Ignore empty aggregates elements when binding aggregate direction (#946)

Previously, including an empty aggregate in a Bundle would cause
a MixedDirectionAggregateException because it has no elements and thus
doesn't have a direction

* Add SampleElementBinding for Vec sample elements
* Add ActualDirection.Empty for bound empty aggregates

1 files changed, 5 insertions, 1 deletions

diff --git a/chiselFrontend/src/main/scala/chisel3/core/Binding.scala b/chiselFrontend/src/main/scala/chisel3/core/Binding.scala
index 55e6bcf9..37ce3f66 100644
--- a/chiselFrontend/src/main/scala/chisel3/core/Binding.scala
+++ b/chiselFrontend/src/main/scala/chisel3/core/Binding.scala
@@ -87,7 +87,7 @@ sealed trait UnconstrainedBinding extends TopBinding {
   def location: Option[BaseModule] = None
 }
 // A constrained binding can only be read/written by specific modules
-// Location will track where this Module is
+// Location will track where this Module is, and the bound object can be referenced in FIRRTL
 sealed trait ConstrainedBinding extends TopBinding {
   def enclosure: BaseModule
   def location: Option[BaseModule] = Some(enclosure)
@@ -107,6 +107,10 @@ case class WireBinding(enclosure: RawModule) extends ConstrainedBinding
 case class ChildBinding(parent: Data) extends Binding {
   def location: Option[BaseModule] = parent.topBinding.location
 }
+/** Special binding for Vec.sample_element */
+case class SampleElementBinding[T <: Data](parent: Vec[T]) extends Binding {
+  def location = parent.topBinding.location
+}
 // A DontCare element has a specific Binding, somewhat like a literal.
 // It is a source (RHS). It may only be connected/applied to sinks.
 case class DontCareBinding() extends UnconstrainedBinding