Add support for marking things as readOnly

Also add internal support for marking things as "deprecated read only".

core/src/main/scala/chisel3/Aggregate.scala

@@ -48,7 +48,7 @@ sealed abstract class Aggregate extends Data {
       // Don't accidentally invent a literal value for a view that is empty
       case Some(_: AggregateViewBinding) if this.getElements.isEmpty =>
         reifyIdentityView(this) match {
-          case Some(target: Aggregate) => target.checkingLitOption(checkForDontCares)
+          case Some((target: Aggregate, _)) => target.checkingLitOption(checkForDontCares)
           // This can occur with empty Vecs or Bundles
           case _ => None
         }
@@ -355,7 +355,7 @@ sealed class Vec[T <: Data] private[chisel3] (gen: => T, val length: Int) extend
         // Views complicate things a bit, but views that correspond exactly to an identical Vec can just forward the
         // dynamic indexing to the target Vec
         // In theory, we could still do this forwarding if the sample element were different by deriving a DataView
-        case Some(target: Vec[T @unchecked])
+        case Some((target: Vec[T @unchecked], _))
             if this.length == target.length &&
               this.sample_element.typeEquivalent(target.sample_element) =>
           return target.apply(p)

core/src/main/scala/chisel3/Data.scala

@@ -7,7 +7,7 @@ import chisel3.experimental.dataview.reify
 import scala.language.experimental.macros
 import chisel3.experimental.{requireIsChiselType, requireIsHardware, Analog, BaseModule}
 import chisel3.experimental.{prefix, SourceInfo, UnlocatableSourceInfo}
-import chisel3.experimental.dataview.{reifyIdentityView, reifySingleTarget}
+import chisel3.experimental.dataview.{reifyIdentityView, reifySingleTarget, DataViewable}
 import chisel3.internal.Builder.pushCommand
 import chisel3.internal._
 import chisel3.internal.binding._
@@ -629,8 +629,8 @@ abstract class Data extends HasId with NamedComponent with SourceInfoDoc {
       case Some(pb: PortBinding)
           if mod.flatMap(Builder.retrieveParent(_, Builder.currentModule.get)) == Builder.currentModule =>
         true
-      case Some(ViewBinding(target))           => target.isVisibleFromModule
-      case Some(AggregateViewBinding(mapping)) => mapping.values.forall(_.isVisibleFromModule)
+      case Some(ViewBinding(target, _))           => target.isVisibleFromModule
+      case Some(AggregateViewBinding(mapping, _)) => mapping.values.forall(_.isVisibleFromModule)
       case Some(pb: SecretPortBinding) => true // Ignore secret to not require visibility
       case Some(_: UnconstrainedBinding) => true
       case _ => false
@@ -651,13 +651,16 @@ abstract class Data extends HasId with NamedComponent with SourceInfoDoc {
   }
 
   // Internal API: returns a ref that can be assigned to, if consistent with the binding
-  private[chisel3] def lref: Node = {
+  private[chisel3] def lref(implicit info: SourceInfo): Node = {
     requireIsHardware(this)
     requireVisible()
     topBindingOpt match {
       case Some(binding: ReadOnlyBinding) =>
         throwException(s"internal error: attempted to generate LHS ref to ReadOnlyBinding $binding")
-      case Some(ViewBinding(target)) => reify(target).lref
+      case Some(ViewBinding(target1, wr1)) =>
+        val (target2, wr2) = reify(target1)
+        val writability = wr1.combine(wr2)
+        writability.reportIfReadOnly(target2.lref)(Wire(chiselTypeOf(target2)).lref)
       case Some(binding: TopBinding) => Node(this)
       case opt => throwException(s"internal error: unknown binding $opt in generating LHS ref")
     }
@@ -677,11 +680,11 @@ abstract class Data extends HasId with NamedComponent with SourceInfoDoc {
     requireIsHardware(this)
     topBindingOpt match {
       // DataView
-      case Some(ViewBinding(target)) => reify(target).ref
+      case Some(ViewBinding(target, _)) => reify(target)._1.ref
       case Some(_: AggregateViewBinding) =>
         reifyIdentityView(this) match {
           // If this is an identity view (a view of something of the same type), return ref of target
-          case Some(target) => target.ref
+          case Some((target, _)) => target.ref
           // Otherwise, we need to materialize hardware of the correct type
           case _ => materializeWire()
         }
@@ -812,7 +815,9 @@ abstract class Data extends HasId with NamedComponent with SourceInfoDoc {
   def do_asTypeOf[T <: Data](that: T)(implicit sourceInfo: SourceInfo): T = {
     val thatCloned = Wire(that.cloneTypeFull)
     thatCloned.connectFromBits(this.asUInt)
-    thatCloned
+    thatCloned.viewAsReadOnlyDeprecated(siteInfo =>
+      Warning(WarningID.AsTypeOfReadOnly, s"Return values of asTypeOf will soon be read-only")(siteInfo)
+    )
   }
 
