Entry::insert

I'm currently in a position of wanting an Entry::insert (or other bikesheddable name) for when you want to conditionally update a value - inserting a value if there is none and updating it if it exists but is "worse" by some metric. This would be nicely captured with code similar to the following:

let was_updated = match map.entry(key) {
    Entry::Occupied(ref entry) if !new_val.better_than(entry.get()) => false,
    otherwise => { otherwise.upsert(new_val); true },
};

Instead, you have to do:

let was_updated = match map.entry(key) {
    Entry::Occupied(ref entry) if !new_val.better_than(entry.get()) => false,
    Entry::Occupied(entry) => { entry.insert(new_val); true },
    Entry::Vacant(entry) => { entry.insert(new_val); true },
};

Which is redundant. The two insert methods have different signatures but I would imagine that Entry::insert would return an Result<&mut Value, Value> to pass ownership in the case that the entry is occupied (or maybe a CowMut if that was to exist).

Read the Entry::insert proposal and its examples in this issue, then compare the existing Entry::Occupied and Entry::Vacant paths described there. Clarify the intended ownership, return behavior, and method name through the RFC process; done means the API design is settled and recorded.