7ef74bc8b9
Without this change it's going through `&mut impl Iterator`, which handles `?Sized` and thus currently can't forward generics. Here's the test added, to see that it fails before this PR (once a new enough nightly is out): https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=462f2896f2fed2c238ee63ca1a7e7c56