   /** Assigns this node from Bits type. Internal implementation for asTypeOf.
@@ -1017,6 +1022,18 @@ object Data {
       }
     }
   }
+
+  implicit class AsReadOnly[T <: Data](self: T) {
+
+    /** Returns a read-only view of this Data
+      *
+      * It is illegal to connect to the return value of this method.
+      * This Data this method is called on must be a hardware type.
+      */
+    def readOnly(implicit sourceInfo: SourceInfo): T = {
+      self.viewAsReadOnly(_ => "Cannot connect to read-only value")
+    }
+  }
 }
 
 trait WireFactory {

core/src/main/scala/chisel3/Element.scala

@@ -39,9 +39,12 @@ abstract class Element extends Data {
         case _            => Some(DontCareBinding())
       }
     // TODO Do we even need this? Looking up things in the AggregateViewBinding is fine
-    case Some(b @ AggregateViewBinding(viewMap)) =>
+    case Some(b @ AggregateViewBinding(viewMap, _)) =>
       viewMap.get(this) match {
-        case Some(elt: Element) => Some(ViewBinding(elt))
+        case Some(elt: Element) =>
+          // Very important to use this instead of elt as "this" is the key to the viewMap
+          val wr = b.lookupWritability(this)
+          Some(ViewBinding(elt, wr))
         // Children of Probes won't be in viewMap, just return the binding
         case _ => Some(b)
       }
@@ -51,7 +54,7 @@ abstract class Element extends Data {
   private[chisel3] def litArgOption: Option[LitArg] = topBindingOpt match {
     case Some(ElementLitBinding(litArg)) => Some(litArg)
     case Some(_: ViewBinding) =>
-      reify(this).litArgOption
+      reify(this)._1.litArgOption
     case _ => None
   }
 

core/src/main/scala/chisel3/experimental/dataview/package.scala

@@ -20,15 +20,21 @@ package object dataview {
     * Calling `viewAs` also requires an implementation of [[DataView]] for the target type
     */
   implicit class DataViewable[T](target: T) {
-    def viewAs[V <: Data](implicit dataproduct: DataProduct[T], dataView: DataView[T, V], sourceInfo: SourceInfo): V = {
+    private def _viewAsImpl[V <: Data](
+      writability: ViewWriteability
+    )(
+      implicit dataproduct: DataProduct[T],
+      dataView:             DataView[T, V],
+      sourceInfo:           SourceInfo
+    ): V = {
       // TODO put a try catch here for ExpectedHardwareException and perhaps others
       // It's likely users will accidentally use chiselTypeOf or something that may error,
       // The right thing to use is DataMirror...chiselTypeClone because of composition with DataView.andThen
       // Another option is that .andThen could give a fake binding making chiselTypeOfs in the user code safe
       val result: V = dataView.mkView(target)
       requireIsChiselType(result, "viewAs")
 
-      doBind(target, result, dataView)
+      doBind(target, result, dataView, writability)
 
       // Setting the parent marks these Data as Views
       result.setAllParents(Some(ViewParent))
@@ -38,6 +44,28 @@ package object dataview {
       result.forceName("view", Builder.viewNamespace)
       result
     }
+
+    def viewAs[V <: Data](
+      implicit dataproduct: DataProduct[T],
+      dataView:             DataView[T, V],
+      sourceInfo:           SourceInfo
+    ): V = _viewAsImpl(ViewWriteability.Default)
+
+    private[chisel3] def viewAsReadOnlyDeprecated[V <: Data](
+      getWarning: SourceInfo => Warning
+    )(
+      implicit dataproduct: DataProduct[T],
+      dataView:             DataView[T, V],
+      sourceInfo:           SourceInfo
+    ): V = _viewAsImpl(ViewWriteability.ReadOnlyDeprecated(getWarning))
+
+    private[chisel3] def viewAsReadOnly[V <: Data](
+      getError: SourceInfo => String
+    )(
+      implicit dataproduct: DataProduct[T],
+      dataView:             DataView[T, V],
+      sourceInfo:           SourceInfo
+    ): V = _viewAsImpl(ViewWriteability.ReadOnly(getError))
   }
 
   /** Provides `viewAsSupertype` for subclasses of [[Record]] */
@@ -75,9 +103,10 @@ package object dataview {
 
