Disallow assignment to op results (#698)

2 files changed, 13 insertions, 2 deletions

diff --git a/chiselFrontend/src/main/scala/chisel3/core/Binding.scala b/chiselFrontend/src/main/scala/chisel3/core/Binding.scala
index 2b6f10f6..b09bab0e 100644
--- a/chiselFrontend/src/main/scala/chisel3/core/Binding.scala
+++ b/chiselFrontend/src/main/scala/chisel3/core/Binding.scala
@@ -88,11 +88,14 @@ sealed trait ConstrainedBinding extends TopBinding {
   def location = Some(enclosure)
 }
 
+// A binding representing a data that cannot be (re)assigned to.
+sealed trait ReadOnlyBinding extends TopBinding
+
 // TODO literal info here
-case class LitBinding() extends UnconstrainedBinding
+case class LitBinding() extends UnconstrainedBinding with ReadOnlyBinding
 // TODO(twigg): Ops between unenclosed nodes can also be unenclosed
 // However, Chisel currently binds all op results to a module
-case class OpBinding(enclosure: UserModule) extends ConstrainedBinding
+case class OpBinding(enclosure: UserModule) extends ConstrainedBinding with ReadOnlyBinding
 case class MemoryPortBinding(enclosure: UserModule) extends ConstrainedBinding
 case class PortBinding(enclosure: BaseModule) extends ConstrainedBinding
 case class RegBinding(enclosure: UserModule) extends ConstrainedBinding
diff --git a/chiselFrontend/src/main/scala/chisel3/core/Data.scala b/chiselFrontend/src/main/scala/chisel3/core/Data.scala
index f61478c8..b5c9d319 100644
--- a/chiselFrontend/src/main/scala/chisel3/core/Data.scala
+++ b/chiselFrontend/src/main/scala/chisel3/core/Data.scala
@@ -267,6 +267,10 @@ abstract class Data extends HasId {
     if (connectCompileOptions.checkSynthesizable) {
       requireIsHardware(this, "data to be connected")
       requireIsHardware(that, "data to be connected")
+      this.topBinding match {
+        case _: ReadOnlyBinding => throwException(s"Cannot reassign to read-only $this")
+        case _ =>  // fine
+      }
       try {
         MonoConnect.connect(sourceInfo, connectCompileOptions, this, that, Builder.forcedUserModule)
       } catch {
@@ -283,6 +287,10 @@ abstract class Data extends HasId {
     if (connectCompileOptions.checkSynthesizable) {
       requireIsHardware(this, s"data to be bulk-connected")
       requireIsHardware(that, s"data to be bulk-connected")
+      (this.topBinding, that.topBinding) match {
+        case (_: ReadOnlyBinding, _: ReadOnlyBinding) => throwException(s"Both $this and $that are read-only")
+        case _ =>  // fine
+      }
       try {
         BiConnect.connect(sourceInfo, connectCompileOptions, this, that, Builder.forcedUserModule)
       } catch {