Set-based operations on HashMap

[jethrogb]

HashSet has all kinds of useful set functionality, but these functions are incompatible with HashMap keys:

• difference
• symmetric_difference
• intersection
• union
• is_disjoint
• is_subset
• is_superset

It would be nice to enable the use of these functions between HashMaps and HashSets, either by allowing semi-cheap conversions from HashMap.keys to HashSet or by enabling those functions to take either maps or sets.

[frewsxcv]

How about:

• std::collections::btree_map::BTreeMap::keys_as_set(&self) -> BTreeSet<'a, K> { .. }
• fn std::collections::hash_map::HashMap::keys_as_set(&self) -> HashSet<'a, K> { .. }

or possibly:

• std::collections::btree_map::Keys::as_btree_set(&self) -> BTreeSet<'a, K> { .. }
• fn std::collections::hash_map::Keys::as_hash_set(&self) -> HashSet<'a, K> { .. }

It might not be that trivial implementation-wise because {BTree,Hash}Set internally use their Map counterparts:

https://github.com/rust-lang/rust/blob/956e2bcbaa00c05e051718b1375375915064f1c3/src/libstd/collections/hash/set.rs#L118

https://github.com/rust-lang/rust/blob/956e2bcbaa00c05e051718b1375375915064f1c3/src/libcollections/btree/set.rs#L74

... but I think it should be possible (EDIT: nevermind, after looking into implementing this, it doesn't appear to be so straightforward).

[jethrogb]

I've been thinking about this some more, and I think it would be useful to have the all set operations supported on Map<K,V1> and Map<K,V2>. The difference iterator type is (&K,&V1). The intersection iterator type is (&K,&V1,&V2). The symmetric_difference and union iterator type is &K, or possibly (&K,Occurence<V1,V2>) with

enum Occurence<V1,V2> {
  InSelf(&V1),
  InOther(&V2),
  InBoth(&V1, &V2),
}