Repository: toml-rs/toml (language: rust)

Issue:
Serializing simple enum leads to `UnsupportedType`
Sorry if the answer is obvious, but I cannot understand why `toml` is not able to serialize this struct (but `serde_json` can):

```rust
#[derive(Debug, serde::Serialize, serde::Deserialize)]
struct Struct {
    field: Enum,
}

#[derive(Debug, serde::Serialize, serde::Deserialize)]
enum Enum {
    Variant(u8),
}

let ser = Struct {
    field: Enum::Variant(21),
};

println!("json: {:?}", serde_json::to_string(&ser));

println!("toml: {:?}", toml::to_string(&ser));
```

```
json: Ok("{\"field\":{\"Variant\":21}}")
toml: Err(Error { inner: UnsupportedType(Some("Enum")) })
```

Even mor surprising, the same struct can be deserialized without any problem:

```rust
let de: Result<Struct, toml::de::Error> = toml::from_str("[field]\nVariant = 21");

println!("toml: {:?}", de);
```

```
toml: Ok(Struct { field: Variant(21) })
```


Discussion / hints:
May be linked to https://github.com/toml-rs/toml/issues/534.
All in one reproduction case:
```rust
#!/usr/bin/env rust-script

//! ```cargo
//! [dependencies]
//! toml = "0.7.3"
//! serde = { version = "1", features = ["derive"] }
//! serde_json = "1"
//! ```

#[derive(Debug, serde::Serialize, serde::Deserialize)]
struct Struct {
    field: Enum,
}

#[derive(Debug, serde::Serialize, serde::Deserialize)]
enum Enum {
    Variant(u8),
}

fn main() {
    let ser = Struct {
        field: Enum::Variant(21),
    };
    println!("json: {:?}", serde_json::to_string(&ser));
    println!("toml: {:?}", toml::to_string(&ser));

    let de: Result<Struct, toml::de::Error> = toml::from_str("[field]\nVariant = 21");
    println!("toml: {:?}", de);
}
```

Fix patch:
diff --git a/crates/toml/src/value.rs b/crates/toml/src/value.rs
index 8ed75e81..2785d9de 100644
--- a/crates/toml/src/value.rs
+++ b/crates/toml/src/value.rs
@@ -584,7 +584,7 @@ impl<'de> de::Deserializer<'de> for Value {
     #[inline]
     fn deserialize_enum<V>(
         self,
-        _name: &str,
+        _name: &'static str,
         _variants: &'static [&'static str],
         visitor: V,
     ) -> Result<V::Value, crate::de::Error>
@@ -593,6 +593,21 @@ impl<'de> de::Deserializer<'de> for Value {
     {
         match self {
             Value::String(variant) => visitor.visit_enum(variant.into_deserializer()),
+            Value::Table(variant) => {
+                use de::Error;
+                if variant.is_empty() {
+                    Err(crate::de::Error::custom(
+                        "wanted exactly 1 element, found 0 elements",
+                    ))
+                } else if variant.len() != 1 {
+                    Err(crate::de::Error::custom(
+                        "wanted exactly 1 element, more than 1 element",
+                    ))
+                } else {
+                    let deserializer = MapDeserializer::new(variant);
+                    visitor.visit_enum(deserializer)
+                }
+            }
             _ => Err(de::Error::invalid_type(
                 de::Unexpected::UnitVariant,
                 &"string only",
@@ -712,6 +727,132 @@ impl<'de> de::MapAccess<'de> for MapDeserializer {
     }
 }
 
+impl<'de> de::EnumAccess<'de> for MapDeserializer {
+    type Error = crate::de::Error;
+    type Variant = MapEnumDeserializer;
+
+    fn variant_seed<V>(mut self, seed: V) -> Result<(V::Value, Self::Variant), Self::Error>
+    where
+        V: serde::de::DeserializeSeed<'de>,
+    {
+        use de::Error;
+        let (key, value) = match self.iter.next() {
+            Some(pair) => pair,
+            None => {
+                return Err(Error::custom(
+                    "expected table with exactly 1 entry, found empty table",
+                ));
+            }
+        };
+
+        let val = seed.deserialize(key.into_deserializer())?;
+
+        let variant = MapEnumDeserializer::new(value);
+
+        Ok((val, variant))
+    }
+}
+
+/// Deserializes table values into enum variants.
+pub(crate) struct MapEnumDeserializer {
+    value: Value,
+}
+
+impl MapEnumDeserializer {
+    pub(crate) fn new(value: Value) -> Self {
+        MapEnumDeserializer { value }
+    }
+}
+
+impl<'de> serde::de::VariantAccess<'de> for MapEnumDeserializer {
+    type Error = crate::de::Error;
+
+    fn unit_variant(self) -> Result<(), Self::Error> {
+        use de::Error;
+        match self.value {
+            Value::Table(values) => {
+                if values.is_empty() {
+                    Ok(())
+                } else {
+                    Err(Error::custom("expected empty table"))
+                }
+            }
+            e => Err(Error::custom(format!(
+                "expected table, found {}",
+                e.type_str()
+            ))),
+        }
+    }
+
+    fn newtype_variant_seed<T>(self, seed: T) -> Result<T::Value, Self::Error>
+    where
+        T: serde::de::DeserializeSeed<'de>,
+    {
+        seed.deserialize(self.value.into_deserializer())
+    }
+
+    fn tuple_variant<V>(self, len: usize, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: serde::de::Visitor<'de>,
+    {
+        use de::Error;
+        match self.value {
+            Value::Table(values) => {
+                let tuple_values = values
+                    .into_iter()
+                    .enumerate()
+                    .map(|(index, (key, value))| match key.parse::<usize>() {
+                        Ok(key_index) if key_index == index => Ok(value),
+                        Ok(_) | Err(_) => Err(Error::custom(format!(
+                            "expected table key `{}`, but was `{}`",
+                            index, key
+                        ))),
+                    })
+                    // Fold all values into a `Vec`, or return the first error.
+                    .fold(Ok(Vec::with_capacity(len)), |result, value_result| {
+                        result.and_then(move |mut tuple_values| match value_result {
+                            Ok(value) => {
+                                tuple_values.push(value);
+                                Ok(tuple_values)
+                            }
+                            // `Result<de::Value, Self::Error>` to `Result<Vec<_>, Self::Error>`
+                            Err(e) => Err(e),
+                        })
+                    })?;
+
+                if tuple_values.len() == len {
+                    serde::de::Deserializer::deserialize_seq(
+                        tuple_values.into_deserializer(),
+                        visitor,
+                    )
+                } else {
+                    Err(Error::custom(format!("expected tuple with length {}", len)))
+                }
+            }
+            e => Err(Error::custom(format!(
+                "expected table, found {}",
+                e.type_str()
+            ))),
+        }
+    }
+
+    fn struct_variant<V>(
+        self,
+        fields: &'static [&'static str],
+        visitor: V,
+    ) -> Result<V::Value, Self::Error>
+    where
+        V: serde::de::Visitor<'de>,
+    {
+        serde::de::Deserializer::deserialize_struct(
+            self.value.into_deserializer(),
+            "", // TODO: this should be the variant name
+            fields,
+            visitor,
+        )
+    }
+}
+
 impl<'de> de::IntoDeserializer<'de, crate::de::Error> for Value {
     type Deserializer = Self;
 
