Note that Rust code can interoperate with other languages by exposing a C interface and having the host language call it as though it were C. There's no runtime to start up or anything, so to the host platform there's no discernible difference (though you may need some machinery in your build system). This is how Rust-written libraries have been integrated into Firefox (which is obviously still integrated into Firefox), and there are companies that have been using Rust to extend Ruby for years now.