Rename does not work for local variables bound inside a pattern

[aochagavia]

To reproduce, write the following code and attempt to rename a.

let (a, b) = (42, 54);
let y = a;

[aochagavia]

cc @kjeremy

[matklad]

Note that this is an interesting problem: in Rust, identifier in a pattern could be either a new name, or a reference to existing constant:

x: Option<i32> = None;
match x {
    None => (), // reference to `None` constant
    Something => (), // a name binding, equivalent to `_` more or less
}

So, this needs some special handling.

[DJMcNab]

This currently works for the first example, but doesn't work for the second. At the moment, this is the correct approach given the current state of name resolution and our lack of knowledge about std, although there might be some cases where we do the wrong thing as we aren't conservative enough.

[daboross]

Note that even without a plain single-identifier pattern, identifiers can refer to existing constants:

struct Constant;

let x = Constant;
// refers to Constant struct
// (does not move 'x' nor trigger 'unused variable' warning)
let Constant = x;
// vs. variable (moves and triggers unused variable warning)
let C = x;
// let z = x; // cannot do due to moving 'x' into 'C'

It's pretty niche so I don't expect it needs to be handled if it'd be extra work, but worth noting that this isn't just for special patterns but for all bindings.

[kiljacken]

This works on current master. Closing :)