error[E0666]: nested `impl Trait` is not allowed
  --> $DIR/issue-57979-nested-impl-trait-in-assoc-proj.rs:9:41
   |
LL | pub fn demo(_: impl Quux<Assoc=impl Foo<impl Bar>>) { }
   |                                ---------^^^^^^^^-
   |                                |        |
   |                                |        nested `impl Trait` here
   |                                outer `impl Trait`

error: aborting due to previous error

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