Parametrize serde_json::value::Serializer with Strategy to serialize values

[mineichen]

Feature request:

Currently, the serde_json::value::Serializer uses to_value() at various points. It would be very convenient to allow configuring this via Generics. Today, this Serializer cannot collect metadata or have special logic, which is applied on all nesting-levels. This pseudo code shows a solution to overcome this limitation:

pub trait SerializeToValue {
   fn serialize_value<S: Serialize>(&mut self, key: &str, item: S) -> Result<serde_json::Value, serde_json::error::Error>;
}

#[derive(Default)]
struct DefaultSerializeToValue;
impl SerializeToValue for DefaultSerializeToValue  {
   fn serialize_value<S: Serialize>(&mut self, key: &str, item: S) -> Result<serde_json::Value, serde_json::error::Error> {
      to_value(item)
   }
}

// In the next major version, this could be added:
// type Serializer = GenericSerializer<DefaultSerializeToValue>;
struct GenericSerializer<T=DefaultToValue> {
   strategy: T
}

Instead of to_value, we could then use self.strategy.serialize_value (e.g. in serialize_newtype_variant). The runtime performance should not change, but there might be some build-time regressions.

As you can see, the above solution doesn't work without breaking changes. Would you consider to mark Serializer deprecated and add a GenericSerializer<T>? For now, the serde_json::value::Serializer and serde_json::value::GenericSerializer could be generated with a macro to avoid Code-Duplication. An alternative would be to ship this Serializer in a separate crate and duplicate the serde_json::value::Serializer code.

[dtolnay]

I would prefer not to build this into this crate, so you would need to put it in a different crate.