Would love to see benchmarks vs competition considering the high performance claim.
Foyer is another rust hybrid cache, and quick-cache is the fastest in-memory impl I'm aware of.
Sane defaults and easy of setting a memory limit are two other things I look for in caches.
On your example, without reading into the implementation, I'm wondering if the comment is wrong, or if the comment is telling us about a hidden default, but then what does the 2 mean.
// Create an LRU cache with a capacity of 100 entries
let mut cache = LRUKCache::new(2);
Another time I wish rust had named parameters. For reading code without an IDE (which is a lot of the experience in discovering a library, learning, navigating in code on github...) it would be useful.
https://rust-lang.github.io/api-guidelines/naming.html#casin...
Sane defaults and easy of setting a memory limit are two other things I look for in caches.