Implemented #[doc(cfg(...))].

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.

1 files changed, 13 insertions, 0 deletions

diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index aeb574bc3af..668732e6855 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -364,6 +364,9 @@ 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! (
@@ -1157,6 +1160,16 @@ 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
         }