@@ -827,15 +968,18 @@ impl ser::Serializer for ValueSerializer {
 
     fn serialize_newtype_variant<T: ?Sized>(
         self,
-        name: &'static str,
+        _name: &'static str,
         _variant_index: u32,
-        _variant: &'static str,
-        _value: &T,
+        variant: &'static str,
+        value: &T,
     ) -> Result<Value, crate::ser::Error>
     where
         T: ser::Serialize,
     {
-        Err(crate::ser::Error::unsupported_type(Some(name)))
+        let value = value.serialize(ValueSerializer)?;
+        let mut table = Table::new();
+        table.insert(variant.to_owned(), value);
+        Ok(table.into())
     }
 
     fn serialize_none(self) -> Result<Value, crate::ser::Error> {
@@ -1005,15 +1149,18 @@ impl ser::Serializer for TableSerializer {
 
     fn serialize_newtype_variant<T: ?Sized>(
         self,
-        name: &'static str,
+        _name: &'static str,
         _variant_index: u32,
-        _variant: &'static str,
-        _value: &T,
+        variant: &'static str,
+        value: &T,
     ) -> Result<Table, crate::ser::Error>
     where
         T: ser::Serialize,
     {
-        Err(crate::ser::Error::unsupported_type(Some(name)))
+        let value = value.serialize(ValueSerializer)?;
+        let mut table = Table::new();
+        table.insert(variant.to_owned(), value);
+        Ok(table)
     }
 
     fn serialize_none(self) -> Result<Table, crate::ser::Error> {
diff --git a/crates/toml_edit/src/ser/value.rs b/crates/toml_edit/src/ser/value.rs
index cc7dfb77..d29390a4 100644
--- a/crates/toml_edit/src/ser/value.rs
+++ b/crates/toml_edit/src/ser/value.rs
@@ -167,15 +167,18 @@ impl serde::ser::Serializer for ValueSerializer {
 
     fn serialize_newtype_variant<T: ?Sized>(
         self,
-        name: &'static str,
+        _name: &'static str,
         _variant_index: u32,
-        _variant: &'static str,
-        _value: &T,
+        variant: &'static str,
+        value: &T,
     ) -> Result<Self::Ok, Self::Error>
     where
         T: serde::ser::Serialize,
     {
-        Err(Error::UnsupportedType(Some(name)))
+        let value = value.serialize(self)?;
+        let mut table = crate::InlineTable::new();
+        table.insert(variant, value);
+        Ok(table.into())
     }
 
     fn serialize_seq(self, len: Option<usize>) -> Result<Self::SerializeSeq, Self::Error> {


Test patch:
diff --git a/crates/toml/tests/testsuite/serde.rs b/crates/toml/tests/testsuite/serde.rs
index 3b9f65a4..6e927f73 100644
--- a/crates/toml/tests/testsuite/serde.rs
+++ b/crates/toml/tests/testsuite/serde.rs
@@ -613,6 +613,28 @@ fn newtypes2() {
     }
 }
 
+#[test]
+fn newtype_variant() {
+    #[derive(Copy, Clone, Debug, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
+    struct Struct {
+        field: Enum,
+    }
+
+    #[derive(Copy, Clone, Debug, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
+    enum Enum {
+        Variant(u8),
+    }
+
+    equivalent! {
+        Struct { field: Enum::Variant(21) },
+        map! {
+            field: map! {
+                Variant: Value::Integer(21)
+            }
+        },
+    }
+}
+
 #[derive(Debug, Default, PartialEq, Serialize, Deserialize)]
 struct CanBeEmpty {
     a: Option<String>,