When conditionally deriving a trait using cfg_attr, the feature gate is not documented.

(present in 1.66.0-nightly)

• Build the documentation of this project with cargo +nightly doc --all-features --open.
• View the documentation for the struct Test.
• Debug should be documentated as only active with the debug feature flag set.

#![feature(doc_auto_cfg)]

#[derive(Default)]
#[cfg_attr(feature = "debug", derive(Debug))]
pub struct Test();