find_or_insert_with should return a &mut pointer

[metajack]

Trying to satisfy the borrow checker in some cases leads to a little bit clumsy HashMap code. For example, if you want to find_or_insert but need to modify the value.

Here's an example where I had to work around this by using contains_key and then find_mut and insert directly:

https://gist.github.com/metajack/5555485#file-gistfile2-diff-L28-L45

I suggest adding find_or_insert_mut and find_or_insert_with_mut. It might be better to use find_mut_or_insert and find_mut_or_insert_with as names perhaps.

[metajack]

nominating for production ready

[nikomatsakis]

Just to be clear, these methods would be the same as find_or_insert_with, but that they return a mutable pointer? It may be that this could just replace find_or_insert_with altogether; you can always re-borrow the returned &mut pointer as & if that's what you want

[metajack]

If that's the case, then find_mut should disappear as well, and find should return &mut, etc?

[thestinger]

@metajack: find needs to exist because it takes &self

[graydon]

accepted for backwards-compatible milestone

[lilyball]

My latest proposed patch in #6815 fixes this.