Revisit of the representation of adjacently tagged enums tag

serde/src/private/de.rs

@@ -1,10 +1,13 @@
 use crate::lib::*;
 
 use crate::de::value::{BorrowedBytesDeserializer, BytesDeserializer};
-use crate::de::{Deserialize, Deserializer, Error, IntoDeserializer, Visitor};
+use crate::de::{
+    Deserialize, DeserializeSeed, Deserializer, EnumAccess, Error, IntoDeserializer, VariantAccess,
+    Visitor,
+};
 
 #[cfg(any(feature = "std", feature = "alloc"))]
-use crate::de::{DeserializeSeed, MapAccess, Unexpected};
+use crate::de::{MapAccess, Unexpected};
 
 #[cfg(any(feature = "std", feature = "alloc"))]
 pub use self::content::{
@@ -2836,3 +2839,60 @@ fn flat_map_take_entry<'de>(
         None
     }
 }
+
+pub struct AdjacentlyTaggedEnumVariantSeed<F> {
+    pub tag: &'static str,
+    pub variants: &'static [&'static str],
+    pub fields_enum: PhantomData<F>,
+}
+
+pub struct AdjacentlyTaggedEnumVariantVisitor<F> {
+    tag: &'static str,
+    fields_enum: PhantomData<F>,
+}
+
+impl<'de, F> Visitor<'de> for AdjacentlyTaggedEnumVariantVisitor<F>
+where
+    F: Deserialize<'de>,
+{
+    type Value = F;
+
+    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        write!(formatter, "enum {}", self.tag)
+    }
+
+    fn visit_enum<A>(self, data: A) -> Result<Self::Value, A::Error>
+    where
+        A: EnumAccess<'de>,
+    {
+        let (variant, variant_access) = match data.variant() {
+            Ok(values) => values,
+            Err(err) => return Err(err),
+        };
+        if let Err(err) = variant_access.unit_variant() {
+            return Err(err);
+        }
+        Ok(variant)
+    }
+}
+
+impl<'de, F> DeserializeSeed<'de> for AdjacentlyTaggedEnumVariantSeed<F>
+where
+    F: Deserialize<'de>,
+{
+    type Value = F;
+
+    fn deserialize<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        deserializer.deserialize_enum(
+            self.tag,
+            self.variants,
+            AdjacentlyTaggedEnumVariantVisitor {
+                tag: self.tag,
+                fields_enum: PhantomData,
+            },
+        )
+    }
+}

serde/src/private/ser.rs

@@ -1355,3 +1355,28 @@ where
         Ok(())
     }
 }
+
+pub struct AdjacentlyTaggedEnumVariantSerializer {
+    tag: &'static str,
+    variant_index: u32,
+    variant_name: &'static str,
+}
+
+impl AdjacentlyTaggedEnumVariantSerializer {
+    pub fn new(tag: &'static str, variant_index: u32, variant_name: &'static str) -> Self {
+        AdjacentlyTaggedEnumVariantSerializer {
+            tag,
+            variant_index,
+            variant_name,
+        }
+    }
+}
+
+impl Serialize for AdjacentlyTaggedEnumVariantSerializer {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        serializer.serialize_unit_variant(self.tag, self.variant_index, self.variant_name)
+    }
+}

serde_derive/src/de.rs

@@ -1510,6 +1510,14 @@ fn deserialize_adjacently_tagged_enum(
         }
     };
 
+    let variant_seed = quote! {
+        _serde::__private::de::AdjacentlyTaggedEnumVariantSeed::<__Field> {
+            tag: #tag,
+            variants: &VARIANTS,
+            fields_enum: _serde::__private::PhantomData
+        }
+    };
+
     let mut missing_content = quote! {
         _serde::__private::Err(<__A::Error as _serde::de::Error>::missing_field(#content))
     };
@@ -1557,6 +1565,10 @@ fn deserialize_adjacently_tagged_enum(
         _serde::de::MapAccess::next_key_seed(&mut __map, #tag_or_content)?
     };
 
