You can easily write and call functions that abide the C ABI in Rust, but the set of types permitted in those signatures is much smaller (only #[repr(C)]-compat

You can easily write and call functions that abide the C ABI in Rust, but the set of types permitted in those signatures is much smaller (only #[repr(C)]-compatible types) than in ordinary Rust functions. The Rust ABI is more complicated and won't be stabilized anytime soon.