error: unsatisfied lifetime constraints
  --> $DIR/issue-40510-1.rs:18:9
   |
LL |     || {
   |     --
   |     ||
   |     |return type of closure is &'2 mut std::boxed::Box<()>
   |     lifetime `'1` represents this closure's body
LL |         &mut x
   |         ^^^^^^ returning this value requires that `'1` must outlive `'2`
   |
   = note: closure implements `FnMut`, so references to captured variables can't escape the closure

error: aborting due to previous error