rustdoc: Don't show hidden trait methods

By skipping trait items whose attributes include `hidden`, we void
showing
such trait methods.

src/librustdoc/clean/inline.rs

@@ -389,13 +389,45 @@ fn merge_attrs(
         }
     }
 
+    let document_hidden = cx.render_options.document_hidden;
     let predicates = tcx.explicit_predicates_of(did);
     let (trait_items, generics) = match impl_item {
         Some(impl_) => (
             impl_
                 .items
                 .iter()
-                .map(|item| tcx.hir().impl_item(item.id).clean(cx))
+                .map(|item| tcx.hir().impl_item(item.id))
+                .filter(|item| {
+                    // Filter out impl items whose corresponding trait item has `doc(hidden)`
+                    // not to document such impl items.
+                    // For inherent impls, we don't do any filtering.
+
+                    // When `--document-hidden-items` is passed, we don't
+                    // do any filtering, too.
+                    if document_hidden {
+                        return true;
+                    }
+                    if let Some(associated_trait) = associated_trait {
+                        let assoc_kind = match item.kind {
+                            hir::ImplItemKind::Const(..) => ty::AssocKind::Const,
+                            hir::ImplItemKind::Fn(..) => ty::AssocKind::Fn,
+                            hir::ImplItemKind::TyAlias(..) => ty::AssocKind::Type,
+                        };
+                        let trait_item = tcx
+                            .associated_items(associated_trait.def_id)
+                            .find_by_name_and_kind(
+                                tcx,
+                                item.ident,
+                                assoc_kind,
+                                associated_trait.def_id,
+                            )
+                            .unwrap(); // SAFETY: For all impl items there exists trait item that has the same name.
+                        !tcx.get_attrs(trait_item.def_id).lists(sym::doc).has_word(sym::hidden)
+                    } else {
+                        true
+                    }
+                })
+                .map(|item| item.clean(cx))
                 .collect::<Vec<_>>(),
             impl_.generics.clean(cx),
         ),

src/test/rustdoc/hidden-trait-methods-with-document-hidden-items.rs

@@ -0,0 +1,31 @@
+// compile-flags: -Z unstable-options --document-hidden-items
+
+// test for trait methods with `doc(hidden)` with `--document-hidden-items` passed.
+#![crate_name = "foo"]
+
+// @has foo/trait.Trait.html
+// @has - '//*[@id="associatedtype.Foo"]' 'type Foo'
+// @has - '//*[@id="associatedtype.Bar"]' 'type Bar'
+// @has - '//*[@id="tymethod.f"]' 'fn f()'
+// @has - '//*[@id="tymethod.g"]' 'fn g()'
+pub trait Trait {
+    #[doc(hidden)]
+    type Foo;
+    type Bar;
+    #[doc(hidden)]
+    fn f();
+    fn g();
+}
+
+// @has foo/struct.S.html
+// @has - '//*[@id="associatedtype.Foo"]' 'type Foo'
+// @has - '//*[@id="associatedtype.Bar"]' 'type Bar'
+// @has - '//*[@id="method.f"]' 'fn f()'
+// @has - '//*[@id="method.g"]' 'fn g()'
+pub struct S;
+impl Trait for S {
+    type Foo = ();
+    type Bar = ();
+    fn f() {}
+    fn g() {}
+}

src/test/rustdoc/hidden-trait-methods.rs

@@ -0,0 +1,29 @@
+// test for trait methods with `doc(hidden)`.
+#![crate_name = "foo"]
+
+// @has foo/trait.Trait.html
+// @!has - '//*[@id="associatedtype.Foo"]' 'type Foo'
+// @has - '//*[@id="associatedtype.Bar"]' 'type Bar'
+// @!has - '//*[@id="tymethod.f"]' 'fn f()'
+// @has - '//*[@id="tymethod.g"]' 'fn g()'
+pub trait Trait {
+    #[doc(hidden)]
+    type Foo;
+    type Bar;
+    #[doc(hidden)]
+    fn f();
+    fn g();
+}
+
+// @has foo/struct.S.html
+// @!has - '//*[@id="associatedtype.Foo"]' 'type Foo'
+// @has - '//*[@id="associatedtype.Bar"]' 'type Bar'
+// @!has - '//*[@id="method.f"]' 'fn f()'
+// @has - '//*[@id="method.g"]' 'fn g()'
+pub struct S;
+impl Trait for S {
+    type Foo = ();
+    type Bar = ();
+    fn f() {}
+    fn g() {}
+}