+    let variant_from_map = quote! {
+        _serde::de::MapAccess::next_value_seed(&mut __map, #variant_seed)?
+    };
+
     // When allowing unknown fields, we want to transparently step through keys
     // we don't care about until we find `tag`, `content`, or run out of keys.
     let next_relevant_key = if deny_unknown_fields {
@@ -1602,11 +1614,11 @@ fn deserialize_adjacently_tagged_enum(
 
     let finish_content_then_tag = if variant_arms.is_empty() {
         quote! {
-            match _serde::de::MapAccess::next_value::<__Field>(&mut __map)? {}
+            match #variant_from_map {}
         }
     } else {
         quote! {
-            let __ret = match _serde::de::MapAccess::next_value(&mut __map)? {
+            let __ret = match #variant_from_map {
                 // Deserialize the buffered content now that we know the variant.
                 #(#variant_arms)*
             }?;
@@ -1662,7 +1674,7 @@ fn deserialize_adjacently_tagged_enum(
                     // First key is the tag.
                     _serde::__private::Some(_serde::__private::de::TagOrContentField::Tag) => {
                         // Parse the tag.
-                        let __field = _serde::de::MapAccess::next_value(&mut __map)?;
+                        let __field = #variant_from_map;
                         // Visit the second key.
                         match #next_relevant_key {
                             // Second key is a duplicate of the tag.

serde_derive/src/ser.rs

@@ -478,7 +478,14 @@ fn serialize_variant(
                 serialize_internally_tagged_variant(params, variant, cattrs, tag)
             }
             (attr::TagType::Adjacent { tag, content }, false) => {
-                serialize_adjacently_tagged_variant(params, variant, cattrs, tag, content)
+                serialize_adjacently_tagged_variant(
+                    params,
+                    variant,
+                    cattrs,
+                    variant_index,
+                    tag,
+                    content,
+                )
             }
             (attr::TagType::None, _) | (_, true) => {
                 serialize_untagged_variant(params, variant, cattrs)
@@ -634,12 +641,14 @@ fn serialize_adjacently_tagged_variant(
     params: &Parameters,
     variant: &Variant,
     cattrs: &attr::Container,
+    variant_index: u32,
     tag: &str,
     content: &str,
 ) -> Fragment {
     let this_type = &params.this_type;
     let type_name = cattrs.name().serialize_name();
     let variant_name = variant.attrs.name().serialize_name();
+    let variant_serializer = quote!(&_serde::__private::ser::AdjacentlyTaggedEnumVariantSerializer::new(#tag, #variant_index, #variant_name));
 
     let inner = Stmts(if let Some(path) = variant.attrs.serialize_with() {
         let ser = wrap_serialize_variant_with(params, path, variant);
@@ -653,7 +662,7 @@ fn serialize_adjacently_tagged_variant(
                     let mut __struct = _serde::Serializer::serialize_struct(
                         __serializer, #type_name, 1)?;
                     _serde::ser::SerializeStruct::serialize_field(
-                        &mut __struct, #tag, #variant_name)?;
+                        &mut __struct, #tag, #variant_serializer)?;
                     _serde::ser::SerializeStruct::end(__struct)
                 };
             }
@@ -670,7 +679,7 @@ fn serialize_adjacently_tagged_variant(
                     let mut __struct = _serde::Serializer::serialize_struct(
                         __serializer, #type_name, 2)?;
                     _serde::ser::SerializeStruct::serialize_field(
-                        &mut __struct, #tag, #variant_name)?;
+                        &mut __struct, #tag, #variant_serializer)?;
                     #func(
                         &mut __struct, #content, #field_expr)?;
                     _serde::ser::SerializeStruct::end(__struct)
@@ -735,7 +744,7 @@ fn serialize_adjacently_tagged_variant(
         let mut __struct = _serde::Serializer::serialize_struct(
             __serializer, #type_name, 2)?;
         _serde::ser::SerializeStruct::serialize_field(
-            &mut __struct, #tag, #variant_name)?;
+            &mut __struct, #tag, #variant_serializer)?;
         _serde::ser::SerializeStruct::serialize_field(
             &mut __struct, #content, &__AdjacentlyTagged {
                 data: (#(#fields_ident,)*),

