What about using a HashSet<Rc<str>>. And returning a type that implements Drop. The drop method can remove the string from the cache when the last reference is dropped. Since the cache must be accessed by the interning function and drop, I used interior mutability. By putting the cache behind a Rc<RefCell<>>.
https://play.rust-lang.org/?version=stable&mode=debug&editio...
With the same principle, we can use a static, arc and mutex to use a the cache as a global variable. And implement thread-safe string interning. Here, locking the cache at the wrong time during the drop can lead to a race condition.
https://play.rust-lang.org/?version=stable&mode=debug&editio...
To make the InternedStr behaves a bit more like &str or an immutable String, I implemented Clone, Display, Pointer, conversion from &Interned to &str. Ideally it should probably resemble how String de-references to &str for everything (Deref<Target = str>).
You'd need to pick a hash that won't have collisions within your expected use.
Edit: this might be interesting to you: https://docs.rs/crossbeam-epoch/0.9.4/crossbeam_epoch/
I started thinking along the lines of an arena allocator, that would allow a shorter lifetime than static so you could remove unused references by dropping the arena and moving over still live references to a new arena. I guess arenas the wrong term with that behaviour, maybe generational mark and sweep garbage collector is closer to describing this approach.
It a avoids the double reference though of the Rc<> model.