements for the various Mozilla packages in the main tree (for -current), or in /patches (for a stable release).