tests: Actually test skip_debug for prost::Oneof

tests/src/skip_debug.rs

@@ -37,6 +37,11 @@ pub enum OneofWithEnumCustomDebug {
     #[prost(enumeration = "BasicEnumeration", tag = "10")]
     Enumeration(i32),
 }
+impl fmt::Debug for OneofWithEnumCustomDebug {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str("OneofWithEnumCustomDebug {..}")
+    }
+}
 
 #[derive(Clone, PartialEq, prost::Message)]
 #[prost(skip_debug)]
@@ -54,10 +59,12 @@ impl fmt::Debug for MessageWithOneofCustomDebug {
 /// Enumerations inside oneofs
 #[test]
 fn oneof_with_enum_custom_debug() {
+    let of = OneofWithEnumCustomDebug::Enumeration(
+        BasicEnumeration::TWO as i32,
+    );
+    assert_eq!(format!("{:?}", of), "OneofWithEnumCustomDebug {..}");
     let msg = MessageWithOneofCustomDebug {
-        of: Some(OneofWithEnumCustomDebug::Enumeration(
-            BasicEnumeration::TWO as i32,
-        )),
+        of: Some(of),
     };
     assert_eq!(format!("{:?}", msg), "MessageWithOneofCustomDebug {..}");
 }