add missing visit_enum

serde/src/de/mod.rs

@@ -2010,6 +2010,18 @@ pub trait EnumAccess<'de>: Sized {
     }
 }
 
+/// Variant Hint for VariantAccess
+pub enum VariantHint {
+    /// variant with no values
+    Unit,
+    /// variant with a single value
+    Newtype,
+    /// vuple-like variant with the length of the tuple
+    Tuple(usize),
+    /// struct-like variant. with names of the fields of the struct variant
+    Struct(&'static [&'static str]),
+}
+
 /// `VariantAccess` is a visitor that is created by the `Deserializer` and
 /// passed to the `Deserialize` to deserialize the content of a particular enum
 /// variant.
@@ -2127,6 +2139,49 @@ pub trait VariantAccess<'de>: Sized {
         self.newtype_variant_seed(PhantomData)
     }
 
+    /// Variant type hint
+    /// ```edition2021
+    /// # use serde::de::{self, value, DeserializeSeed, Visitor, VariantAccess, VariantHint, Unexpected};
+    /// #
+    /// # enum X {
+    /// #     Unit,
+    /// #     Newtype,
+    /// # }
+    /// #
+    /// # impl<'de> VariantAccess<'de> for X {
+    /// #     type Error = value::Error;
+    /// #
+    /// #     fn unit_variant(self) -> Result<(), Self::Error> {
+    /// #         unimplemented!()
+    /// #     }
+    /// #
+    /// fn hint(&self) -> Option<VariantHint> {
+    ///     Some(match self {
+    ///         Self::Unit => VariantHint::Unit,
+    ///         Self::Newtype => VariantHint::Newtype,
+    ///     })
+    /// }
+    /// #
+    /// #     fn newtype_variant_seed<T>(self, _seed: T) -> Result<T::Value, Self::Error>
+    /// #     where
+    /// #     T: DeserializeSeed<'de>,
+    /// #     { unimplemented!() }
+    /// #
+    /// #     fn tuple_variant<V>(self, _: usize, _: V) -> Result<V::Value, Self::Error>
+    /// #     where
+    /// #         V: Visitor<'de>,
+    /// #     { unimplemented!() }
+    /// #
+    /// #     fn struct_variant<V>(self, _: &[&str], _: V) -> Result<V::Value, Self::Error>
+    /// #     where
+    /// #         V: Visitor<'de>,
+    /// #     { unimplemented!() }
+    /// # }
+    /// ```
+    fn hint(&self) -> Option<VariantHint> {
+        None
+    }
+
     /// Called when deserializing a tuple-like variant.
     ///
     /// The `len` is the number of fields expected in the tuple variant.

serde/src/private/de.rs

@@ -212,7 +212,7 @@ mod content {
     use crate::de::value::{MapDeserializer, SeqDeserializer};
     use crate::de::{
         self, size_hint, Deserialize, DeserializeSeed, Deserializer, EnumAccess, Expected,
-        IgnoredAny, MapAccess, SeqAccess, Unexpected, Visitor,
+        IgnoredAny, MapAccess, SeqAccess, Unexpected, VariantAccess, Visitor,
     };
 
     /// Used from generated code to buffer the contents of the Deserializer when
@@ -525,13 +525,21 @@ mod content {
             Ok(Content::Map(vec))
         }
 
-        fn visit_enum<V>(self, _visitor: V) -> Result<Self::Value, V::Error>
+        fn visit_enum<V>(self, visitor: V) -> Result<Self::Value, V::Error>
         where
             V: EnumAccess<'de>,
         {
-            Err(de::Error::custom(
+            let (key, data) = tri!(visitor.variant::<Content>());
+            let variant_hint = tri!(data.hint().ok_or(de::Error::custom(
                 "untagged and internally tagged enums do not support enum input",
-            ))
+            )));
+            let data = match variant_hint {
+                de::VariantHint::Unit => Content::Unit,
+                de::VariantHint::Newtype => tri!(data.newtype_variant()),
+                de::VariantHint::Tuple(len) => tri!(data.tuple_variant(len, self)),
+                de::VariantHint::Struct(fields) => tri!(data.struct_variant(fields, self)),
+            };
+            Ok(Content::Map([(key, data)].into()))
         }
     }