Distinguish between custom and flattened variant errors

serde/src/private/de.rs

@@ -2587,17 +2587,43 @@ where
 }
 
 #[cfg(any(feature = "std", feature = "alloc"))]
-pub struct FlatMapDeserializer<'a, 'de: 'a, E>(
-    pub &'a mut Vec<Option<(Content<'de>, Content<'de>)>>,
-    pub PhantomData<E>,
-);
+#[cfg_attr(any(feature = "std", feature = "alloc"), derive(Debug))]
+pub enum FlatMapDeserializerError {
+    Message(String),
+    NoVariantFoundInFlattenedData(&'static str),
+}
 
 #[cfg(any(feature = "std", feature = "alloc"))]
-impl<'a, 'de, E> FlatMapDeserializer<'a, 'de, E>
-where
-    E: Error,
-{
-    fn deserialize_other<V>() -> Result<V, E> {
+impl Display for FlatMapDeserializerError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            FlatMapDeserializerError::Message(msg) => write!(f, "{}", msg),
+            FlatMapDeserializerError::NoVariantFoundInFlattenedData(name) => {
+                write!(f, "no variant of enum {} found in flattened data", name)
+            }
+        }
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl std::error::Error for FlatMapDeserializerError {}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl Error for FlatMapDeserializerError {
+    fn custom<T>(msg: T) -> Self
+    where
+        T: Display,
+    {
+        FlatMapDeserializerError::Message(msg.to_string())
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+pub struct FlatMapDeserializer<'a, 'de: 'a>(pub &'a mut Vec<Option<(Content<'de>, Content<'de>)>>);
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<'a, 'de> FlatMapDeserializer<'a, 'de> {
+    fn deserialize_other<V>() -> Result<V, FlatMapDeserializerError> {
         Err(Error::custom("can only flatten structs and maps"))
     }
 }
@@ -2617,11 +2643,8 @@ macro_rules! forward_to_deserialize_other {
 }
 
 #[cfg(any(feature = "std", feature = "alloc"))]
-impl<'a, 'de, E> Deserializer<'de> for FlatMapDeserializer<'a, 'de, E>
-where
-    E: Error,
-{
-    type Error = E;
+impl<'a, 'de> Deserializer<'de> for FlatMapDeserializer<'a, 'de> {
+    type Error = FlatMapDeserializerError;
 
     fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
     where
@@ -2645,10 +2668,9 @@ where
             }
         }
 
-        Err(Error::custom(format_args!(
-            "no variant of enum {} found in flattened data",
-            name
-        )))
+        Err(FlatMapDeserializerError::NoVariantFoundInFlattenedData(
+            name,
+        ))
     }
 
     fn deserialize_map<V>(self, visitor: V) -> Result<V::Value, Self::Error>