I can answer the Rust question. In the ancient era, Rust source code was split into two types of files: .rc files and .rs files. The latter were "Rust source" f

I can answer the Rust question. In the ancient era, Rust source code was split into two types of files: .rc files and .rs files. The latter were "Rust source" files, and the former were "Rust crate" files. The crate files were essentially header files; they defined the public interface of the crate by explicitly listing items to be exported from the crate. Eventually explicit export lists were removed (and replaced with visibility modifiers on items themselves), and so there was no reason for .rc files to stick around, and nobody really ever objected to .rs as the now-universal extension for Rust files, since it's still a valid abbreviation.