Switch to using experimental trait for OpaqueTypes (backport #2783) (#2836)

* Switch to using experimental trait for OpaqueTypes (#2783)

This makes it more clear that the feature is experimental. Users may
still override the opaqueType method for more dynamic control over when
instances of a given Record are OpaqueTypes or not, but they are
discouraged from doing so.

(cherry picked from commit 7525dc71ccc2050d8e4a68b38f3b76920ba693fc)

* Fix cloneType in RecordSpec

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

1 files changed, 27 insertions, 0 deletions

diff --git a/core/src/main/scala/chisel3/experimental/OpaqueType.scala b/core/src/main/scala/chisel3/experimental/OpaqueType.scala
new file mode 100644
index 00000000..e7a2a15d
--- /dev/null
+++ b/core/src/main/scala/chisel3/experimental/OpaqueType.scala
@@ -0,0 +1,27 @@
+// SPDX-License-Identifier: Apache-2.0
+
+package chisel3.experimental
+
+import chisel3._
+
+/** Indicates if this Record represents an "Opaque Type"
+  *
+  * Opaque types provide a mechanism for user-defined types
+  * that do not impose any "boxing" overhead in the emitted FIRRTL and Verilog.
+  * You can think about an opaque type Record as a box around
+  * a single element that only exists at Chisel elaboration time.
+  * Put another way, if this trait is mixed into a Record,
+  * the Record may only contain a single element with an empty name
+  * and there will be no `_` in the name for that element in the emitted Verilog.
+  *
+  * @see RecordSpec in Chisel's tests for example usage and expected output
+  */
+trait OpaqueType { self: Record =>
+
+  /** If set to true, indicates that this Record is an OpaqueType
+    *
+    * Users can override this if they need more dynamic control over the behavior for when
+    * instances of this type are considered opaque
+    */
+  def opaqueType: Boolean = true
+}