The feature your asking for is just plain old function overloading, which Rust does not have. There are a few reasons I think its unlikely Rust will have this feature any time soon:
- It's not usually that useful, since you can just use a different method name.
- Using it is often not a great idea, because if these methods have different signatures and different implementations, like as not they should have a different name.
- In a language with a lot of type parameters flying around like Rust has, proving that the two signatures are actually disjoint is not trivial. We'd have to have a whole coherence system for function signatures, and before long we'd be talking about specialization between signatures.
In fact, I'm pretty sure that these two signatures overlap in the case where the HashMap's key is a reference type, since &T: Copy and &T: Borrow<T> (here K = &T and Q = T).
So yeah, you'd just use a different name. But I'm not sure its more convenient to call index_copy(k) than index(&k).
EDIT: I'm a little embarrassed to admit that I've been programming Rust for two years and until this minute I thought the map types didn't implement Index. 