Expose all OS-specific modules in libstd doc.
1. Uses the special `--cfg dox` configuration passed by rustbuild when running `rustdoc`. Changes the `#[cfg(platform)]` into `#[cfg(any(dox, platform))]` so that platform-specific API are visible to rustdoc.
2. Since platform-specific implementations often won't compile correctly on other platforms, `rustdoc` is changed to apply `everybody_loops` to the functions during documentation and doc-test harness.
3. Since platform-specific code are documented on all platforms now, it could confuse users who found a useful API but is non-portable. Also, their examples will be doc-tested, so must be excluded when not testing on the native platform. An undocumented attribute `#[doc(cfg(...))]` is introduced to serve the above purposed.
Fixes#24658 (Does _not_ fully implement #1998).
This attribute has two effects:
1. Items with this attribute and their children will have the "This is
supported on **** only" message attached in the documentation.
2. The items' doc tests will be skipped if the configuration does not
match.
rustdoc: add [src] links to associated functions inside an impl block
While impl blocks currently have a `[src]` link to show the source for the impl block as a whole, individual methods inside that impl block do not. This can pose a problem for structs with a lot of methods, like many in libstd. This change adds little `[src]` links to individual methods that point directly to the function in the bundled source.
fixes#12932

Previously, the union fields would all render on the same line with
hideous spacing; comparison to the analogous section for structs makes
it undoubtable that `display: block` is the true intent.
Concisely and definitively resolves#43404 and its perfidious
malignancy.
rustdoc: Fix implementors list javascript
* Use a different loop variable, `i` was already taken. This caused
missing items in the implementors list.
* Use `.getAttribute('href')` rather than `.href` to get the relative
URL which is what it needs to actually fix the links.
More fallout from #41307.
r? @GuillaumeGomez
* Use a different loop variable, `i` was already taken. This caused
missing items in the implementors list.
* Use `.getAttribute('href')` rather than `.href` to get the relative
URL which is what it needs to actually fix the links.
Makes the sidebar a light grey and highlights the currently viewed item
in the sidebar more prominently.
All visual design credit goes to @johnwhelchel (#37856)