I'm considering cfg-gating the use of bitflags! in my libraries, and to that end it would be nice if it was possible to apply attributes to the generated impls as well.

The naive solution would be:

pub struct Flags(u8);

#[cfg(feature = "bitflags")]
bitflags! {
    impl Flags: u8 {
        // ...
    }
}

However, that works poorly with the doc_cfg (and doc_auto_cfg) feature, as the attribute is only applied to the macro, and isn't passed down to each output item.

Instead, I would like to be able to do the following:

#![feature(doc_cfg)]

pub struct Flags(u8);

#[cfg(feature = "bitflags")]
bitflags! {
    #[doc(cfg(feature = "bitflags"))]
    impl Flags: u8 {
        // ...
    }
}