Computed from the interface. The metadata does not understand semver or any of the higher level versioning tools Cargo exposes to users.

The metadata just contains info on all the types, and their hashes (or something like that), so if stuff doesn't match you'll know.

This is generally visible from the "expected type Foo but found type Foo" error, which will often mention you have two versions of the same crate.

Worth mentioning that unlike C++ or C Rust doesn't have a global name mangling scheme; so it is totally ok for two crates to have a toplevel struct Foo (unlike in C++ where you are forced to namespace them with uniquely-named namespaces). This has the side effect of it being totally ok to link two versions of the same crate together; and Rust will just complain if you try to mix the types.