feat: Implementation of locals_used in HIR level

[dfireBird]

fixes: #21039

[ChayimFriedman2]

One thing: due to destructuring assignments, patterns can also refer to variables (Pat::Path). So you need to account for that.

[dfireBird]

Just to confirm, you mean for something like this right?

fn foo() {
    let mut a = 3;
    let mut b = 4;

     $0(a, b) = bar(1, 3);
    (a, b) = bar(5, 7);$0
}

fn bar(y: u32, z: u32) -> (u32, u32) {
}

[ChayimFriedman2]

Yeah, exactly.

[dfireBird]

Thanks for confirming. I didn't knew this existed :(

While doing that I found an issue, where variables inside pattern destructing assignments are not marked mutable in the generate function, so I made a fix for that as well.

EDIT: Should I squash the two commits like I did before, just before merging? Please let me know?

[ChayimFriedman2]

Thanks!