error[E0277]: the trait bound `Bar: Hash` is not satisfied
  --> $DIR/issue-21160.rs:8:12
   |
LL | #[derive(Hash)]
   |          ---- in this derive macro expansion
LL | struct Foo(Bar);
   |            ^^^ the trait `Hash` is not implemented for `Bar`
   |
   = note: this error originates in the derive macro `Hash` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider annotating `Bar` with `#[derive(Hash)]`
   |
LL + #[derive(Hash)]
LL | struct Bar;
   |

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0277`.