   // TODO should this be moved to class Aggregate / can it be unified with Aggregate.bind?
   private def doBind[T: DataProduct, V <: Data](
-    target:   T,
-    view:     V,
-    dataView: DataView[T, V]
+    target:      T,
+    view:        V,
+    dataView:    DataView[T, V],
+    writability: ViewWriteability
   )(
     implicit sourceInfo: SourceInfo
   ): Unit = {
@@ -205,7 +234,7 @@ package object dataview {
     }
 
     view match {
-      case elt: Element => view.bind(ViewBinding(elementResult(elt)))
+      case elt: Element => view.bind(ViewBinding(elementResult(elt), writability))
       case agg: Aggregate =>
         val fullResult = elementResult ++ aggregateMappings
 
@@ -222,15 +251,19 @@ package object dataview {
             case _ => // Do nothing
           }
         }
-        agg.bind(AggregateViewBinding(fullResult))
+        val aggWritability = Option.when(writability.isReadOnly)(
+          Map((agg: Data) -> writability)
+        )
+        agg.bind(AggregateViewBinding(fullResult, aggWritability))
     }
   }
 
   // Traces an Element that may (or may not) be a view until it no longer maps
   // Inclusive of the argument
+  // Note that this does *not* include writability so do not use this in place of reify
   private def unfoldView(elt: Element): LazyList[Element] = {
     def rec(e: Element): LazyList[Element] = e.topBindingOpt match {
-      case Some(ViewBinding(target)) => target #:: rec(target)
+      case Some(ViewBinding(target, _)) => target #:: rec(target)
       case Some(avb: AggregateViewBinding) =>
         val target = avb.lookup(e).get
         target #:: rec(target)
@@ -247,19 +280,25 @@ package object dataview {
     * This is the fundamental "unwrapping" or "tracing" primitive operation for handling Views within
     * Chisel.
     */
-  private[chisel3] def reify(elt: Element): Element =
-    reify(elt, elt.topBinding)
+  private[chisel3] def reify(elt: Element): (Element, ViewWriteability) =
+    reify(elt, elt.topBinding, ViewWriteability.Default)
 
   /** Turn any [[Element]] that could be a View into a concrete Element
     *
     * This is the fundamental "unwrapping" or "tracing" primitive operation for handling Views within
     * Chisel.
     */
-  @tailrec private[chisel3] def reify(elt: Element, topBinding: TopBinding): Element =
+  @tailrec private[chisel3] def reify(
+    elt:        Element,
+    topBinding: TopBinding,
+    wrAcc:      ViewWriteability
+  ): (Element, ViewWriteability) = {
     topBinding match {
-      case ViewBinding(target) => reify(target, target.topBinding)
-      case _                   => elt
+      case ViewBinding(target, writeability) =>
+        reify(target, target.topBinding, wrAcc.combine(writeability))
+      case _ => (elt, wrAcc)
     }
+  }
 
   /** Determine the target of a View if the view is an identity mapping.
     *
@@ -269,19 +308,24 @@ package object dataview {
     * @return The identity target of this view or None if not an identity view.
     * @note Returns Some(_) of the argument if it is not a view.
     */
-  private[chisel3] def reifyIdentityView[T <: Data](data: T): Option[T] = {
-    val candidate: Option[Data] =
+  private[chisel3] def reifyIdentityView[T <: Data](
+    data:  T,
+    wrAcc: ViewWriteability = ViewWriteability.Default
+  ): Option[(T, ViewWriteability)] = {
+    val candidate: Option[(Data, ViewWriteability)] =
       data.topBindingOpt match {
-        case None                               => None
-        case Some(ViewBinding(target))          => Some(target)
-        case Some(AggregateViewBinding(lookup)) => lookup.get(data)
-        case Some(_)                            => Some(data)
+        case None                                       => None
+        case Some(ViewBinding(target, wr))              => Some(target -> wr)
+        case Some(vb @ AggregateViewBinding(lookup, _)) => lookup.get(data).map(_ -> vb.lookupWritability(data))
+        case Some(_)                                    => Some(data -> ViewWriteability.Default)
       }
-    candidate.flatMap { d =>
-      // This cast is safe by construction, we only put Data in the view mapping if it is an identity mapping
-      val cast = d.asInstanceOf[T]
-      // Candidate may itself be a view, keep tracing in those cases
-      if (isView(d)) reifyIdentityView(cast) else Some(cast)
+    candidate.flatMap {
+      case (d, wr) =>
+        val wrx = wrAcc.combine(wr)
+        // This cast is safe by construction, we only put Data in the view mapping if it is an identity mapping
+        val cast = d.asInstanceOf[T]
+        // Candidate may itself be a view, keep tracing in those cases
+        if (isView(d)) reifyIdentityView(cast, wrx) else Some(cast -> wrx)
     }
   }
 
