Provide help on closures capturing self causing borrow checker errors

[estebank]

Given a closure that immutably captures self, you can easily cause a lifetime error

struct S;
impl S {
    fn foo(&mut self) {
        let x = || {
            self.bar();
        };
        self.qux();
        x();
    }
    fn bar(&self) {}
    fn qux(&mut self) {}
}

error[E0502]: cannot borrow `*self` as mutable because it is also borrowed as immutable
 --> src/main.rs:7:9
  |
4 |         let x = || {
  |                 -- immutable borrow occurs here
5 |             self.bar();
  |             ---- first borrow occurs due to use of `*self` in closure
6 |         };
7 |         self.qux();
  |         ^^^^^^^^^^ mutable borrow occurs here
8 |         x();
  |         - immutable borrow later used here

The usual solution here is to explicitly pass self into the closure as an this: &Self argument to side-step the issue. We should suggest doing so.

[chenyukang]

@rustbot claim

[estebank]

This is a duplicate of #45634