assist: rename to match field name

[pickx]

$ represents the cursor

fn new() -> MyStruct {
    // ...
    MyStruct {
        foo,
        bar: some_var_name$,
        baz,
    }
}

here it should be suggested to rename some_var_name to bar, to end up with

MyStruct {
    foo,
    bar,
    baz,
}

this is especially useful if the name of the field was previously some_var_name, and then was later renamed to bar so the local variable name has desynced from the field name

(the name of the assist can be bikeshedded, I can't think of a good one right now)

[ChayimFriedman2]

The effort required is just F2 and typing the name. I don't think this justifies a new assist. @rust-lang/rust-analyzer do you agree?

[Veykril]

Yea, if you rename the right hand side we automatically collapse it already. This is not really worth an assist I think

[flodiebold]

It would be nice if we could make suggestions when renaming, then we could suggest this as a name, but I agree it shouldn't be its own assist.

[pickx]

Yea, if you rename the right hand side we automatically collapse it already. This is not really worth an assist I think

in retrospect, this assist isn't the right way of dealing with the problem of local names desyncing from field names.
but it is common enough in my workflows that it bothers me, so I think it's worth discussing this issue (if not here than elsewhere)

[A4-Tacks]

Perhaps it can be renamed as "_", output a suggested name