Well of course a hello world Rust project would just compile fine since Rust 1.0, unlike my recent Rust project which broke again and that just broke on a minor version bump.

As for another ancient Rust project which was also on 1.0 when I was trying the language out, the borrow checker on 1.52.1 screeched at every line of my code.

While C++ is just as bad, the fact that Firefox must always use the latest Rust compiler gives you a hint that it will not work on lower versions; hence the need of a MSRV. (Minimum Supported Rust Version)