Especially since the unstable_features lint is on DUMMY_SPAN, it would seem that it's firing incorrectly. The other possibility is that it's firing on some code you're injecting (without e.g. #[allow_internal_unstable] like handling).
If you don't hit the lint when bootstrapping for a different target, it's something that you're doing for the new target. Grepping for what restricted_std feature gates may help diagnose.