test_suite/tests/test_annotations.rs

@@ -2108,7 +2108,10 @@ fn test_adjacently_tagged_enum_bytes() {
                 len: 2,
             },
             Token::Str("t"),
-            Token::Str("A"),
+            Token::UnitVariant {
+                name: "t",
+                variant: "A",
+            },
             Token::Str("c"),
             Token::Struct { name: "A", len: 1 },
             Token::Str("a"),
@@ -2126,7 +2129,10 @@ fn test_adjacently_tagged_enum_bytes() {
                 len: 2,
             },
             Token::Bytes(b"t"),
-            Token::Str("A"),
+            Token::UnitVariant {
+                name: "t",
+                variant: "A",
+            },
             Token::Bytes(b"c"),
             Token::Struct { name: "A", len: 1 },
             Token::Str("a"),
@@ -2167,7 +2173,10 @@ fn test_adjacently_tagged_enum_containing_flatten() {
                 len: 2,
             },
             Token::Str("t"),
-            Token::Str("A"),
+            Token::UnitVariant {
+                name: "t",
+                variant: "A",
+            },
             Token::Str("c"),
             Token::Map { len: None },
             Token::Str("a"),
@@ -2757,7 +2766,7 @@ fn test_expecting_message_adjacently_tagged_enum() {
     // Check that #[serde(expecting = "...")] doesn't affect variant identifier error message
     assert_de_tokens_error::<Enum>(
         &[Token::Map { len: None }, Token::Str("tag"), Token::Unit],
-        r#"invalid type: unit value, expected variant identifier"#,
+        r#"invalid type: unit value, expected enum tag"#,
     );
 }
 
@@ -2992,7 +3001,10 @@ mod flatten {
                         Token::Str("outer"),
                         Token::U32(42),
                         Token::Str("tag"),
-                        Token::Str("Struct"),
+                        Token::UnitVariant {
+                            name: "tag",
+                            variant: "Struct",
+                        },
                         Token::Str("content"),
                         Token::Struct {
                             len: 2,
@@ -3020,7 +3032,10 @@ mod flatten {
                         Token::Str("outer"),
                         Token::U32(42),
                         Token::Str("tag"),
-                        Token::Str("Newtype"),
+                        Token::UnitVariant {
+                            name: "tag",
+                            variant: "Newtype",
+                        },
                         Token::Str("content"),
                         Token::Struct {
                             len: 1,

