The only runtime performance cost here is one you would need regardless for correctness- you only need to copy if you're going to hold onto the data longer than

The only runtime performance cost here is one you would need regardless for correctness- you only need to copy if you're going to hold onto the data longer than a borrow would allow. In fact, Rust's semantics allows a fewer number of copies than naive use of std::string, since an already-owned value won't be copied, as mentioned above.