rustdoc: reduce the amount of asyncness query executions

src/librustdoc/clean/mod.rs

@@ -1183,7 +1183,12 @@ fn clean_fn_decl_from_did_and_sig<'tcx>(
     // but shouldn't change any code meaning.
     let mut output = clean_middle_ty(sig.output(), cx, None, None);
 
-    if let Some(did) = did && cx.tcx.asyncness(did).is_async() {
+    // If the return type is not an `impl Trait`, we can safely assume that this function isn't async
+    // without needing to execute the `asyncness` query which gives us a slight performance boost.
+    if let Some(did) = did
+        && let Type::ImplTrait(_) = output
+        && cx.tcx.asyncness(did).is_async()
+    {
         output = output.sugared_async_return_type();
     }