@@ -298,15 +342,17 @@ package object dataview {
     *
     * @return The single Data target of this view or None if a single Data doesn't exist.
     * @note Returns Some(_) of the argument if it is not a view.
+    * @note You should never attempt to write the result of this function.
     */
   private[chisel3] def reifySingleTarget(data: Data): Option[Data] = {
     def err(msg: String) = throwException(s"Internal Error! $msg reifySingleTarget($data)")
     // Identity views are obviously single targets.
-    reifyIdentityView(data).orElse {
+    // We ignore writability because the return of this function should never be written.
+    reifyIdentityView(data).map(_._1).orElse {
       // Otherwise, all children of data need to map to all of the children of another Data.
       // This is really expensive, is there a better way?
       data.topBindingOpt.flatMap {
-        case AggregateViewBinding(mapping) =>
+        case AggregateViewBinding(mapping, _) =>
           // Take every single leaf and map to its target
           val leaves = DataMirror.collectLeafMembers(data)
           val targets = leaves.map { l =>

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

@@ -11,7 +11,7 @@ import chisel3._
 import chisel3.experimental.dataview.{isView, reify, reifyIdentityView}
 import chisel3.internal.firrtl.ir.{Arg, ILit, Index, ModuleIO, Slot, ULit}
 import chisel3.internal.{throwException, Builder, ViewParent}
-import chisel3.internal.binding.{AggregateViewBinding, ChildBinding, CrossModuleBinding, ViewBinding}
+import chisel3.internal.binding.{AggregateViewBinding, ChildBinding, CrossModuleBinding, ViewBinding, ViewWriteability}
 
 /** Represents lookup typeclass to determine how a value accessed from an original IsInstantiable
   *   should be tweaked to return the Instance's version
@@ -176,19 +176,25 @@ object Lookupable {
 
     // We have to lookup the target(s) of the view since they may need to be underlying into the current context
     val newBinding = data.topBinding match {
-      case ViewBinding(target) => ViewBinding(lookupData(reify(target)))
-      case avb @ AggregateViewBinding(map) =>
+      case ViewBinding(target, writable1) =>
+        val (reified, writable2) = reify(target)
+        ViewBinding(lookupData(reified), writable1.combine(writable2))
+      case avb @ AggregateViewBinding(map, _) =>
         data match {
-          case e: Element   => ViewBinding(lookupData(reify(avb.lookup(e).get)))
+          case e: Element =>
+            val (reified, writable) = reify(avb.lookup(e).get)
+            ViewBinding(lookupData(reified), avb.lookupWritability(e).combine(writable))
           case _: Aggregate =>
             // We could just call reifyIdentityView, but since we already have avb, it is
             // faster to just use it but then call reifyIdentityView in case the target is itself a view
-            def reifyOpt(data: Data): Option[Data] = map.get(data).flatMap(reifyIdentityView(_))
+            def reifyOpt(data: Data): Option[(Data, ViewWriteability)] = map.get(data).flatMap(reifyIdentityView(_))
             // Just remap each Data present in the map
-            val newMap = coiterate(result, data).flatMap {
-              case (res, from) => reifyOpt(from).map(res -> lookupData(_))
-            }.toMap
-            AggregateViewBinding(newMap)
+            val mapping = coiterate(result, data).flatMap {
+              case (res, from) => reifyOpt(from).map { case (t, w) => (res, lookupData(t), w) }
+            }
+            val newMap = mapping.map { case (from, to, _) => from -> to }.toMap
+            val wrMap = mapping.flatMap { case (from, _, wr) => Option.when(wr.isReadOnly)(from -> wr) }.toMap
+            AggregateViewBinding(newMap, Option.when(wrMap.nonEmpty)(wrMap))
         }
       case _ => throw new InternalErrorException("Match error: data.topBinding=${data.topBinding}")
     }
@@ -197,7 +203,7 @@ object Lookupable {
     // We must also mark any non-identity Aggregates as unnammed
     newBinding match {
       case _: ViewBinding => // Do nothing
-      case AggregateViewBinding(childMap) =>
+      case AggregateViewBinding(childMap, _) =>
         // TODO we could do reifySingleTarget instead of just marking non-identity mappings
         getRecursiveFields.lazily(result, "_").foreach {
           case (agg: Aggregate, _) if !childMap.contains(agg) =>