error[E0502]: cannot borrow `self.thing` as mutable because it is also borrowed as immutable
  --> $DIR/issue-51268.rs:28:9
   |
LL |            self.thing.bar(|| {
   |            ^              -- immutable borrow occurs here
   |   _________|
   |  |_________|
   | ||
LL | ||         //~^ ERROR cannot borrow `self.thing` as mutable because it is also borrowed as immutable [E0502]
LL | ||             &self.number;
   | ||              ---- first borrow occurs due to use of `self` in closure
LL | ||         });
   | ||          ^
   | ||__________|
   | |___________mutable borrow occurs here
   |             borrow later used here

error: aborting due to previous error

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