rustdoc: doc_cfg on trait impls needs more visibility

[yaahc]

I'm trying to annotate all the parts of a crate that are available based on feature flags and I've had the following experience when annotating a trait impl.

#[cfg(feature = "registry")]
#[cfg_attr(docsrs, doc(cfg(feature = "registry")))]
impl<'a, L, S> LookupSpan<'a> for Layered<L, S>
where
    S: Subscriber + LookupSpan<'a>,
{
    type Data = S::Data;

    fn span_data(&'a self, id: &span::Id) -> Option<Self::Data> {
        self.inner.span_data(id)
    }
}

This is what it looks like with the default visibility

After expanding a single [+]

Fully expanded

Ideally the default visibility with everything minimized should still indicate that this trait impl is dependent upon the given feature.

[WaffleLapkin]

(except I'm missing something) On the latest nightly the messages couldn't be seen at all

Cargo version

% cargo --version
cargo 1.46.0-nightly (c26576f9a 2020-06-23)

The code:

#![cfg_attr(docsrs, feature(doc_cfg))]

pub struct SomeType;

pub trait SomeTrait {
    type SomeAssoc;

    fn some_method(&self);
}

#[cfg(feature = "some_feature")]
#[cfg_attr(docsrs, doc(cfg(feature = "some_feature")))]
impl SomeTrait for SomeType {
    type SomeAssoc = i32;

    fn some_method(&self) {}
}

The command:

% RUSTDOCFLAGS="--cfg docsrs" cargo doc --features "some_feature" --open

The rendered documentation:

Expanded: