Add support to include `JsonPolymorph` metadata in generated types

[atrauzzi]

System.Text.Json includes support for defining polymorphic types, along with a customizable discriminator field and value:

[JsonPolymorphic(TypeDiscriminatorPropertyName = "?")]
[JsonDerivedType(typeof(Subtype), "subtype")]
public interface MyType
{
    // ...
}

public class Subtype : MyType
{
    // ...
}

When outputting the type, this information is not included:

export interface MyType
{
    // ...
}

export interface Subtype extends MyType
{
    // ...
}

What would be nice, is if it was! 🎊

// ...

export interface Subtype extends MyType
{
    "?": "subtype";
    // ...
}

It's extremely valuable and important to support this because any system that is sending out types like this will need those discriminator keys and values populated. Failure to include them will result in parse errors, or create additional toil/manual overhead or bloat to communicate type information outside of the serialization pipeline.

[atrauzzi]

I guess what I'm after is something out-of-the-box that does this:

public class DiscriminatorGenerator : InterfaceCodeGenerator
{
    public override RtInterface GenerateNode(Type element, RtInterface result, TypeResolver resolver)
    {
        var node = base.GenerateNode(element, result, resolver);
        
        var polymorphicInterface = element.GetInterfaces()
            .SingleOrDefault((interfaces) => interfaces.GetCustomAttributes(typeof(JsonPolymorphicAttribute), false).Any())
            ?? throw new InvalidOperationException($"Type {element.Name} is not JsonPolymorphic, or has it specified more than once!");

        var polymorphicAttribute = polymorphicInterface.GetCustomAttributes(typeof(JsonPolymorphicAttribute), false)
            .OfType<JsonPolymorphicAttribute>()
            .Single();

        var discriminatorPropertyName = polymorphicAttribute.TypeDiscriminatorPropertyName ?? "$type";

        var derivedTypeAttribute = polymorphicInterface
            .GetCustomAttributes(typeof(JsonDerivedTypeAttribute), false)
            .OfType<JsonDerivedTypeAttribute>()
            .Single(attribute => attribute.DerivedType == element);

        var discriminatorValue = derivedTypeAttribute?.TypeDiscriminator?.ToString() ?? throw new($"Cannot infer discriminator value for {element.Name}!");
        
        node.Members.Add(new RtField
        {
            Identifier = new($"\"{discriminatorPropertyName}\""),
            AccessModifier = AccessModifier.Public,
            Type = new RtSimpleTypeName($"\"{discriminatorValue}\""),
            Order = -1,
        });

        return node;
    }
}

(this only supports interfaces, it can be easily modified to work with class parent types as well)

[pavel-b-novikov]

Yup