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>>) { } | ---------^^^^^^^^- | | | | | nested `impl Trait` here | outer `impl Trait` error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0666`.