WebAssembly implies a model with no syscalls. WASI provides a basic set of POSIX like syscalls which is used by the various standard libraries when the compilation target is wasm32-wasi.
So C, C++, Rust code using their respective standard libraries compiled to wasm32-wasi can run on Deno out of the box.
There is a type-checker in the works tho https://stc.dudy.dev/docs/status/