error[E0596]: cannot borrow `c` as mutable, as it is not declared as mutable --> $DIR/issue-82438-mut-without-upvar.rs:27:27 | LL | A.f(participant_name, &mut c); | ^^^^^^ cannot borrow as mutable | help: consider changing this to be mutable | LL | let mut c = |a, b, c, d| {}; | +++ error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0596`.