error[E0502]: cannot borrow `self.thing` as mutable because it is also borrowed as immutable
  --> $DIR/issue-51268.rs:16:9
   |
LL |           self.thing.bar(|| {
   |           ^          --- -- immutable borrow occurs here
   |           |          |
   |  _________|          immutable borrow later used by call
   | |
LL | |
LL | |             &self.number;
   | |              ---- first borrow occurs due to use of `self` in closure
LL | |         });
   | |__________^ mutable borrow occurs here

error: aborting due to previous error

For more information about this error, try `rustc --explain E0502`.