Code compiles on playground but fails when passed via stdin to rustc

Did you try when set via CARGO_UNSTABLE_ALLOW_FEATURES too? That’s probably the more common interface to the functionality now that it exists, and that’s what I’ve been running into a lot of buggy probe handling for recently.

(And it’s kind of an eternal fact that every time the interface for this sort of thing improves, build scripts will fail to handle it because they’re kind of fail open, instead of fail closed; I remember long ago when none handled RUSTFLAGS correctly either).