warning: captured variable cannot escape `FnMut` closure body
  --> $DIR/issue-40510-3.rs:11:9
   |
LL |       || {
   |        - inferred to be a `FnMut` closure
LL | /         || {
LL | |             x.push(())
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
   = warning: this error has been downgraded to a warning for backwards compatibility with previous releases
   = warning: this represents potential undefined behavior in your code and this warning will become a hard error in the future
   = note: for more information, try `rustc --explain E0729`

error: compilation successful
  --> $DIR/issue-40510-3.rs:22:1
   |
LL | fn main() {}
   | ^^^^^^^^^^^^

error: aborting due to previous error