Document the doc attribute

cc #42322

src/doc/rustdoc/src/SUMMARY.md

@@ -2,7 +2,7 @@
 
 - [What is rustdoc?](what-is-rustdoc.md)
 - [Command-line arguments](command-line-arguments.md)
-- [In-source directives](in-source-directives.md)
+- [The `#[doc]` attribute](the-doc-attribute.md)
 - [Documentation tests](documentation-tests.md)
 - [Plugins](plugins.md)
 - [Passes](passes.md)

src/doc/rustdoc/src/the-doc-attribute.md

@@ -0,0 +1,108 @@
+# The `#[doc]` attribute
+
+The `#[doc]` attribute lets you control various aspects of how `rustdoc` does
+its job. 
+
+The most basic job of `#[doc]` is to be the way that the text of the documentation
+is handled. That is, `///` is syntax sugar for `#[doc]`. This means that these two
+are the same:
+
+```rust,ignore
+/// This is a doc comment.
+#[doc = "This is a doc comment."]
+```
+
+In most cases, `///` is easier to use than `#[doc]`. One case where the latter is easier is
+when generating documentation in macros; the `collapse-docs` pass will combine multiple
+`#[doc]` attributes into a single doc comment, letting you generate code like this:
+
+```rust,ignore
+#[doc = "This is"]
+#[doc = " a "]
+#[doc = "doc comment"]
+```
+
+Which can feel more flexible.
+
+The `doc` attribute has more options though! These don't involve the text of
+the output, but instead, various aspects of the presentation of the output.
+We've split them into two kinds below: attributes that are useful at the
+crate level, and ones that are useful at the item level.
+
+## At the crate level
+
+These options control how the docs look at a macro level.
+
+### `html_favicon_url`
+
+This form of the `doc` attribute lets you control the favicon of your docs.
+
+```rust,ignore
+#![doc(html_favicon_url = "https://foo.com/favicon.ico")]
+```
+
+This will put `<link rel="shortcut icon" href="{}">` into your docs, where
+the string for the attribute goes into the `{}`.
+
+### `html_logo_url`
+
+This form of the `doc` attribute lets you control the logo in the upper
+left hand side of the docs.
+
+```rust,ignore
+#![doc(html_logo_url = "https://foo.com/logo.jpg")]
+```
+
+This will put `<a href='index.html'><img src='{}' alt='logo' width='100'></a>` into
+your docs, where the string for the attribute goes into the `{}`.
+
+### `html_playground_url`
+
+This form of the `doc` attribute lets you control where the "run" buttons
+on your documentation examples make requests to.
+
+```rust,ignore
+#![doc(html_playground_url = "https://playground.foo.com/")]
+```
+
+Now, when you press "run", the button will make a request to this domain.
+
+### `issue_tracker_base_url`
+
+This form of the `doc` attribute is mostly only useful for the standard library;
+When a feature is unstable, an issue number for tracking the feature must be
+given. `rustdoc` uses this number, plus the base URL given here, to link to
+the tracking issue.
+
+```rust,ignore
+#![doc(issue_tracker_base_url = "https://github.com/foo/foo/issues/")]
+```
+
+### `html_no_source`
+
+By default, `rustdoc` will include the source code of your program, with links
+to it in the docs. But if you include this:
+
+```rust,ignore
+#![doc(html_no_source)]
+```
+
+it will not.
+
+## At the item level
+
+These forms of the `#[doc]` attribute are used on individual items, to control how
+they are documented.
+
+## `#[doc(no_inline)]`
+
+## `#[doc(hidden)]`
+
+Any item annotated with `#[doc(hidden)]` will not appear in the documentation, unless
+the `strip-hidden` pass is removed.
+
+## `#[doc(primitive)]`
+
+Since primitive types are defined in the compiler, there's no place to attach documentation
+attributes. This attribute is used by the standard library to provide a way to generate
+documentation for primitive types.