Add chapter for re-exports in the rustdoc book

Author: GuillaumeGomez

src/doc/rustdoc/src/SUMMARY.md

@@ -7,6 +7,7 @@
 - [How to write documentation](how-to-write-documentation.md)
     - [What to include (and exclude)](write-documentation/what-to-include.md)
     - [The `#[doc]` attribute](write-documentation/the-doc-attribute.md)
+    - [Re-exports](write-documentation/re-exports.md)
     - [Linking to items by name](write-documentation/linking-to-items-by-name.md)
     - [Documentation tests](write-documentation/documentation-tests.md)
 - [Rustdoc-specific lints](lints.md)

src/doc/rustdoc/src/write-documentation/re-exports.md

@@ -0,0 +1,172 @@
+# Re-exports
+
+Let's start by explaining what are re-exports. To do so, we will use an example where we are
+writing a library (named `lib`) with some types dispatched in sub-modules:
+
+```rust
+pub mod sub_module1 {
+    pub struct Foo;
+}
+pub mod sub_module2 {
+    pub struct AnotherFoo;
+}
+```
+
+Users can import them like this:
+
+```rust,ignore (inline)
+use lib::sub_module1::Foo;
+use lib::sub_module2::AnotherFoo;
+```
+
+But what if you want the types to be available directly at the crate root or if we don't want the
+modules to be visible for users? That's where re-exports come in:
+
+```rust,ignore (inline)
+// `sub_module1` and `sub_module2` are not visible outside.
+mod sub_module1 {
+    pub struct Foo;
+}
+mod sub_module2 {
+    pub struct AnotherFoo;
+}
+// We re-export both types:
+pub use crate::sub_module1::Foo;
+pub use crate::sub_module2::AnotherFoo;
+```
+
+And now users will be able to do:
+
+```rust,ignore (inline)
+use lib::{Foo, AnotherFoo};
+```
+
+And since both `sub_module1` and `sub_module2` are private, users won't be able to import them.
+
+Now what's interesting is that the generated documentation for this crate will show both `Foo` and
+`AnotherFoo` directly at the crate root, meaning they have been inlined. There are a few rules to
+know whether or not a re-exported item will be inlined.
+
+## Inlining rules
+
+If a public item comes from a private module, it will be inlined:
+
+```rust,ignore (inline)
+mod private_module {
+    pub struct Public;
+}
+pub mod public_mod {
+    // `Public` will inlined here since `private_module` is private.
+    pub use super::private_module::Public;
+}
+// `Public` will not be inlined here since `public_mod` is public.
+pub use self::public_mod::Public;
+```
+
+Likewise, if an item inherits `#[doc(hidden)]` from any of its ancestors, it will be inlined:
+
+```rust,ignore (inline)
+#[doc(hidden)]
+pub mod public_mod {
+    pub struct Public;
+}
+// `Public` be inlined since its parent (`public_mod`) has `#[doc(hidden)]`.
+pub use self::public_mod::Public;
+```
+
+If an item has `#[doc(hidden)]`, it won't be inlined (nor visible in the generated documentation):
+
+```rust,ignore (inline)
+// This struct won't be visible.
+#[doc(hidden)]
+pub struct Hidden;
+
+// This re-export won't be visible.
+pub use self::Hidden as InlinedHidden;
+```
+
+The same applies on re-exports themselves: if you have multiple re-exports and some of them have
+`#[doc(hidden)]`, then these ones (and only these) own't appear in the documentation:
+
+```rust,ignore (inline)
+mod private_mod {
+    /// First
+    pub struct InPrivate;
+}
+
+/// Second
+#[doc(hidden)]
+pub use self::private_mod::InPrivate as Hidden;
+/// Third
+pub use self::Hidden as Visible;
+```
+
+In this case, `InPrivate` will be inlined as `Visible`. However, its documentation will be
+`First Third` and not `First Second Third` because the re-export with `Second` as documentation has
+`#[doc(hidden)]`, therefore, all its attributes are ignored.
+
+## Inlining with `#[doc(inline)]`
+
+You can use the `#[doc(inline)]` attribute if you want to force an item to be inlined:
+
+```rust,ignore (inline)
+pub mod public_mod {
+    pub struct Public;
+}
+#[doc(inline)]
+pub use self::public_mod::Public;
+```
+
+With this code, even though `public_mod::Public` is public and present in the documentation, the
+`Public` type will be present both at the crate root and in the `public_mod` module.
+
+## Preventing inlining with `#[doc(no_inline)]`
+
+On the opposite of the `#[doc(inline)]` attribute, if you want to prevent an item from being
+inlined, you can use `#[doc(no_inline)]`:
+
+```rust,ignore (inline)
+mod private_mod {
+    pub struct Public;
+}
+#[doc(no_inline)]
+pub use self::private_mod::Public;
+```
+
+In the generated documentation, you will see a re-export at the crate root and not the type
+directly.
+
+## Attributes
+
+When an item is inlined, its doc comments and most of its attributes will be inlined along with it:
+
+```rust,ignore (inline)
+mod private_mod {
+    /// First
+    #[cfg(a)]
+    pub struct InPrivate;
+    /// Second
+    #[cfg(b)]
+    pub use self::InPrivate as Second;
+}
+
+/// Third
+#[doc(inline)]
+#[cfg(c)]
+pub use self::private_mod::Second as Visible;
+```
+
+In this case, `Visible` will have as documentation `First Second Third` and will also have as `cfg`:
+`#[cfg(a, b, c)]`.
+
+[Intra-doc links](./linking-to-items-by-name.md) are resolved relative to where the doc comment is
+defined.
+
+There are a few attributes which are not inlined though:
+ * `#[doc(alias="")]`
+ * `#[doc(inline)]`
+ * `#[doc(no_inline)]`
+ * `#[doc(hidden)]` (because the re-export itself and its attributes are ignored).
+
+All other attributes are inherited when inlined, so that the documentation matches the behavior if
+the inlined item was directly defined at the spot where it's shown.

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

@@ -223,12 +223,18 @@ Now we'll have a `Re-exports` line, and `Bar` will not link to anywhere.
 One special case: In Rust 2018 and later, if you `pub use` one of your dependencies, `rustdoc` will
 not eagerly inline it as a module unless you add `#[doc(inline)]`.
 
+If you want to know more about inlining rules, take a look at the
+[`re-exports` chapter](./re-exports.md).
+
 ### `hidden`
 
 <span id="dochidden"></span>
 
 Any item annotated with `#[doc(hidden)]` will not appear in the documentation, unless
-the `strip-hidden` pass is removed.
+the `strip-hidden` pass is removed. Re-exported items where one of its ancestors has
+`#[doc(hidden)]` will be considered the same as private.
+
+You can find more information in the [`re-exports` chapter](./re-exports.md).
 
 ### `alias`