Merge pull request #723 from freechipsproject/autoclonetype

Auto clone type 🦆

12 files changed, 401 insertions, 31 deletions

diff --git a/chiselFrontend/src/main/scala/chisel3/core/Aggregate.scala b/chiselFrontend/src/main/scala/chisel3/core/Aggregate.scala
index b89961aa..14011cd9 100644
--- a/chiselFrontend/src/main/scala/chisel3/core/Aggregate.scala
+++ b/chiselFrontend/src/main/scala/chisel3/core/Aggregate.scala
@@ -556,31 +556,173 @@ class Bundle(implicit compileOptions: CompileOptions) extends Record {
     case _ => None
   }
 
+  // If this Data is an instance of an inner class, record a *guess* at the outer object.
+  // This is only used for cloneType, and is mutable to allow autoCloneType to try guesses other
+  // than the module that was current when the Bundle was constructed.
+  private var _outerInst: Option[Object] = Builder.currentModule
+
   override def cloneType : this.type = {
-    // If the user did not provide a cloneType method, try invoking one of
-    // the following constructors, not all of which necessarily exist:
-    // - A zero-parameter constructor
-    // - A one-paramater constructor, with null as the argument
-    // - A one-parameter constructor for a nested Bundle, with the enclosing
-    //   parent Module as the argument
-    val constructor = this.getClass.getConstructors.head
-    try {
-      val args = Seq.fill(constructor.getParameterTypes.size)(null)
-      constructor.newInstance(args:_*).asInstanceOf[this.type]
+    // This attempts to infer constructor and arguments to clone this Bundle subtype without
+    // requiring the user explicitly overriding cloneType.
+    import scala.language.existentials
+
+    def reflectError(desc: String): Nothing = {
+      Builder.exception(s"Unable to automatically infer cloneType on $this: $desc").asInstanceOf[Nothing]
+    }
+
+    import scala.reflect.runtime.universe._
+
+    // Check if this is an inner class, and if so, try to get the outer instance
+    val clazz = this.getClass
+    val outerClassInstance = Option(clazz.getEnclosingClass).map { outerClass =>
+      def canAssignOuterClass(x: Object) = outerClass.isAssignableFrom(x.getClass)
+      val outerInstance = try {
+        clazz.getDeclaredField("$outer").get(this)  // doesn't work in all cases, namely anonymous inner Bundles
+      } catch {
+        case _: NoSuchFieldException =>
+          // First check if this Bundle is bound and an anonymous inner Bundle of another Bundle
+          this.bindingOpt.collect { case ChildBinding(p) if canAssignOuterClass(p) => p }
+              .orElse(this._outerInst)
+              .getOrElse(reflectError(s"Unable to determine instance of outer class $outerClass"))
+      }
+      if (!canAssignOuterClass(outerInstance)) {
+        reflectError(s"Unable to determine instance of outer class $outerClass," +
+            s" guessed $outerInstance, but is not assignable to outer class $outerClass")
+      }
+      // Record the outer instance for future cloning
+      this._outerInst = Some(outerInstance)
+      (outerClass, outerInstance)
+    }
+
+    // If possible (constructor with no arguments), try Java reflection first
+    // This handles two cases that Scala reflection doesn't:
+    // 1. getting the ClassSymbol of a class with an anonymous outer class fails with a
+    //    CyclicReference exception
+    // 2. invoking the constructor of an anonymous inner class seems broken (it expects the outer
+    //    class as an argument, but fails because the number of arguments passed in is incorrect)
+    if (clazz.getConstructors.size == 1) {
+      var ctor = clazz.getConstructors.head
+      val argTypes = ctor.getParameterTypes.toList
+      val clone = (argTypes, outerClassInstance) match {
+        case (Nil, None) => // no arguments, no outer class, invoke constructor directly
+          Some(ctor.newInstance().asInstanceOf[this.type])
+        case (argType :: Nil, Some((_, outerInstance))) =>
+          if (argType isAssignableFrom outerInstance.getClass) {
+            Some(ctor.newInstance(outerInstance).asInstanceOf[this.type])
+          } else {
+            None
+          }
+        case _ => None
+      }
+      clone match {
+        case Some(clone) =>
+          clone._outerInst = this._outerInst
+          if (!clone.typeEquivalent(this)) {
+            reflectError(s"Automatically cloned $clone not type-equivalent to base $this." +
+            " Constructor argument values were not inferred, ensure constructor is deterministic.")
+          }
+          return clone.asInstanceOf[this.type]
+        case None =>
+      }
+    }
+
+    // Get constructor parameters and accessible fields
+    val mirror = runtimeMirror(clazz.getClassLoader)
+    val classSymbol = try {
+      mirror.reflect(this).symbol
     } catch {
-      case e: java.lang.reflect.InvocationTargetException if e.getCause.isInstanceOf[java.lang.NullPointerException] =>
-        try {
-          constructor.newInstance(_parent.get).asInstanceOf[this.type]
-        } catch {
-          case _: java.lang.reflect.InvocationTargetException | _: java.lang.IllegalArgumentException =>
-            Builder.exception(s"Parameterized Bundle ${this.getClass} needs cloneType method. You are probably using " +
-              "an anonymous Bundle object that captures external state and hence is un-cloneTypeable")
-            this
-        }