test_suite/tests/test_macros.rs

@@ -472,7 +472,10 @@ fn test_adjacently_tagged_newtype_struct() {
             },
             Token::U32(5),
             Token::Str("t"),
-            Token::Str("Newtype"),
+            Token::UnitVariant {
+                name: "t",
+                variant: "Newtype",
+            },
             Token::StructEnd,
         ],
     );
@@ -1066,7 +1069,10 @@ fn test_adjacently_tagged_enum() {
                 len: 1,
             },
             Token::Str("t"),
-            Token::Str("Unit"),
+            Token::UnitVariant {
+                name: "t",
+                variant: "Unit",
+            },
             Token::StructEnd,
         ],
     );
@@ -1080,7 +1086,10 @@ fn test_adjacently_tagged_enum() {
                 len: 2,
             },
             Token::Str("t"),
-            Token::Str("Unit"),
+            Token::UnitVariant {
+                name: "t",
+                variant: "Unit",
+            },
             Token::StructEnd,
         ],
     );
@@ -1094,7 +1103,10 @@ fn test_adjacently_tagged_enum() {
                 len: 2,
             },
             Token::Str("t"),
-            Token::Str("Unit"),
+            Token::UnitVariant {
+                name: "t",
+                variant: "Unit",
+            },
             Token::Str("c"),
             Token::Unit,
             Token::StructEnd,
@@ -1112,7 +1124,10 @@ fn test_adjacently_tagged_enum() {
             Token::Str("c"),
             Token::Unit,
             Token::Str("t"),
-            Token::Str("Unit"),
+            Token::UnitVariant {
+                name: "t",
+                variant: "Unit",
+            },
             Token::StructEnd,
         ],
     );
@@ -1128,7 +1143,10 @@ fn test_adjacently_tagged_enum() {
             Token::Str("f"),
             Token::Unit,
             Token::Str("t"),
-            Token::Str("Unit"),
+            Token::UnitVariant {
+                name: "t",
+                variant: "Unit",
+            },
             Token::Str("g"),
             Token::Unit,
             Token::Str("c"),
@@ -1148,7 +1166,10 @@ fn test_adjacently_tagged_enum() {
                 len: 2,
             },
             Token::Str("t"),
-            Token::Str("Newtype"),
+            Token::UnitVariant {
+                name: "t",
+                variant: "Newtype",
+            },
             Token::Str("c"),
             Token::U8(1),
             Token::StructEnd,
@@ -1166,7 +1187,10 @@ fn test_adjacently_tagged_enum() {
             Token::Str("c"),
             Token::U8(1),
             Token::Str("t"),
-            Token::Str("Newtype"),
+            Token::UnitVariant {
+                name: "t",
+                variant: "Newtype",
+            },
             Token::StructEnd,
         ],
     );
@@ -1180,7 +1204,10 @@ fn test_adjacently_tagged_enum() {
                 len: 1,
             },
             Token::Str("t"),
-            Token::Str("Newtype"),
+            Token::UnitVariant {
+                name: "t",
+                variant: "Newtype",
+            },
             Token::StructEnd,
         ],
     );
@@ -1194,7 +1221,10 @@ fn test_adjacently_tagged_enum() {
                 len: 2,
             },
             Token::Str("t"),
-            Token::Str("Tuple"),
+            Token::UnitVariant {
+                name: "t",
+                variant: "Tuple",
+            },
             Token::Str("c"),
             Token::Tuple { len: 2 },
             Token::U8(1),
@@ -1218,7 +1248,10 @@ fn test_adjacently_tagged_enum() {
             Token::U8(1),
             Token::TupleEnd,
             Token::Str("t"),
-            Token::Str("Tuple"),
+            Token::UnitVariant {
+                name: "t",
+                variant: "Tuple",
+            },
             Token::StructEnd,
         ],
     );
@@ -1232,7 +1265,10 @@ fn test_adjacently_tagged_enum() {
                 len: 2,
             },
             Token::Str("t"),
-            Token::Str("Struct"),
+            Token::UnitVariant {
+                name: "t",
+                variant: "Struct",
+            },
             Token::Str("c"),
             Token::Struct {
                 name: "Struct",
@@ -1262,7 +1298,10 @@ fn test_adjacently_tagged_enum() {
             Token::U8(1),
             Token::StructEnd,
             Token::Str("t"),
-            Token::Str("Struct"),
+            Token::UnitVariant {
+                name: "t",
+                variant: "Struct",
+            },
             Token::StructEnd,
         ],
     );
@@ -1278,7 +1317,10 @@ fn test_adjacently_tagged_enum() {
             Token::U64(1), // content field
             Token::U8(1),
             Token::U64(0), // tag field
-            Token::Str("Newtype"),
+            Token::UnitVariant {
+                name: "t",
+                variant: "Newtype",
+            },
             Token::StructEnd,
         ],
     );
@@ -1294,7 +1336,10 @@ fn test_adjacently_tagged_enum() {
             Token::Bytes(b"c"),
             Token::U8(1),
             Token::Bytes(b"t"),
-            Token::Str("Newtype"),
+            Token::UnitVariant {
+                name: "t",
+                variant: "Newtype",
+            },
             Token::StructEnd,
         ],
     );
@@ -1316,7 +1361,10 @@ fn test_adjacently_tagged_enum_deny_unknown_fields() {
                 len: 2,
             },
             Token::Str("t"),
-            Token::Str("Unit"),
+            Token::UnitVariant {
+                name: "t",
+                variant: "Unit",
+            },
             Token::Str("c"),
             Token::Unit,
             Token::StructEnd,
@@ -1330,7 +1378,10 @@ fn test_adjacently_tagged_enum_deny_unknown_fields() {
                 len: 2,
             },
             Token::Str("t"),
-            Token::Str("Unit"),
+            Token::UnitVariant {
+                name: "t",
+                variant: "Unit",
+            },
             Token::Str("c"),
             Token::Unit,
             Token::Str("h"),
@@ -1369,7 +1420,10 @@ fn test_adjacently_tagged_enum_deny_unknown_fields() {
                 len: 2,
             },
             Token::U64(0), // tag field
-            Token::Str("Unit"),
+            Token::UnitVariant {
+                name: "t",
+                variant: "Unit",
+            },
             Token::U64(3),
         ],
         r#"invalid value: integer `3`, expected "t" or "c""#,
@@ -1565,7 +1619,10 @@ fn test_internally_tagged_struct_with_flattened_field() {
             Token::Str("tag_struct"),
             Token::Str("Struct"),
             Token::Str("tag_enum"),
-            Token::Str("A"),
+            Token::UnitVariant {
+                name: "tag_enum",
+                variant: "A",
+            },
             Token::Str("content"),
             Token::U64(0),
             Token::MapEnd,