There's really an endless list of these optimizations. A few I've used (though not necessarily in rust):

Atoms: Each string can be referenced with a single u32 or even u16, and they're inherently deduplicated.

Bump allocator: your strings are &str, allocation is super fast with limited fragmentation.

Single pointer strings (this has a name, I can't think of it right now): you store the length inside the allocation instead of in each reference, so your strings are a single pointer.