-      case _: java.lang.reflect.InvocationTargetException | _: java.lang.IllegalArgumentException =>
-        Builder.exception(s"Parameterized Bundle ${this.getClass} needs cloneType method")
-        this
+      case e: scala.reflect.internal.Symbols#CyclicReference => reflectError(s"Scala cannot reflect on $this, got exception $e." +
+          " This is known to occur with inner classes on anonymous outer classes." +
+          " In those cases, autoclonetype only works with no-argument constructors, or you can define a custom cloneType.")
+    }
+
+    val decls = classSymbol.typeSignature.decls
+    val ctors = decls.collect { case meth: MethodSymbol if meth.isConstructor => meth }
+    if (ctors.size != 1) {
+      reflectError(s"found multiple constructors ($ctors)." +
+          " Either remove all but the default constructor, or define a custom cloneType method.")
+    }
+    val ctor = ctors.head
+    val ctorParamss = ctor.paramLists
+    val ctorParams = ctorParamss match {
+      case Nil => List()
+      case ctorParams :: Nil => ctorParams
+      case ctorParams :: ctorImplicits :: Nil => ctorParams ++ ctorImplicits
+      case _ => reflectError(s"internal error, unexpected ctorParamss = $ctorParamss")
     }
+    val ctorParamsNames = ctorParams.map(_.name.toString)
+
+    // Special case for anonymous inner classes: their constructor consists of just the outer class reference
+    // Scala reflection on anonymous inner class constructors seems broken
+    if (ctorParams.size == 1 && outerClassInstance.isDefined &&
+        ctorParams.head.typeSignature == mirror.classSymbol(outerClassInstance.get._1).toType) {
+      // Fall back onto Java reflection
+      val ctors = clazz.getConstructors
+      require(ctors.size == 1)  // should be consistent with Scala constructors
+      try {
+        val clone = ctors.head.newInstance(outerClassInstance.get._2).asInstanceOf[this.type]
+        clone._outerInst = this._outerInst
+        return clone
+      } catch {
+        case e @ (_: java.lang.reflect.InvocationTargetException | _: IllegalArgumentException) =>
+          reflectError(s"Unexpected failure at constructor invocation, got $e.")
+      }
+    }
+
+    // Get all the class symbols up to (but not including) Bundle and get all the accessors.
+    // (each ClassSymbol's decls only includes those declared in the class itself)
+    val bundleClassSymbol = mirror.classSymbol(classOf[Bundle])
+    val superClassSymbols = classSymbol.baseClasses.takeWhile(_ != bundleClassSymbol)
+    val superClassDecls = superClassSymbols.map(_.typeSignature.decls).flatten
+    val accessors = superClassDecls.collect { case meth: MethodSymbol if meth.isParamAccessor => meth }
+
+    // Get constructor argument values
+    // Check that all ctor params are immutable and accessible. Immutability is required to avoid
+    // potential subtle bugs (like values changing after cloning).
+    // This also generates better error messages (all missing elements shown at once) instead of
+    // failing at the use site one at a time.
+    val accessorsName = accessors.filter(_.isStable).map(_.name.toString)
+    val paramsDiff = ctorParamsNames.toSet -- accessorsName.toSet
+    if (!paramsDiff.isEmpty) {
+      reflectError(s"constructor has parameters $paramsDiff that are not both immutable and accessible." +
+          " Either make all parameters immutable and accessible (vals) so cloneType can be inferred, or define a custom cloneType method.")
+    }
+
+    // Get all the argument values
+    val accessorsMap = accessors.map(accessor => accessor.name.toString -> accessor).toMap
+    val instanceReflect = mirror.reflect(this)
+    val ctorParamsNameVals = ctorParamsNames.map {
+      paramName => paramName -> instanceReflect.reflectMethod(accessorsMap(paramName)).apply()
+    }
+
+    // Opportunistic sanity check: ensure any arguments of type Data is not bound
+    // (which could lead to data conflicts, since it's likely the user didn't know to re-bind them).
+    // This is not guaranteed to catch all cases (for example, Data in Tuples or Iterables).
+    val boundDataParamNames = ctorParamsNameVals.collect {
+      case (paramName, paramVal: Data) if paramVal.hasBinding => paramName
+    }
+    if (boundDataParamNames.nonEmpty) {
+      reflectError(s"constructor parameters ($boundDataParamNames) have values that are hardware types, which is likely to cause subtle errors." +
+          " Use chisel types instead: use the value before it is turned to a hardware type (with Wire(...), Reg(...), etc) or use chiselTypeOf(...) to extract the chisel type.")
+    }
+
+    val ctorParamsVals = ctorParamsNameVals.map{ case (_, paramVal) => paramVal }
+
+    // Invoke ctor
+    val classMirror = outerClassInstance match {
+      case Some((_, outerInstance)) => mirror.reflect(outerInstance).reflectClass(classSymbol)
+      case None => mirror.reflectClass(classSymbol)
+    }
+    val clone = classMirror.reflectConstructor(ctor).apply(ctorParamsVals:_*).asInstanceOf[this.type]
+    clone._outerInst = this._outerInst
+
+    if (!clone.typeEquivalent(this)) {
+      reflectError(s"Automatically cloned $clone not type-equivalent to base $this." +
+          " Constructor argument values were inferred: ensure that variable names are consistent and have the same value throughout the constructor chain," +
+          " and that the constructor is deterministic.")
+    }
+
+    clone
   }
 
