Iterator over entries of Hash*

[nagisa]

An iterator over entries of HashMap/Set could be added?

[eefriedman]

HashMap already has a method iter_mut(); could you describe what you want a bit more precisely?

[nagisa]

@eefriedman Consider following code which could potentially be a poor-man’s implementation of pop:

for entry in map.entries() {
    if let Entry::Occupied(o) = entry {
        return o.remove();
    } else { unreachable!() }
}

That cannot be implemented with iter_mut() today.

[apasel422]

I'm not sure that the current HashMap design would support the existence of multiple OccupiedEntrys at the same time (due to the relocation of entries during removal), but if such an iterator were added, I expect that its item type would be OccupiedEntry rather than Entry, as a VacantEntry cannot be created without a key.

[dtolnay]

I agree with @apasel422 that I don't think this will work with an iterator of entries. The Iterator trait permits the caller to hold on to multiple items at the same time, which in this case would mean multiple items that each have mutable access to the same HashMap.

I think the use case of iterating over key-value pairs and conditionally removing them efficiently is addressed by HashMap::retain which was added in #36648, after this issue was opened, so I think this can be considered done.