[choice] Adds a means to declare a choice group without use

core/src/main/scala/chisel3/choice/package.scala

@@ -3,8 +3,13 @@
 package chisel3
 
 import chisel3.experimental.{BaseModule, SourceInfo}
+import chisel3.internal.Builder
 import chisel3.util.simpleClassName
 
+import scala.collection.mutable
+import scala.reflect.runtime.universe._
+import scala.reflect.runtime.{currentMirror => cm}
+
 /** This package contains Chisel language definitions for describing configuration options and their accepted values.
   */
 package object choice {
@@ -13,7 +18,7 @@ package object choice {
     *
     * @example
     * {{{
-    * import chisel3.option.{Group, Case}
+    * import chisel3.choice.{Group, Case}
     * object Platform extends Group {
     *   object FPGA extends Case
     *   object ASIC extends Case
@@ -23,6 +28,22 @@ package object choice {
   abstract class Group(implicit _sourceInfo: SourceInfo) {
     self: Singleton =>
 
+    private[choice] def registerCases(): Unit = {
+      // Grab a symbol for the derived class (a concrete Group)
+      val instanceMirror = cm.reflect(this)
+      val symbol = instanceMirror.symbol
+
+      symbol.typeSignature.members.collect {
+        // Look only for inner objects in the Group. Note, this is not recursive.
+        case m: ModuleSymbol if m.isStatic =>
+          val instance = cm.reflectModule(m.asModule).instance
+          // Confirms the instance is a subtype of Case
+          if (cm.classSymbol(instance.getClass).toType <:< typeOf[Case]) {
+            Builder.options += instance.asInstanceOf[Case]
+          }
+      }
+    }
+
     private[chisel3] def sourceInfo: SourceInfo = _sourceInfo
 
     private[chisel3] def name: String = simpleClassName(this.getClass())
@@ -47,4 +68,14 @@ package object choice {
       */
     def ->[T](module: => T): (Case, () => T) = (this, () => module)
   }
+
+  /** Registers all options in a group with the Builder.
+    * This lets Chisel know  that this layer should be emitted into FIRRTL text.
+    *
+    * This API can be used to guarantee that a design will always have certain
+    * group defined.  This is analagous in spirit to [[layer.addLayer]].
+    */
+  def addGroup(group: Group): Unit = {
+    group.registerCases()
+  }
 }

src/test/scala/chiselTests/ModuleChoiceSpec.scala

@@ -3,7 +3,7 @@
 package chiselTests
 
 import chisel3._
-import chisel3.choice.{Case, Group, ModuleChoice}
+import chisel3.choice.{addGroup, Case, Group, ModuleChoice}
 import chiselTests.{ChiselFlatSpec, MatchesAndOmits, Utils}
 import _root_.circt.stage.ChiselStage
 
@@ -23,7 +23,7 @@ class ASICTarget extends FixedIOExtModule[TargetIO](new TargetIO(8))
 
 class VerifTarget extends FixedIORawModule[TargetIO](new TargetIO(8))
 
-class ModuleChoiceSpec extends ChiselFlatSpec with Utils with MatchesAndOmits {
+class ModuleChoiceSpec extends ChiselFlatSpec with Utils with FileCheck {
   it should "emit options and cases" in {
     class ModuleWithChoice extends Module {
       val out = IO(UInt(8.W))
@@ -35,17 +35,14 @@ class ModuleChoiceSpec extends ChiselFlatSpec with Utils with MatchesAndOmits {
       out := inst.out
     }
 
-    val chirrtl = ChiselStage.emitCHIRRTL(new ModuleWithChoice, Array("--full-stacktrace"))
-
-    info("CHIRRTL emission looks correct")
-    matchesAndOmits(chirrtl)(
-      "option Platform :",
-      "FPGA",
-      "ASIC",
-      "instchoice inst of VerifTarget, Platform :",
-      "FPGA => FPGATarget",
-      "ASIC => ASICTarget"
-    )()
+    generateFirrtlAndFileCheck(new ModuleWithChoice)(
+      """|CHECK: option Platform :
+         |CHECK-NEXT: FPGA
+         |CHECK-NEXT: ASIC
+         |CHECK: instchoice inst of VerifTarget, Platform :
+         |CHECK-NEXT: FPGA => FPGATarget
+         |CHECK-NEXT: ASIC => ASICTarget""".stripMargin
+    )
   }
 
   it should "require that all cases are part of the same option" in {
@@ -129,3 +126,20 @@ class ModuleChoiceSpec extends ChiselFlatSpec with Utils with MatchesAndOmits {
   }
 
 }
+class AddGroupSpec extends ChiselFlatSpec with Utils with FileCheck {
+  it should "emit options for a registered group even if there are no consumers" in {
+    class ModuleWithoutChoice extends Module {
+      addGroup(Platform)
+      val out = IO(UInt(8.W))
+      val in = IO(UInt(8.W))
+      out := in
+    }
+
+    generateFirrtlAndFileCheck(new ModuleWithoutChoice)(
+      """|CHECK: option Platform :
+         |CHECK-NEXT: ASIC @[
+         |CHECK-NEXT: FPGA @[
+         |""".stripMargin
+    )
+  }
+}