Elision never assumes 'static, since that's an unusual case that has very few real-world uses.

This leaves the only remaining possibility that references in the return type must have the same lifetime as references in function's arguments. Without a GC, or leaking memory, you can't make a Rust reference/lifetime out of thin air, so there's always such relationship.

When a function takes multiple arguments by reference, the compiler will ask to disambiguate which one is used for the return type. Borrow checker intentionally checks borrowing against function prototypes, not function bodies.