error[E0596]: cannot borrow `v` as mutable, as it is not declared as mutable --> $DIR/issue-33819.rs:4:34 | LL | Some(ref v) => { let a = &mut v; }, | ^^^^^^ | | | cannot borrow as mutable | help: try removing `&mut` here error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0596`.