Add support for type parameters in annotations

[pgrandjean]

Follow up on issue #922 and pull request #925. I would like to add support for type parameters in annotations, both variable and type annotations. For example, define an annotation such as:

case class MyAnnotation[T](t: T) extends scala.annotation.StaticAnnotation

The out type of Annotations and TypeAnnotations would have the complete "reference" to type T.

[pgrandjean]

Moreover, what about generalizing Annotations to return all annotations instead of just one annotation type A? Example:

case class HelloWorld(
  field1: String @A11[T111, T112, ..., T11n] @A12[T121, T122, ..., T12n] ... @A1m[T1m1, T1m2, ..., T1mn],
  field2: String @A21[T211, T212, ..., T21n] @A22[T221, T222, ..., T22n] ... @A2m[T2m1, T2m2, ..., T2mn],
  ...,
  fieldk: String @Ak1[Tk11, Tk12, ..., Tk1n] @Ak2[Tk21, Tk22, ..., Tk2n] ... @Akm[Tkm1, Tkm2, ..., Tkmn],
)

Would be nice to have a single macro to extract in one pass all annotations, no?

The trait would look like the following:

trait AllAnnotations[T] extends DepFn0 with Serializable {
  // no more type A
  type Out <: HList
}

For the example above, the Out type would be an HList of HLists:

(A11[T111, T112, ..., T11n] :: A12[T121, T122, ..., T12n] :: ... :: A1m[T1m1, T1m2, ..., T1mn] :: HNil) ::
(A21[T211, T212, ..., T21n] :: A22[T221, T222, ..., T22n] :: ... :: A2m[T2m1, T2m2, ..., T2mn] :: HNil) ::
... ::
(Ak1[Tk11, Tk12, ..., Tk1n] :: Ak2[Tk21, Tk22, ..., Tk2n] :: ... :: Akm[Tkm1, Tkm2, ..., Tkmn] :: HNil) :: HNil

[alexarchambault]

AFAIC, I think it would be nice to have.

[pgrandjean]

I have a working prototype. Will try to submit a PR in the coming days.

[pgrandjean]

@joroKr21 just wondering, is this going to work with Scala 3?

[joroKr21]

@joroKr21 just wondering, is this going to work with Scala 3?

The Scala 3 story is a bit messy for shapeless because it uses a lot of macros.

This particular implementation won't work on Scala 3.
There is Shapeless 3 which works only with Scala 3 on the shapeless-3 branch.
I don't know much about it yet, so I can't say what it offers in terms of annotations.
Separately from that we have an issue to port shapeless 2 in some form to Scala 3: #1043
But nobody is working on that at the moment.

[joroKr21]

At least there is a step in that direction: #1019
Next would be, write a scalafix rule to replace Lazy and Strict with by-name implicits in the source code.