Implement Lookupable for Unit

[jackkoenig]

An oft-overlooked primitive type but an important one nonetheless!

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• Feature (or new API)

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[debs-sifive]

Thanks!

[mwachs5]

I approved, but isn't there an error message about what the types are allowed that we should add Unit to:

@public is only legal within a class or trait marked @instantiable, and only on vals of type Data, BaseModule, MemBase, IsInstantiable, IsLookupable, or Instance[_], or in an Iterable, Option, Either, or Tuple2

[jackkoenig]

I approved, but isn't there an error message about what the types are allowed that we should add Unit to:

@public is only legal within a class or trait marked @instantiable, and only on vals of type Data, BaseModule, MemBase, IsInstantiable, IsLookupable, or Instance[_], or in an Iterable, Option, Either, or Tuple2

We ought to, but note there are a lot of other types that should also be listed (if we're to list them all), including Int, Float, Double, BigInt`...

We could link to the object Lookupable ScalaDoc for the complete list: https://www.chisel-lang.org/api/latest/chisel3/experimental/hierarchy/core/Lookupable$.html

But more fundamentally, I think we should make it easier to implement Lookupable for your own type, which needs some improvements on the Chisel API side and needs docs (probably on this page: https://www.chisel-lang.org/docs/cookbooks/hierarchy) in which case we should just link to that.