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<dyn Layer>`
   |
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`.