error[E0596]: cannot borrow `bar` as mutable, as it is not declared as mutable --> $DIR/issue-81700-mut-borrow.rs:3:5 | LL | let bar = || { foo(x); }; | - calling `bar` requires mutable binding due to mutable borrow of `x` LL | bar(); | ^^^ cannot borrow as mutable | help: consider changing this to be mutable | LL | let mut bar = || { foo(x); }; | +++ error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0596`.