You wanted a Map<String, String>, and that's exactly what you should have made. A String is a convenience wrapper for storing a &str on the heap, so storing a String on the heap is redundant. The other options are for performance concerns, which you haven't mentioned any.
Every time I need a static mutable thread safe map in rust for a cache or registry or some such thing, I groan.
I have a love hate relationship with rust. I love a lot of things about it, but sometimes I get so fed up with the borrow checker I drop into unsafe and just write the problem off as something better not done safely.
[0] https://doc.rust-lang.org/std/sync/struct.RwLock.html [1] https://crates.io/crates/lazy_static
I'm using 'static references for the map keys, but they're not really 'static, so I use unsafe transmute to create them and I rebuild the map when the memory map changes to a different address.