make value roundtrip

[wiiznokes]

previous pr: #553

todo:

• add more test cases (currently, only Enum::A pass)
• handle all struct type in handle_any_struct
• finish implementing visit_enum
• impl Serialize for Value
• clean up code

• I've included my change in CHANGELOG.md

[wiiznokes]

honestly, after having passed way to much time on this, i think its just not possible.
We just need to attach a name to Tuple and Map.
To create a Value, we need a Visitor. The only method capable of this in Visitor is visit_enum.

But,

• the visitor must be generic. It doesn't know in advance the type of the enum.
• the VariantAccess trait have

  fn unit_variant(self) -> Result<(), Self::Error>;
  fn newtype_variant_seed<T>(self, seed: T) -> Result<T::Value, Self::Error>
  fn tuple_variant<V>(self, len: usize, visitor: V) -> Result<V::Value, Self::Error>
  fn struct_variant<V>(
        self,
        fields: &'static [&'static str],
        visitor: V,
     ) -> Result<V::Value, Self::Error>

  all this method take self, so we can't all call them to know what is the type of the enum.

So we are cooked. I wonder if a language succeed to do this. Maybe i can be proven wrong.

I think we just need another method on VariantAccess:

fn enum_type(&self) -> Option<EnumType>;
enum EnumType {
    Unit,
    NewType,
    Tuple,
    Struct
}

[wiiznokes]

serde-rs/serde#2874

[juntyr]

Thank you for exploring it further @wiiznokes! I personally think that many improvements are still blocked on serde, perhaps we can come back to this once ron is given more agency in deserialising data without type information

[wiiznokes]

Another solution could be to not implement Deserialize for Value. I don't think it's very used in its current shape anyway since it doesn't store enum