By linking both and not allowing mixing types, i.e. it considers types from a totally unrelated with types from b.
Also, Rust compiles the whole world at once, so any ABI breakage from mixing code from different compiler versions doesn't happen. (Editions are different thing from compiler versions, a single version of the compiler supports multiple editions.)
Rust does not compile the whole world at once. Each crate is compiled separately, and then they’re all linked together at the end.
Yeah, I know, but I mean that it's normal to link together crates compiled with the same compiler, unlike with C, where ABIs are stabler and binary dependencies are more common.