This is a quick heads up for people keeping track of hypothetical code breakage due to warnings being promoted to errors.
We revised the macro-future proofing rules some time ago, back in RFC amendment https://github.com/rust-lang/rfcs/pull/1384, and to ease the transition between the two sets of rules, we explicitly allows macros that passed the old macro-checking rules but did not pass the new rules to be allowed through, with a stern warning that the macro was not future-compatible and that this warning would become a hard error in the future.
The last step in the recorded plan for that amendment was to upgrade that warning to a hard error (i.e., to take out the old code that checked conformance against the old macro-checking rules).
The new macro rules have been in place for several release cycles now, so we want to move ahead with the change. (There is a whole process around upgrading these warnings to hard errors that has sprouted up in the meantime, but I think this bug predates that process.)
So, this post is meant to be a warning that we are indeed planning to make the shift from warning to error in the near future; e.g., it will likely hit nightly sometime soon during this cycle, and propagate on through the release channel train after that.
Tracking issue https://github.com/rust-lang/rust/issues/30450
Pull request upgrading to a hard error: https://github.com/rust-lang/rust/pull/33982