error: captured variable cannot escape `FnMut` closure body --> $DIR/regions-return-ref-to-upvar-issue-17403.rs:7:24 | LL | let mut x = 0; | ----- variable defined here LL | let mut f = || &mut x; | - ^^^^^- | | | | | | | variable captured here | | returns a reference to a captured variable which escapes the closure body | inferred to be a `FnMut` closure | = 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