   /** Default "pretty-print" implementation
diff --git a/chiselFrontend/src/main/scala/chisel3/core/Data.scala b/chiselFrontend/src/main/scala/chisel3/core/Data.scala
index 19adf01b..1cf50e9f 100644
--- a/chiselFrontend/src/main/scala/chisel3/core/Data.scala
+++ b/chiselFrontend/src/main/scala/chisel3/core/Data.scala
@@ -240,6 +240,7 @@ abstract class Data extends HasId {
   // This information is supplemental (more than is necessary to generate FIRRTL) and is used to
   // perform checks in Chisel, where more informative error messages are possible.
   private var _binding: Option[Binding] = None
+  private[core] def bindingOpt = _binding
   private[core] def hasBinding = _binding.isDefined
   // Only valid after node is bound (synthesizable), crashes otherwise
   private[core] def binding = _binding.get
diff --git a/src/main/scala/chisel3/testers/BasicTester.scala b/src/main/scala/chisel3/testers/BasicTester.scala
index 6d1a4913..7bb441ba 100644
--- a/src/main/scala/chisel3/testers/BasicTester.scala
+++ b/src/main/scala/chisel3/testers/BasicTester.scala
@@ -13,7 +13,7 @@ import internal.sourceinfo.SourceInfo
 
 class BasicTester extends Module() {
   // The testbench has no IOs, rather it should communicate using printf, assert, and stop.
-  val io = IO(new Bundle())
+  val io = IO(new Bundle() {})
 
   def popCount(n: Long): Int = n.toBinaryString.count(_=='1')
 
diff --git a/src/test/scala/chiselTests/AnalogIntegrationSpec.scala b/src/test/scala/chiselTests/AnalogIntegrationSpec.scala
index 952d3872..a3e6e643 100644
--- a/src/test/scala/chiselTests/AnalogIntegrationSpec.scala
+++ b/src/test/scala/chiselTests/AnalogIntegrationSpec.scala
@@ -19,7 +19,7 @@ class AnalogBlackBoxPort extends Bundle {
 
