Add "make mutable" diagnostic / quick fix for local variables

[jplatte]

Example:

let foo = Vec::new();

loop {
    // other code
    foo.<|>
}

To reproduce:

• Type pu => get Vec::push suggested
• Accept suggestion => line changes to foo.push(value)

Desired outcome: let foo = Vec::new(); is changed to let mut foo = Vec::new(); when accepting the suggestion
Actual outcome: let foo = Vec::new(); is not changed

I would love if ra did this because that would require less jumping around the code.

[matklad]

I don't think completion is the right place for this. This should be a quick fix for the subsequent diagnostic instead.

[lnicola]

#10658 has more info.

[HKalbasi]

The diagnostic is implemented in #14232 so I'm going to close this issue. About auto inserting mut, I think we should have a general fix-on-format or fix-on-save for diagnostics.