Revamp rustdoc docs about documentation using cfg

- Move `cfg(doc)` out of `unstable-features`. It's not unstable.
- Remove outdated reference to `everybody_loops`.
- Improve wording in various places
- Give an example of code this allows (and does not allow)
- Link to `cfg(doc)` in `doc(cfg)` documentation. Since one is stable
and the other is not, don't combine them.
- Cleanup wording for `doc(cfg)`
- Incorporate changes from #76849
- Mention that `doc(cfg)` is also for features

src/doc/rustdoc/src/advanced-features.md

@@ -2,7 +2,7 @@
 
 The features listed on this page fall outside the rest of the main categories.
 
-## `#[cfg(doc)]`: Documenting platform-/feature-specific information
+## `#[cfg(doc)]`: Documenting platform-specific or feature-specific information
 
 For conditional compilation, Rustdoc treats your crate the same way the compiler does. Only things
 from the host target are available (or from the given `--target` if present), and everything else is
@@ -17,7 +17,7 @@ with other `#[cfg]` filters on it, you can write something like `#[cfg(any(windo
 This will preserve the item either when built normally on Windows, or when being documented
 anywhere.
 
-Please note that this feature is not passed to doctests.
+Please note that this `cfg` is not passed to doctests.
 
 Example:
 
@@ -33,6 +33,40 @@ pub struct UnixToken;
 Here, the respective tokens can only be used by dependent crates on their respective platforms, but
 they will both appear in documentation.
 
+### Interactions between platform-specific docs
+
+Rustdoc does not have a magic way to compile documentation 'as-if' you'd run it once for each
+platform (such a magic wand has been called the ['holy grail of rustdoc'][#1998]). Instead,
+it sees *all* of your code at once, the same way the Rust compiler would if you passed it
+`--cfg doc`. However, Rustdoc has a trick up its sleeve to handle platform-specific code if it
+*does* receive it.
+
+To document your crate, Rustdoc only needs to know the public signature of your functions.
+In particular, it doesn't have to know how any of your functions are implemented, so it ignores
+all type errors and name resolution errors with function bodies. Note that this does *not*
+work for anything outside a function body: since Rustdoc documents your types, it has to
+know what those types are! For example, this code will work regardless of the platform:
+
+<!-- `ignore` because doc-tests are run with `rustc`, not `rustdoc` -->
+```ignore
+pub fn f() {
+    use std::os::windows::ffi::OsStrExt;
+}
+```
+
+but this will not, because the unknown type is part of the function signature:
+
+```ignore
+pub fn f() -> std::os::windows::ffi::EncodeWide<'static> {
+    unimplemented!()
+}
+```
+
+For a more realistic example of code this allows, see [the rustdoc test suite][realistic-async].
+
+[#1998]: https://github.com/rust-lang/rust/issues/1998
+[realistic-async]: https://github.com/rust-lang/rust/blob/b146000e910ccd60bdcde89363cb6aa14ecc0d95/src/test/rustdoc-ui/error-in-impl-trait/realistic-async.rs
+
 ## Add aliases for an item in documentation search
 
 This feature allows you to add alias(es) to an item when using the `rustdoc` search through the

src/doc/rustdoc/src/unstable-features.md

@@ -43,28 +43,16 @@ plain text.
 These features operate by extending the `#[doc]` attribute, and thus can be caught by the compiler
 and enabled with a `#![feature(...)]` attribute in your crate.
 
-### Documenting platform-/feature-specific information
+### `#[doc(cfg)]`: Recording what platforms or features are required for code to be present
 
-Because of the way Rustdoc documents a crate, the documentation it creates is specific to the target
-rustc compiles for. Anything that's specific to any other target is dropped via `#[cfg]` attribute
-processing early in the compilation process. However, Rustdoc has a trick up its sleeve to handle
-platform-specific code if it *does* receive it.
+You can use `#[doc(cfg(...))]` to tell Rustdoc exactly which platform items appear on.
+This has two effects:
 
-Because Rustdoc doesn't need to fully compile a crate to binary, it replaces function bodies with
-`loop {}` to prevent having to process more than necessary. This means that any code within a
-function that requires platform-specific pieces is ignored. Combined with a special attribute,
-`#[doc(cfg(...))]`, you can tell Rustdoc exactly which platform something is supposed to run on,
-ensuring that doctests are only run on the appropriate platforms.
-
-The `#[doc(cfg(...))]` attribute has another effect: When Rustdoc renders documentation for that
-item, it will be accompanied by a banner explaining that the item is only available on certain
-platforms.
-
-For Rustdoc to document an item, it needs to see it, regardless of what platform it's currently
-running on. To aid this, Rustdoc sets the flag `#[cfg(doc)]` when running on your crate.
-Combining this with the target platform of a given item allows it to appear when building your crate
-normally on that platform, as well as when building documentation anywhere.
+1. doctests will only run on the appropriate platforms, and
+2. When Rustdoc renders documentation for that item, it will be accompanied by a banner explaining
+   that the item is only available on certain platforms.
 
+`#[doc(cfg)]` is intended to be used alongside [`#[cfg(doc)]`][cfg-doc].
 For example, `#[cfg(any(windows, doc))]` will preserve the item either on Windows or during the
 documentation process. Then, adding a new attribute `#[doc(cfg(windows))]` will tell Rustdoc that
 the item is supposed to be used on Windows. For example:
@@ -81,6 +69,12 @@ pub struct WindowsToken;
 #[cfg(any(unix, doc))]
 #[doc(cfg(unix))]
 pub struct UnixToken;
+
+/// Token struct that is only available with the `serde` feature
+#[cfg(feature = "serde")]
+#[doc(cfg(feature = "serde"))]
+#[derive(serde::Deserialize)]
+pub struct SerdeToken;
 ```
 
 In this sample, the tokens will only appear on their respective platforms, but they will both appear
@@ -90,6 +84,7 @@ in documentation.
 `#![feature(doc_cfg)]` feature gate. For more information, see [its chapter in the Unstable
 Book][unstable-doc-cfg] and [its tracking issue][issue-doc-cfg].
 
+[cfg-doc]: ./advanced-features.md
 [unstable-doc-cfg]: ../unstable-book/language-features/doc-cfg.html
 [issue-doc-cfg]: https://github.com/rust-lang/rust/issues/43781