Right now, I have an enum for various error types. Display is in core, so that's implemented unconditionally. However, Error only exists in std, so I've put it behind a feature flag (named "std") to allow compatibility with #![no_std]. Having the ability to do this programmatically would be helpful for this use case.

Something like this would be workable, I think.

#[derive(Error)]
#[error_feature = "std"]
enum Foo { ... }