How to Use cfg_attr to Conditionally Document Features

Use the #[cfg_attr] attribute on your item to conditionally apply the doc attribute when a specific feature is enabled. Place #[cfg_attr(feature = "your_feature", doc = "Your documentation text")] immediately before the item definition to include the documentation only when that feature is active.

#[cfg_attr(feature = "nightly", doc = "This is only documented when the nightly feature is enabled.")]
pub struct MyStruct {
    pub value: i32,
}

Alternatively, to conditionally include the entire item in documentation (and compilation), use #[cfg_attr(feature = "nightly", doc(hidden))] to hide it or #[cfg(feature = "nightly")] to exclude it entirely from builds without the feature.

cfg_attr lets you add documentation to code that only exists or matters when a specific build option is turned on. It's like putting a sticky note on a tool that only appears when you unlock a special mode in your workshop. You use it to keep your public docs clean while still providing context for advanced or unstable features.