You can assume that unless you have some specific information to the contrary, any Rust binary you encounter in real life was built with rustc. mrustc is not us

You can assume that unless you have some specific information to the contrary, any Rust binary you encounter in real life was built with rustc. mrustc is not used for any mainstream purpose other than in the bootstrap chain of rustc in distros like Guix that care about reproducibility, and even then, the build of rustc they distribute to users will be re-built from the bootstrapped rustc, it won’t be one compiled by mrustc directly.