Apply `doc(cfg)` from parent items while collecting trait impls

Because trait impls bypass the standard `clean` hierarchy they do not
participate in the `propagate_doc_cfg` pass, so instead we need to
pre-collect all possible `doc(cfg)` attributes that will apply to them
when cleaning.

src/librustdoc/clean/inline.rs

@@ -339,9 +339,6 @@ crate fn build_impl(
         return;
     }
 
-    let attrs = merge_attrs(cx, parent_module.into(), load_attrs(cx, did), attrs);
-    debug!("merged_attrs={:?}", attrs);
-
     let tcx = cx.tcx;
     let associated_trait = tcx.impl_trait_ref(did);
 
@@ -435,7 +432,7 @@ crate fn build_impl(
 
     debug!("build_impl: impl {:?} for {:?}", trait_.def_id(), for_.def_id());
 
-    ret.push(clean::Item::from_def_id_and_parts(
+    let mut item = clean::Item::from_def_id_and_parts(
         did,
         None,
         clean::ImplItem(clean::Impl {
@@ -450,7 +447,10 @@ crate fn build_impl(
             blanket_impl: None,
         }),
         cx,
-    ));
+    );
+    item.attrs = merge_attrs(cx, parent_module.into(), load_attrs(cx, did), attrs);
+    debug!("merged_attrs={:?}", item.attrs);
+    ret.push(item);
 }
 
 fn build_module(cx: &DocContext<'_>, did: DefId, visited: &mut FxHashSet<DefId>) -> clean::Module {

src/librustdoc/passes/collect_trait_impls.rs

@@ -5,6 +5,7 @@ use crate::fold::DocFolder;
 
 use rustc_data_structures::fx::FxHashSet;
 use rustc_hir::def_id::{DefId, LOCAL_CRATE};
+use rustc_middle::ty::DefIdTree;
 use rustc_span::symbol::sym;
 
 crate const COLLECT_TRAIT_IMPLS: Pass = Pass {
@@ -90,7 +91,32 @@ crate fn collect_trait_impls(krate: Crate, cx: &DocContext<'_>) -> Crate {
         for &impl_node in cx.tcx.hir().trait_impls(trait_did) {
             let impl_did = cx.tcx.hir().local_def_id(impl_node);
             cx.tcx.sess.time("build_local_trait_impl", || {
-                inline::build_impl(cx, None, impl_did.to_def_id(), None, &mut new_items);
+                let mut extra_attrs = Vec::new();
+                let mut parent = cx.tcx.parent(impl_did.to_def_id());
+                while let Some(did) = parent {
+                    extra_attrs.extend(
+                        cx.tcx
+                            .get_attrs(did)
+                            .iter()
+                            .filter(|attr| attr.has_name(sym::doc))
+                            .filter(|attr| {
+                                if let Some([attr]) = attr.meta_item_list().as_deref() {
+                                    attr.has_name(sym::cfg)
+                                } else {
+                                    false
+                                }
+                            })
+                            .cloned(),
+                    );
+                    parent = cx.tcx.parent(did);
+                }
+                inline::build_impl(
+                    cx,
+                    None,
+                    impl_did.to_def_id(),
+                    Some(&extra_attrs),
+                    &mut new_items,
+                );
             });
         }
     }

src/test/rustdoc/doc-cfg.rs

@@ -25,12 +25,13 @@ pub mod unix_only {
     // @has doc_cfg/unix_only/trait.ArmOnly.html \
     //  '//*[@id="main"]/*[@class="stability"]/*[@class="stab portability"]' \
     //  'This is supported on Unix and ARM only.'
-    // @count - '//*[@class="stab portability"]' 2
+    // @count - '//*[@class="stab portability"]' 1
     #[doc(cfg(target_arch = "arm"))]
     pub trait ArmOnly {
         fn unix_and_arm_only_function();
     }
 
+    #[doc(cfg(target_arch = "arm"))]
     impl ArmOnly for super::Portable {
         fn unix_and_arm_only_function() {}
     }

src/test/rustdoc/issue-79201.rs

@@ -0,0 +1,41 @@
+#![feature(doc_cfg)]
+
+// @has 'issue_79201/trait.Foo.html'
+// @count   - '//*[@class="stab portability"]' 6
+// @matches - '//*[@class="stab portability"]' 'crate feature foo-root'
+// @matches - '//*[@class="stab portability"]' 'crate feature foo-public-mod'
+// @matches - '//*[@class="stab portability"]' 'crate feature foo-private-mod'
+// @matches - '//*[@class="stab portability"]' 'crate feature foo-fn'
+// @matches - '//*[@class="stab portability"]' 'crate feature foo-method'
+
+pub trait Foo {}
+
+#[doc(cfg(feature = "foo-root"))]
+impl crate::Foo for usize {}
+
+#[doc(cfg(feature = "foo-public-mod"))]
+pub mod public {
+    impl crate::Foo for u8 {}
+}
+
+#[doc(cfg(feature = "foo-private-mod"))]
+mod private {
+    impl crate::Foo for u16 {}
+}
+
+#[doc(cfg(feature = "foo-const"))]
+const _: () = {
+    impl crate::Foo for u32 {}
+};
+
+#[doc(cfg(feature = "foo-fn"))]
+fn __() {
+    impl crate::Foo for u64 {}
+}
+
+#[doc(cfg(feature = "foo-method"))]
+impl dyn Foo {
+    fn __() {
+        impl crate::Foo for u128 {}
+    }
+}