error[E0596]: cannot borrow `**layer` as mutable, as it is behind a `&` reference --> $DIR/issue-115259-suggest-iter-mut.rs:15:65 | LL | self.layers.iter().fold(0, |result, mut layer| result + layer.process()) | --------- ^^^^^ `layer` is a `&` reference, so the data it refers to cannot be borrowed as mutable | | | consider changing this binding's type to be: `&mut Box` | help: you may want to use `iter_mut` here | LL | self.layers.iter_mut().fold(0, |result, mut layer| result + layer.process()) | ~~~~~~~~ error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0596`.