Support flattened externally tagged enums with default attributes

serde/src/private/de.rs

@@ -7,7 +7,7 @@ use crate::de::{
 };
 
 #[cfg(any(feature = "std", feature = "alloc"))]
-use crate::de::{MapAccess, Unexpected};
+use crate::de::{Expected, MapAccess, StdError, Unexpected};
 
 #[cfg(any(feature = "std", feature = "alloc"))]
 pub use self::content::{
@@ -2586,6 +2586,72 @@ where
     }
 }
 
+#[cfg(any(feature = "std", feature = "alloc"))]
+#[derive(Debug)]
+pub enum FlatMapDeserializerError<E> {
+    Inner(E),
+    NoVariantFoundInFlattenedData(&'static str),
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<E> Display for FlatMapDeserializerError<E>
+where
+    E: Display,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            FlatMapDeserializerError::Inner(e) => write!(f, "{}", e),
+            FlatMapDeserializerError::NoVariantFoundInFlattenedData(name) => {
+                write!(f, "no variant of enum {} found in flattened data", name)
+            }
+        }
+    }
+}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<E> StdError for FlatMapDeserializerError<E> where E: StdError {}
+
+#[cfg(any(feature = "std", feature = "alloc"))]
+impl<E> Error for FlatMapDeserializerError<E>
+where
+    E: Error,
+{
+    fn custom<T>(msg: T) -> Self
+    where
+        T: Display,
+    {
+        FlatMapDeserializerError::Inner(Error::custom(msg))
+    }
+
+    fn invalid_type(unexp: Unexpected, exp: &Expected) -> Self {
+        FlatMapDeserializerError::Inner(Error::invalid_type(unexp, exp))
+    }
+
+    fn invalid_value(unexp: Unexpected, exp: &Expected) -> Self {
+        FlatMapDeserializerError::Inner(Error::invalid_value(unexp, exp))
+    }
+
+    fn invalid_length(len: usize, exp: &Expected) -> Self {
+        FlatMapDeserializerError::Inner(Error::invalid_length(len, exp))
+    }
+
+    fn unknown_variant(variant: &str, exp: &'static [&'static str]) -> Self {
+        FlatMapDeserializerError::Inner(Error::unknown_variant(variant, exp))
+    }
+
+    fn unknown_field(field: &str, exp: &'static [&'static str]) -> Self {
+        FlatMapDeserializerError::Inner(Error::unknown_field(field, exp))
+    }
+
+    fn missing_field(field: &'static str) -> Self {
+        FlatMapDeserializerError::Inner(Error::missing_field(field))
+    }
+
+    fn duplicate_field(field: &'static str) -> Self {
+        FlatMapDeserializerError::Inner(Error::duplicate_field(field))
+    }
+}
+
 #[cfg(any(feature = "std", feature = "alloc"))]
 pub struct FlatMapDeserializer<'a, 'de: 'a, E>(
     pub &'a mut Vec<Option<(Content<'de>, Content<'de>)>>,
@@ -2597,7 +2663,7 @@ impl<'a, 'de, E> FlatMapDeserializer<'a, 'de, E>
 where
     E: Error,
 {
-    fn deserialize_other<V>() -> Result<V, E> {
+    fn deserialize_other<V>() -> Result<V, FlatMapDeserializerError<E>> {
         Err(Error::custom("can only flatten structs and maps"))
     }
 }
@@ -2621,7 +2687,7 @@ impl<'a, 'de, E> Deserializer<'de> for FlatMapDeserializer<'a, 'de, E>
 where
     E: Error,
 {
-    type Error = E;
+    type Error = FlatMapDeserializerError<E>;
 
     fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
     where
@@ -2645,10 +2711,9 @@ 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>

serde_derive/src/de.rs

@@ -2594,11 +2594,36 @@ fn deserialize_map(
                 }
                 Some(path) => quote!(#path),
             };
+
+            let no_variant_expr = if field.attrs.default().is_none() && cattrs.default().is_none() {
+                let span = field.original.span();
+                quote_spanned!(span=>
+                    return _serde::__private::Err(
+                        _serde::de::Error::custom(_e)
+                    );
+                )
+            } else {
+                let is_missing = Expr(expr_is_missing(field, cattrs));
+                quote!(#is_missing)
+            };
+
             quote! {
-                let #name: #field_ty = #func(
+                let #name: #field_ty = match #func(
                     _serde::__private::de::FlatMapDeserializer(
                         &mut __collect,
-                        _serde::__private::PhantomData))?;
+                        _serde::__private::PhantomData)) {
+                        _serde::__private::Ok(#name) => #name,
+                        _serde::__private::Err(
+                            _e @ _serde::__private::de::FlatMapDeserializerError::NoVariantFoundInFlattenedData(..)
+                        ) => {
+                            #no_variant_expr
+                        }
+                        _serde::__private::Err(
+                            _serde::__private::de::FlatMapDeserializerError::Inner(e)
+                        ) => {
+                            return _serde::__private::Err(e);
+                        }
+                    };
             }
         });