Deprecate calling .viewAs on non-hardware (backport #3395)

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

@@ -102,6 +102,9 @@ package object dataview {
       // that are actually members of the target or view
       val tex = unfoldView(te).find(x => targetContains(x) || x.isLit || x == DontCare).getOrElse(err("Target", te))
       val vex = unfoldView(ve).find(viewFieldLookup.contains).getOrElse(err("View", ve))
+      if (!tex.isSynthesizable) {
+        Builder.deprecated(s".viewAs should only be called on hardware, this will become an error in Chisel 6.0.0")
+      }
 
       (tex, vex) match {
         /* Allow views where the types are equal. */

core/src/main/scala/chisel3/internal/Builder.scala

@@ -815,9 +815,18 @@ private[chisel3] object Builder extends LazyLogging {
       val absTarget = view.toAbsoluteTarget
       val elts = getRecursiveFields.lazily(view, "").collect { case (elt: Element, _) => elt }
       for (elt <- elts) {
-        val targetOfView = reify(elt)
-        renames.record(localTarget, targetOfView.toTarget)
-        renames.record(absTarget, targetOfView.toAbsoluteTarget)
+        // This is a hack to not crash when .viewAs is called on non-hardware
+        // It can be removed in Chisel 6.0.0 when it becomes illegal to call .viewAs on non-hardware
+        val targetOfViewOpt =
+          try {
+            Some(reify(elt))
+          } catch {
+            case _: NoSuchElementException => None
+          }
+        targetOfViewOpt.foreach { targetOfView =>
+          renames.record(localTarget, targetOfView.toTarget)
+          renames.record(absTarget, targetOfView.toAbsoluteTarget)
+        }
       }
     }
     renames

src/test/scala/chiselTests/experimental/DataView.scala

@@ -737,6 +737,17 @@ class DataViewSpec extends ChiselFlatSpec {
     }
   }
 
+  // In Chisel 6.0.0 this will become an error, but for now it at least needs to not crash
+  it should "not crash when calling .viewAs on unbound hardware" in {
+    class MyBundle(val foo: UInt, val bar: UInt) extends Bundle
+    implicit val view =
+      DataView[(UInt, UInt), MyBundle](x => new MyBundle(x._1.cloneType, x._2.cloneType), _._1 -> _.foo, _._2 -> _.bar)
+    class MyModule extends Module {
+      (UInt(8.W), UInt(8.W)).viewAs[MyBundle]
+    }
+    ChiselStage.emitCHIRRTL(new MyModule)
+  }
+
   it should "support literals as part of the target" in {
     import ValidExtensions._
     class MyModule extends Module {