I think there would be value of such an option, as long as it would be completely forbidden to publish to crates.io with code that only works with those flags.

I think there would be value of such an option, as long as it would be completely forbidden to publish to crates.io with code that only works with those flags. It could work by replacing any function with type errors (including those caused by type definitions) to panic on use, but otherwise still generate a binary. For type errors it is less useful than for lifetime errors because usually the problems of the former are very local while for the later it they are not.