Fix generated code for deserializing untagged newtype variant

serde_derive/src/de.rs

@@ -1802,10 +1802,8 @@ fn deserialize_untagged_newtype_variant(
         }
         Some(path) => {
             quote_block! {
-                let __value: #field_ty = _serde::export::Result::map(
-                    #path(#deserializer),
-                    #this::#variant_ident);
-                __value
+                let __value: _serde::export::Result<#field_ty, _> = #path(#deserializer);
+                _serde::export::Result::map(__value, #this::#variant_ident)
             }
         }
     }

test_suite/tests/test_gen.rs

@@ -643,6 +643,16 @@ fn test_gen() {
     struct ImpliciltyBorrowedOption<'a> {
         option: std::option::Option<&'a str>,
     }
+
+    #[derive(Serialize, Deserialize)]
+    #[serde(untagged)]
+    enum UntaggedNewtypeVariantWith {
+        Newtype(
+            #[serde(serialize_with = "ser_x")]
+            #[serde(deserialize_with = "de_x")]
+            X,
+        ),
+    }
 }
 
 //////////////////////////////////////////////////////////////////////////