[core] Add private Layer.layerSeq member function

Add a function that returns that sequence of all layers from the current
layer up to the root.  This is part of forthcoming support for different
layer convention ABIs which need to use this information in their ABI
utility functions.

Signed-off-by: Schuyler Eldridge <[email protected]>

Author: seldridge

core/src/main/scala/chisel3/Layer.scala

@@ -8,7 +8,7 @@ import chisel3.internal.firrtl.ir.{LayerBlock, Node}
 import chisel3.util.simpleClassName
 import java.nio.file.{Path, Paths}
 import scala.annotation.tailrec
-import scala.collection.mutable.LinkedHashSet
+import scala.collection.mutable.{ArrayBuffer, LinkedHashSet}
 
 /** This object contains Chisel language features for creating layers.  Layers
   * are collections of hardware that are not always present in the circuit.
@@ -120,6 +120,19 @@ object layer {
       case _ if this == that => true
       case _                 => this.canWriteTo(that.parent)
     }
+
+    /** Return a list containg this layer and all its parents, excluding the root
+      * layer.  The deepest layer (this layer) is the first element in the list.
+      *
+      * @return a sequence of all layers
+      */
+    private[chisel3] def layerSeq: List[Layer] = {
+      def rec(current: Layer, acc: List[Layer]): List[Layer] = current match {
+        case Layer.root => acc
+        case _          => rec(current.parent, current :: acc)
+      }
+      rec(this, Nil)
+    }
   }
 
   object Layer {