 // This IO can be used for a single BlackBox or to group multiple
 // Has multiple ports for driving and checking but only one shared bus
-class AnalogBlackBoxIO(n: Int) extends Bundle {
+class AnalogBlackBoxIO(val n: Int) extends Bundle {
   require(n > 0)
   val bus = Analog(32.W)
   val port = Vec(n, new AnalogBlackBoxPort)
diff --git a/src/test/scala/chiselTests/AutoClonetypeSpec.scala b/src/test/scala/chiselTests/AutoClonetypeSpec.scala
new file mode 100644
index 00000000..1170ed12
--- /dev/null
+++ b/src/test/scala/chiselTests/AutoClonetypeSpec.scala
@@ -0,0 +1,115 @@
+// See LICENSE for license details.
+
+package chiselTests
+
+import chisel3._
+
+import chisel3.testers.BasicTester
+
+class BundleWithIntArg(val i: Int) extends Bundle {
+  val out = UInt(i.W)
+}
+
+class BundleWithImplicit()(implicit val ii: Int) extends Bundle {
+  val out = UInt(ii.W)
+}
+
+class BundleWithArgAndImplicit(val i: Int)(implicit val ii: Int) extends Bundle {
+  val out1 = UInt(i.W)
+  val out2 = UInt(ii.W)
+}
+
+class BaseBundleVal(val i: Int) extends Bundle {
+  val inner = UInt(i.W)
+}
+class SubBundle(i: Int, val i2: Int) extends BaseBundleVal(i) {
+  val inner2 = UInt(i2.W)
+}
+class SubBundleInvalid(i: Int, val i2: Int) extends BaseBundleVal(i+1) {
+  val inner2 = UInt(i2.W)
+}
+
+class BaseBundleNonVal(i: Int) extends Bundle {
+  val inner = UInt(i.W)
+}
+class SubBundleVal(val i: Int, val i2: Int) extends BaseBundleNonVal(i) {
+  val inner2 = UInt(i2.W)
+}
+
+class ModuleWithInner extends Module {
+  class InnerBundle(val i: Int) extends Bundle {
+    val out = UInt(i.W)
+  }
+
+  val io = IO(new Bundle{})
+
+  val myWire = Wire(new InnerBundle(14))
+  require(myWire.i == 14)
+}
+
+
+class AutoClonetypeSpec extends ChiselFlatSpec {
+  "Bundles with Scala args" should "not need clonetype" in {
+    elaborate { new Module {
+      val io = IO(new Bundle{})
+
+      val myWire = Wire(new BundleWithIntArg(8))
+      assert(myWire.i == 8)
+    } }
+  }
+
+  "Bundles with Scala implicit args" should "not need clonetype" in {
+    elaborate { new Module {
+      val io = IO(new Bundle{})
+
+      implicit val implicitInt: Int = 4
+      val myWire = Wire(new BundleWithImplicit())
+
+      assert(myWire.ii == 4)
+    } }
+  }
+
+  "Bundles with Scala explicit and impicit args" should "not need clonetype" in {
+    elaborate { new Module {
+      val io = IO(new Bundle{})
+
+      implicit val implicitInt: Int = 4
+      val myWire = Wire(new BundleWithArgAndImplicit(8))
+
+      assert(myWire.i == 8)
+      assert(myWire.ii == 4)
+    } }
+  }
+
+  "Subtyped Bundles" should "not need clonetype" in {
+    elaborate { new Module {
+      val io = IO(new Bundle{})
+
+      val myWire = Wire(new SubBundle(8, 4))
+
+      assert(myWire.i == 8)
+      assert(myWire.i2 == 4)
+    } }
+    elaborate { new Module {
+      val io = IO(new Bundle{})
+
+      val myWire = Wire(new SubBundleVal(8, 4))
+
+      assert(myWire.i == 8)
+      assert(myWire.i2 == 4)
+    } }
+  }
+
+  "Subtyped Bundles that don't clone well" should "be caught" in {
+    a [ChiselException] should be thrownBy {
+      elaborate { new Module {
+        val io = IO(new Bundle{})
+        val myWire = Wire(new SubBundleInvalid(8, 4))
+      } }
+    }
+  }
+
+  "Inner bundles with Scala args" should "not need clonetype" in {
+    elaborate { new ModuleWithInner }
+  }
+}
diff --git a/src/test/scala/chiselTests/AutoNestedCloneSpec.scala b/src/test/scala/chiselTests/AutoNestedCloneSpec.scala
new file mode 100644
index 00000000..746780be
--- /dev/null
+++ b/src/test/scala/chiselTests/AutoNestedCloneSpec.scala
@@ -0,0 +1,108 @@
+// See LICENSE for license details.
+
+package chiselTests
+import Chisel.ChiselException
+import org.scalatest._
+import chisel3._
