error[E0666]: nested `impl Trait` is not allowed
  --> $DIR/issue-57979-deeply-nested-impl-trait-in-assoc-proj.rs:14:48
   |
LL | pub fn demo(_: impl Quux<Assoc=Deeper<impl Foo<impl Bar>>>) { }
   |                                       ---------^^^^^^^^-
   |                                       |        |
   |                                       |        nested `impl Trait` here
   |                                       outer `impl Trait`

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0666`.