Type-parameterized annotations are not deserializable

Type of issue: bug report

What is the use case for changing the behavior? If a user tries to build a type-parameterized annotation, chisel will emit it without complaint, but it will not be deserializable by firrtl.

Impact: API modification

Development Phase: request

Other information

For example:

trait MyCoolTrait {
  val foo: Int
}

case class MyConcreteClassA(foo: Int) extends MyCoolTrait
case class MyConcreteClassB(foo: Int, bar: String) extends MyCoolTrait

case class MyAnnotation[T <: MyCoolTrait](target: ModuleTarget, data: T)
  extends SingleTargetAnnotation[ModuleTarget] { /* snipped boilerplate */ }

This is serialized by chisel without issue, but cannot be deserialized by firrtl because no type hints for data are provided. I think there are basically two options:

1. Figure out a smart way to enable this
2. Explicitly disallow it somehow so that chisel errors out when building such a thing

But… I currently have no suggestions on how to do either of these. 😃

[davidbiancolin]

I modified the JsonProtocol (firrtl) to let annotations supply additional TypeHints to get this working in Golden Gate. The fully qualified classnames for everything you need the extra type hint for is captured in an additional annotation, so that deserialization can proceed. I think it would be trivial to let annotations also supply a custom serdes, though i suspect in your case that extra type hints are all you need.

I was going to open a draft PR later today.

[albert-magyar]

Closed by #1211