+
+class BundleWithAnonymousInner(val w: Int) extends Bundle {
+  val inner = new Bundle {
+    val foo = Input(UInt(w.W))
+  }
+}
+
+class AutoNestedCloneSpec extends ChiselFlatSpec with Matchers {
+  behavior of "autoCloneType of inner Bundle in Chisel3"
+
+  it should "clone a doubly-nested inner bundle successfully" in {
+    elaborate {
+      class Outer(val w: Int) extends Module {
+        class Middle(val w: Int) {
+          class InnerIOType extends Bundle {
+            val in = Input(UInt(w.W))
+          }
+          def getIO = new InnerIOType
+        }
+        val io = IO(new Bundle {})
+        val myWire = Wire((new Middle(w)).getIO)
+      }
+      new Outer(2)
+    }
+  }
+
+  it should "clone an anonymous inner bundle successfully" in {
+    elaborate {
+      class TestTop(val w: Int) extends Module {
+        val io = IO(new Bundle {})
+        val myWire = Wire(new Bundle{ val a = UInt(w.W) })
+      }
+      new TestTop(2)
+    }
+  }
+
+  it should "pick the correct $outer instance for an anonymous inner bundle" in {
+    elaborate {
+      class Inner(val w: Int) extends Module {
+        val io = IO(new Bundle{
+          val in = Input(UInt(w.W))
+          val out = Output(UInt(w.W))
+        })
+      }
+      class Outer(val w: Int) extends Module {
+        val io = IO(new Bundle{
+          val in = Input(UInt(w.W))
+          val out = Output(UInt(w.W))
+        })
+        val i = Module(new Inner(w))
+        val iw = Wire(chiselTypeOf(i.io))
+        iw <> io
+        i.io <> iw
+      }
+      new Outer(2)
+    }
+  }
+
+  it should "clone an anonymous, bound, inner bundle of another bundle successfully" in {
+    elaborate {
+      class TestModule(w: Int) extends Module {
+        val io = IO(new BundleWithAnonymousInner(w) )
+        val w0 = WireInit(io)
+        val w1 = WireInit(io.inner)
+      }
+      new TestModule(8)
+    }
+  }
+
+  it should "clone an anonymous, inner bundle of a Module, bound to another bundle successfully" in {
+    elaborate {
+      class TestModule(w: Int) extends Module {
+        val bun = new Bundle {
+          val foo = UInt(w.W)
+        }
+        val io = IO(new Bundle {
+          val inner = Input(bun)
+        })
+        val w0 = WireInit(io)
+        val w1 = WireInit(io.inner)
+      }
+      new TestModule(8)
+    }
+  }
+
+  behavior of "anonymous doubly-nested inner bundle fails with clear error"
+  ( the[ChiselException] thrownBy {
+    elaborate {
+      class Outer(val w: Int) extends Module {
+        class Middle(val w: Int) {
+          def getIO = new Bundle {
+            val in = Input(UInt(w.W))
+          }
+        }
+        val io = IO(new Bundle {})
+        val myWire = Wire((new Middle(w)).getIO)
+      }
+      new Outer(2)
+    }
+  }).getMessage should include("Unable to determine instance")
+
+}
diff --git a/src/test/scala/chiselTests/CompatibilityInteroperabilitySpec.scala b/src/test/scala/chiselTests/CompatibilityInteroperabilitySpec.scala
index 457f26de..95dc87c1 100644
--- a/src/test/scala/chiselTests/CompatibilityInteroperabilitySpec.scala
+++ b/src/test/scala/chiselTests/CompatibilityInteroperabilitySpec.scala
@@ -12,6 +12,8 @@ object CompatibilityComponents {
   class ChiselBundle extends Bundle {
     val a = UInt(width = 32)
     val b = UInt(width = 32).flip
+
+    override def cloneType = (new ChiselBundle).asInstanceOf[this.type]
   }
   class ChiselRecord extends Record {
     val elements = ListMap("a" -> UInt(width = 32), "b" -> UInt(width = 32).flip)
@@ -46,6 +48,8 @@ object Chisel3Components {
   class Chisel3Bundle extends Bundle {
     val a = Output(UInt(32.W))
     val b = Input(UInt(32.W))
+
+    override def cloneType = (new Chisel3Bundle).asInstanceOf[this.type]
   }
 
