error[E0596]: cannot borrow `*container` as mutable, as it is behind a `&` reference
  --> $DIR/issue-62387-suggest-iter-mut-2.rs:30:45
   |
LL |             vec.iter().flat_map(|container| container.things()).cloned().collect::<Vec<PathBuf>>();
   |                                  ---------  ^^^^^^^^^ `container` is a `&` reference, so the data it refers to cannot be borrowed as mutable
   |                                  |
   |                                  consider changing this binding's type to be: `&mut Container`
   |
help: you may want to use `iter_mut` here
   |
LL |             vec.iter_mut().flat_map(|container| container.things()).cloned().collect::<Vec<PathBuf>>();
   |                 ~~~~~~~~

error: aborting due to previous error

For more information about this error, try `rustc --explain E0596`.