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