This is exaggerated re. C, but it is true that the important thing for practical security is whether memory safety is violated in practice, and obscure compiler bugs you have to go out of your way to try to exploit don't affect that. That being said, this bug is pretty bad, and it should be fixed.

I should point out that the reason the bug is difficult seems to mainly be because of backwards compatibility concerns. If compatibility weren't an issue, Rust could just ban function contravariance and nobody would care.