Constant CACHE_LEN

const CACHE_LEN: usize = 32;

Exactly what cache size we should use is a difficult trade-off. There will always be some workload which has a BorTag working set which exceeds the size of the cache, and ends up falling back to linear searches of the borrow stack very often. The cost of making this value too large is that the loop in Stack::insert which ensures the entries in the cache stay correct after an insert becomes expensive.