Don't collect return-position impl traits for documentation

src/librustdoc/visit_ast.rs

@@ -421,12 +421,20 @@ impl<'a, 'tcx> RustdocVisitor<'a, 'tcx> {
             | hir::ItemKind::Struct(..)
             | hir::ItemKind::Union(..)
             | hir::ItemKind::TyAlias(..)
-            | hir::ItemKind::OpaqueTy(..)
+            | hir::ItemKind::OpaqueTy(hir::OpaqueTy {
+                origin: hir::OpaqueTyOrigin::TyAlias, ..
+            })
             | hir::ItemKind::Static(..)
             | hir::ItemKind::Trait(..)
             | hir::ItemKind::TraitAlias(..) => {
                 self.add_to_current_mod(item, renamed, import_id);
             }
+            hir::ItemKind::OpaqueTy(hir::OpaqueTy {
+                origin: hir::OpaqueTyOrigin::AsyncFn(_) | hir::OpaqueTyOrigin::FnReturn(_),
+                ..
+            }) => {
+                // return-position impl traits are never nameable, and should never be documented.
+            }
             hir::ItemKind::Const(..) => {
                 // Underscore constants do not correspond to a nameable item and
                 // so are never useful in documentation.

tests/rustdoc/async-fn-opaque-item.rs

@@ -0,0 +1,15 @@
+// compile-flags: --document-private-items --crate-type=lib
+// edition: 2021
+
+// Issue 109931 -- test against accidentally documenting the `impl Future`
+// that comes from an async fn desugaring.
+
+// Check that we don't document an unnamed opaque type
+// @!has async_fn_opaque_item/opaque..html
+
+// Checking there is only a "Functions" header and no "Opaque types".
+// @has async_fn_opaque_item/index.html
+// @count - '//*[@class="small-section-header"]' 1
+// @has - '//*[@class="small-section-header"]' 'Functions'
+
+pub async fn test() {}