Source Generator to Create Concrete Constant<T> Subclasses

[zacharylayne]

Really loving the constant generic arguments, but it feels like busy work to define Constant<T> subclasses. I'd like to suggest creating a source generator to do that instead.

My idea is that usage would be similar to the following code:

[Constant<ulong>(UInt64.Max, "Max")]
[Constant<ulong>(0, "Zero")]
public partial abstract class UInt64Const : Constant<ulong>() { }

The ConstantAttribute<T> must be applied to a partial abstract class that derives from Constant<T>. This would create nested subclasses with their names & constant values specified along with the attribute. The attribute would have a static constructor to validate that T values can be constant, and a protected instance constructor to validate class names.

Feel free to let me know if this is a dumb idea! :|

[sakno]

Currently, I'm not investing time in this feature. .NET 8 is coming, and starting from November I'll be focused on releasing next major version of .NEXT 5.x. Since .NET 7 (and even .NET 6, with enabled preview features) constant generic argument can be efficiently replaced with static abstract interface property, e.g.:

public interface IConstant<out T>
{
    static abstract T Value { get; }
}

static void MethodWithGenericConstant<T>() where T : IConstant<int>
{
    int value = T.Value;
}

Therefore, I have a plan to remove constant generic arguments in the next version.

[sakno]

Related: dotnet/runtime#89730