Display note for "unused variable" warning suggesting that the warning can be disabled via underscores

[nham]

For reference, see this Reddit thread

It seems to me that if you have code like this

for x in foo {
    // code that does not use `x`...
}

Then in addition to the "unused variable" warning that you currently get:

unused_var.rs:2:9: 2:10 warning: unused variable: `x`, #[warn(unused_variables)] on by default
unused_var.rs:2     for x in [1,2,3].iter() {
                        ^

it should also display a note, perhaps saying

note: to disable this warning, consider using `_x` or `_` instead

Barring that, the lint could have an error code and the error explanation could mention that you can disable the warning in either of these ways.

I'm not sure if this note should only be displayed with for loops (and similar situations), or if it should be unconditionally displayed alongside the "unused variable" warning.

[steveklabnik]

Triage: no change.

[Mark-Simulacrum]

I'm not sure how to add notes to lints, but in general I think the changes would need to be done somewhere around here. Perhaps someone could write up some instructions?

[GuillaumeGomez]

Sounds like fun. Doing it.