Auto merge of #43348 - kennytm:fix-24658-doc-every-platform, r=alexcrichton

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).

1 files changed, 13 insertions, 0 deletions

diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index 46ee126b9d9..2b8a733b852 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -372,6 +372,9 @@ declare_features! (
     // global allocators and their internals
     (active, global_allocator, "1.20.0", None),
     (active, allocator_internals, "1.20.0", None),
+
+    // #[doc(cfg(...))]
+    (active, doc_cfg, "1.21.0", Some(43781)),
 );
 
 declare_features! (
@@ -1172,6 +1175,16 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
             self.context.check_attribute(attr, false);
         }
 
+        if attr.check_name("doc") {
+            if let Some(content) = attr.meta_item_list() {
+                if content.len() == 1 && content[0].check_name("cfg") {
+                    gate_feature_post!(&self, doc_cfg, attr.span,
+                        "#[doc(cfg(...))] is experimental"
+                    );
+                }
+            }
+        }
+
         if self.context.features.proc_macro && attr::is_known(attr) {
             return
         }