error: captured variable cannot escape `FnMut` closure body
  --> $DIR/issue-40510-3.rs:7:9
   |
LL |       let mut x: Vec<()> = Vec::new();
   |           ----- variable defined here
LL |
LL |       || {
   |        - inferred to be a `FnMut` closure
LL | /         || {
LL | |             x.push(())
   | |             - variable captured here
LL | |         }
   | |_________^ returns a closure that contains a reference to a captured variable, which then 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
help: consider adding 'move' keyword before the nested closure
   |
LL |         move || {
   |         ++++

error: aborting due to previous error