Add PartialDataView.supertype (backport #2826) (#2827)

* Add PartialDataView.supertype (#2826)

This factory method makes it easy to create PartialDataViews from a
Bundle type to its supertype. Because of the typing relationship, there
is no need to provide a mapping between fields. The only thing necessary
is to provide a function for constructing an instance of the supertype
from an instance of the subtype.

(cherry picked from commit 251d454a224e5a961438ba0ea41134d7da7a5992)

# Conflicts:
#	core/src/main/scala/chisel3/experimental/dataview/package.scala
#	src/test/scala/chiselTests/experimental/DataView.scala

* Resolve backport conflicts

Co-authored-by: Jack Koenig <koenig@sifive.com>

2 files changed, 25 insertions, 12 deletions

diff --git a/core/src/main/scala/chisel3/experimental/dataview/DataView.scala b/core/src/main/scala/chisel3/experimental/dataview/DataView.scala
index 7f20964d..cc555b11 100644
--- a/core/src/main/scala/chisel3/experimental/dataview/DataView.scala
+++ b/core/src/main/scala/chisel3/experimental/dataview/DataView.scala
@@ -592,4 +592,27 @@ object PartialDataView {
     implicit sourceInfo: SourceInfo
   ): DataView[T, V] =
     new DataView[T, V](mkView, mapping, _total = false)
+
+  /** Constructs a non-total [[DataView]] mapping from a [[Bundle]] type to a parent [[Bundle]] type
+    *
+    * @param mkView a function constructing an instance `V` from an instance of `T`
+    * @return the [[DataView]] that enables viewing instances of a [[Bundle]] as instances of a parent type
+    */
+  def supertype[T <: Bundle, V <: Bundle](
+    mkView: T => V
+  )(
+    implicit ev: SubTypeOf[T, V],
+    sourceInfo:  SourceInfo
+  ): DataView[T, V] =
+    mapping[T, V](
+      mkView,
+      {
+        case (a, b) =>
+          val aElts = a.elements
+          val bElts = b.elements
+          val bKeys = bElts.keySet
+          val keys = aElts.keysIterator.filter(bKeys.contains)
+          keys.map(k => aElts(k) -> bElts(k)).toSeq
+      }
+    )
 }
diff --git a/core/src/main/scala/chisel3/experimental/dataview/package.scala b/core/src/main/scala/chisel3/experimental/dataview/package.scala
index 71ae2d8f..a52e88cf 100644
--- a/core/src/main/scala/chisel3/experimental/dataview/package.scala
+++ b/core/src/main/scala/chisel3/experimental/dataview/package.scala
@@ -43,24 +43,14 @@ package object dataview {
     "${A} is not a subtype of ${B}! Did you mean .viewAs[${B}]? " +
       "Please see https://www.chisel-lang.org/chisel3/docs/cookbooks/dataview"
   )
-  private type SubTypeOf[A, B] = A <:< B
+  private[dataview] type SubTypeOf[A, B] = A <:< B
 
   /** Provides `viewAsSupertype` for subclasses of [[Bundle]] */
   implicit class BundleUpcastable[T <: Bundle](target: T) {
 
     /** View a [[Bundle]] or [[Record]] as a parent type (upcast) */
     def viewAsSupertype[V <: Bundle](proto: V)(implicit ev: SubTypeOf[T, V], sourceInfo: SourceInfo): V = {
-      implicit val dataView = PartialDataView.mapping[T, V](
-        _ => proto,
-        {
-          case (a, b) =>
-            val aElts = a.elements
-            val bElts = b.elements
-            val bKeys = bElts.keySet
-            val keys = aElts.keysIterator.filter(bKeys.contains)
-            keys.map(k => aElts(k) -> bElts(k)).toSeq
-        }
-      )
+      implicit val dataView = PartialDataView.supertype[T, V](_ => proto)
       target.viewAs[V]
     }
   }