Implement Lookupable for Unit (#4497) (#4498)

(cherry picked from commit 48866a9)

Co-authored-by: Jack Koenig <[email protected]>

Author: mergify[bot]

core/src/main/scala/chisel3/experimental/hierarchy/core/Lookupable.scala

@@ -543,4 +543,5 @@ object Lookupable {
   implicit val lookupString:  SimpleLookupable[String] = new SimpleLookupable[String]()
   implicit val lookupBoolean: SimpleLookupable[Boolean] = new SimpleLookupable[Boolean]()
   implicit val lookupBigInt:  SimpleLookupable[BigInt] = new SimpleLookupable[BigInt]()
+  implicit val lookupUnit:    SimpleLookupable[Unit] = new SimpleLookupable[Unit]()
 }

src/test/scala/chiselTests/experimental/hierarchy/Examples.scala

@@ -369,4 +369,11 @@ object Examples {
     out := copy.out
 
   }
+
+  @instantiable
+  class HasPublicUnit extends Module {
+    @public val x: Unit = ()
+    // Should also work in type-parameterized lookupable things
+    @public val y: (Data, Unit) = (Wire(UInt(3.W)), ())
+  }
 }

src/test/scala/chiselTests/experimental/hierarchy/InstanceSpec.scala

@@ -449,6 +449,18 @@ class InstanceSpec extends ChiselFunSpec with Utils {
         MarkAnnotation("~Top|Top/i:HasHasTarget>sram_mem".rt, "x")
       )
     }
+    it("(3.s): should work on Unit") {
+      class Top extends Module {
+        val i = Instance(Definition(new HasPublicUnit))
+        i.x should be(())
+        mark(i.y._1, "y_1")
+        i.y._2 should be(())
+      }
+      val (_, annos) = getFirrtlAndAnnos(new Top)
+      annos.collect { case c: MarkAnnotation => c } should contain(
+        MarkAnnotation("~Top|Top/i:HasPublicUnit>y_1".rt, "y_1")
+      )
+    }
   }
   describe("(4) toInstance") {
     it("(4.a): should work on modules") {