> Theoretically it could tell by introspecting the function body, but the compiler only works on signatures

Note that this is an intentional choice rather than a limitation, because if the compiler analyzed the function body to determine lifetimes of parameters and return values, then changing the body of a function could be a non-obvious breaking API change. If lifetimes are only dependent on the signature, then its explicit what promises you are or are not making to callers of a function about object lifetimes, and changing those promises must be done intentionally by changing the signature rather than implicitly.