error[E0599]: no method named `len` found for struct `Wrapper<T>` in the current scope
  --> $DIR/deref-constrains-self-ty.rs:22:32
   |
LL | struct Wrapper<T>(T);
   | ----------------- method `len` not found for this struct
...
LL |         let _ = Wrapper(foo()).len();
   |                                ^^^ method not found in `Wrapper<impl Sized>`
   |
   = help: items from traits can only be used if the trait is implemented and in scope
   = note: the following trait defines an item `len`, perhaps you need to implement it:
           candidate #1: `ExactSizeIterator`

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0599`.