I've been writing Rust professionally for a few years now and if there's one thing I've learned it's that if you ever write a function that takes a parameter of `impl Fn(&Vec<&'a str>) -> &'a str` you are going to be in for some pain. Just make it `impl Fn(&Vec<&str>) -> String`. It is highly unlikely that the extra allocation is ever going to be noticed in the performance.
Just because Rust pretty much forces you to be explicit about your allocations doesn't mean you have to avoid them at all costs.
Just because Rust pretty much forces you to be explicit about your allocations doesn't mean you have to avoid them at all costs.