First example of non-compiling `impl Trait` does compile

[rodrimati1992]

The first example of non-compiling uses of impl Trait

impl-trait-initiative/explainer/where_not_ok.md

Line 8 in 87428cd

fn foo(x: &mut dyn Iterator<Item = impl Debug>)

fn foo(x: &mut dyn Iterator<Item = impl Debug>)

Does compile when written in full:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=bd81ce5a64eda9175ea1001723a12127

use std::fmt::Debug;

fn foo(x: &mut dyn Iterator<Item = impl Debug>){}