Suppress some dead code warnings

Works around #1645.

IMO we should merge this (and other similar fragments in other crates as needed) right away (well, subject to the tree being open wrt the release). Getting probably-spurious warnings while doing unrelated work is intolerable.

If people think we should go and do all the cfg-decoration, that should be checked in CI, so that it ends up being done by the person introducing the new cfg code, not accidentally turned into deferred yaks that interrupt unrelated development. In that case, these allows could be reverted when the tests are in place.

We should have the conversation about the ultimate resolution in #1645.