Add take_dropped to lazy_lru as a template parameter

[tomerfiliba]

Hi @behzadnouri, I really like lazy_lru but I need a way to obtain the dropped elements. The "main" LruCache impl allows you to get the dropped value upon insert, but your crate doesn't. I added put_with_evicted() which does the trick

[behzadnouri]

I will take a look, but on a cursory initial look:

• Can you please provide some context when/why you would need the evicted cached entries?
• Have you checked if this code has any impact on performance? there are already some benchmarks in the crate.

[tomerfiliba]

i'm using the LRU for dedup purposes, to reduce the number of events sent. once an event gets dropped from the LRU i want to pick it up and send it. the events that remain in the LRU are drained every interval.

[kskalski]

The performance concern here is valid, this implementation will allocate a vector every time the eviction happens, which will make the impact of latency spikes (puts that have to evict as opposed to those staying within capacity) higher.

I think it could return an impl Iterator instead, such that the existing entries are split into those that stay and those that are returned to the caller.

As for use-cases for such API, there could be many - we gather stats in the cache value or we need to do some some extra work / finalization for dropped values. In general if the implementation of some API removes a series of elements, it is nice to allow obtaining those elements back somewhere through that or supplementing API.