error[E0368]: binary assignment operation `+=` cannot be applied to type `&isize` --> $DIR/issue-5239-1.rs:4:30 | LL | let x = |ref x: isize| { x += 1; }; | -^^^^^ | | | cannot use `+=` on type `&isize` error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0368`.