error: captured variable cannot escape `FnMut` closure body
  --> $DIR/issue-40510-1.rs:7:9
   |
LL |     let mut x: Box<()> = Box::new(());
   |         ----- variable defined here
LL |
LL |     || {
   |      - inferred to be a `FnMut` closure
LL |         &mut x
   |         ^^^^^-
   |         |    |
   |         |    variable captured here
   |         returns a reference to a captured variable which escapes the closure body
   |
   = note: `FnMut` closures only have access to their captured variables while they are executing...
   = note: ...therefore, they cannot allow references to captured variables to escape

error: aborting due to 1 previous error