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);
Why 100? Why not 2?
derp, It should be LRU instead of LRU-K.