This is for all intents and purposes exactly what the cryptography maintainers did, except they did one better: they made sure this release is capable of valida

This is for all intents and purposes exactly what the cryptography maintainers did, except they did one better: they made sure this release is capable of validating the new configuration. They made rust an optional, but on-by-default, dependency. That is the equivalent of #error plus validation. They made it possible to turn this dependency off. That is the equivalent of -DI_UNDERSTAND.