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

programmerjake:

IMO Rust never guaranteed it wouldn't add more warnings for code that was warning-free, so if you use deny(warnings), you're opting-in to breakage.

Yes, and probes really should aim to use a specific #[forbid(...)] etc. to change the specific case they are testing.

And yet ... In this case we are looking at potentially breaking build scripts silently with hard-to-spot and hard-to-debug consequences, not only for the crate in question but for anyone downstream and without any action needed on anyone's part (other than updating rustc) for the breakage to occur.

So, while I agree that "you really shouldn't have done that, so you have no-one else to blame" is the technically right call, I also don't feel comfortable putting an MCP in for this without a thumbs-up form someone more senior.