   class Chisel3Record extends Record {
diff --git a/src/test/scala/chiselTests/MissingCloneBindingExceptionSpec.scala b/src/test/scala/chiselTests/MissingCloneBindingExceptionSpec.scala
index 43f2b0fd..fc9346a0 100644
--- a/src/test/scala/chiselTests/MissingCloneBindingExceptionSpec.scala
+++ b/src/test/scala/chiselTests/MissingCloneBindingExceptionSpec.scala
@@ -28,7 +28,7 @@ class MissingCloneBindingExceptionSpec extends ChiselFlatSpec with Matchers {
     }
 
     elaborate(new TestTop)
-  }).getMessage should include("needs cloneType method")
+  }).getMessage should include("make all parameters immutable")
 
   behavior of "missing cloneType in Chisel2"
   ( the[ChiselException] thrownBy {
@@ -53,5 +53,5 @@ class MissingCloneBindingExceptionSpec extends ChiselFlatSpec with Matchers {
     }
 
     elaborate(new TestTop)
-  }).getMessage should include("needs cloneType method")
+  }).getMessage should include("make all parameters immutable")
 }
diff --git a/src/test/scala/chiselTests/NamingAnnotationTest.scala b/src/test/scala/chiselTests/NamingAnnotationTest.scala
index 699dc0fb..a7b9b75c 100644
--- a/src/test/scala/chiselTests/NamingAnnotationTest.scala
+++ b/src/test/scala/chiselTests/NamingAnnotationTest.scala
@@ -12,7 +12,7 @@ import chisel3.testers.BasicTester
 import scala.collection.mutable.ListBuffer
 
 trait NamedModuleTester extends Module {
-  val io = IO(new Bundle())  // Named module testers don't need IO
+  val io = IO(new Bundle() {})  // Named module testers don't need IO
 
   val expectedNameMap = ListBuffer[(InstanceId, String)]()
   val expectedModuleNameMap = ListBuffer[(Module, String)]()
diff --git a/src/test/scala/chiselTests/OptionBundle.scala b/src/test/scala/chiselTests/OptionBundle.scala
index 2ac661ea..03b08385 100644
--- a/src/test/scala/chiselTests/OptionBundle.scala
+++ b/src/test/scala/chiselTests/OptionBundle.scala
@@ -6,7 +6,7 @@ import org.scalatest._
 import chisel3._
 import chisel3.testers.BasicTester
 
-class OptionBundle(hasIn: Boolean) extends Bundle {
+class OptionBundle(val hasIn: Boolean) extends Bundle {
   val in = if (hasIn) {
     Some(Input(Bool()))
   } else {
@@ -15,7 +15,7 @@ class OptionBundle(hasIn: Boolean) extends Bundle {
   val out = Output(Bool())
 }
 
-class OptionBundleModule(hasIn: Boolean) extends Module {
+class OptionBundleModule(val hasIn: Boolean) extends Module {
   val io = IO(new OptionBundle(hasIn))
   if (hasIn) {
     io.out := io.in.get
diff --git a/src/test/scala/chiselTests/VectorPacketIO.scala b/src/test/scala/chiselTests/VectorPacketIO.scala
index 7745db57..4a8c6f78 100644
--- a/src/test/scala/chiselTests/VectorPacketIO.scala
+++ b/src/test/scala/chiselTests/VectorPacketIO.scala
@@ -27,7 +27,7 @@ class Packet extends Bundle {
   * lines also.
   * The problem does not occur if the Vec is taken out
   */
-class VectorPacketIO(n: Int) extends Bundle {
+class VectorPacketIO(val n: Int) extends Bundle {
   val ins  = Vec(n, chisel3.util.DeqIO(new Packet()))
   val outs = Vec(n, chisel3.util.EnqIO(new Packet()))
 }
diff --git a/src/test/scala/examples/VendingMachineGenerator.scala b/src/test/scala/examples/VendingMachineGenerator.scala
index 55a17d9f..0a9dc3e6 100644
--- a/src/test/scala/examples/VendingMachineGenerator.scala
+++ b/src/test/scala/examples/VendingMachineGenerator.scala
@@ -9,7 +9,7 @@ import chisel3.util._
 
 import VendingMachineUtils._
 
-class VendingMachineIO(legalCoins: Seq[Coin]) extends Bundle {
+class VendingMachineIO(val legalCoins: Seq[Coin]) extends Bundle {
   require(legalCoins.size >= 1, "The vending machine must accept at least 1 coin!")
   // Order of coins by value
   val coins: Seq[Coin] = legalCoins sortBy (_.value)