doc_cfg - The Rust Unstable Book

# `doc_cfg`

The tracking issue for this feature is: #43781

The `doc_cfg` feature allows an API be documented as only available in some specific platforms. This attribute has two effects:

1. In the annotated item's documentation, there will be a message saying "This is supported on (platform) only".

2. The item's doc-tests will only run on the specific platform.

In addition to allowing the use of the `#[doc(cfg)]` attribute, this feature enables the use of a special conditional compilation flag, `#[cfg(rustdoc)]`, set whenever building documentation on your crate.

This feature was introduced as part of PR #43348 to allow the platform-specific parts of the standard library be documented.

    #![allow(unused_variables)]
    #![feature(doc_cfg)]

    fn main() {
    #[cfg(any(windows, rustdoc))]
    #[doc(cfg(windows))]
    /// The application's icon in the notification area (a.k.a. system tray).
    ///
    /// # Examples
    ///
    /// ```no_run
    /// extern crate my_awesome_ui_library;
    /// use my_awesome_ui_library::current_app;
    /// use my_awesome_ui_library::windows::notification;
    ///
    /// let icon = current_app().get::<notification::Icon>();
    /// icon.show();
    /// icon.show_message("Hello");
    /// ```
    pub struct Icon {
        // ...
    }
    }