Add private Module API and internal DataMirror API for moduleIOs.

core/src/main/scala/chisel3/Module.scala

@@ -574,6 +574,12 @@ package experimental {
       _ports.toSeq
     }
 
+    /** Get IOs that are currently bound to this module.
+      */
+    private[chisel3] def getIOs: Seq[Data] = {
+      _ports.map(_._1).toSeq
+    }
+
     // These methods allow checking some properties of ports before the module is closed,
     // mainly for compatibility purposes.
     protected def portsContains(elem: Data): Boolean = {

core/src/main/scala/chisel3/reflect/DataMirror.scala

@@ -212,6 +212,17 @@ object DataMirror {
     def chiselTypeClone[T <: Data](target: T): T = {
       target.cloneTypeFull
     }
+
+    /** Returns the current ports of an in-progress module.
+      *
+      * This method does not necessarily return the final ports of the target module. It consults Chisel's internal data
+      * structures to extract the module's IOs. For this reason, it is generally not safe, and users should prefer
+      * [[DataMirror.modulePorts]], but this method may be used for certain use cases that want the current list of
+      * ports before the module is closed.
+      *
+      * @param target BaseModule to get IOs from
+      */
+    def currentModulePorts(target: BaseModule): Seq[Data] = target.getIOs
   }
 
   // Old definition of collectLeafMembers

src/test/scala/chiselTests/reflect/DataMirrorSpec.scala

@@ -245,4 +245,24 @@ class DataMirrorSpec extends ChiselFlatSpec {
     DataMirror.getLayerColor(foo.c) should be(Some(A))
   }
 
+  "moduleIOs" should "return an in-progress module's IOs" in {
+    class Foo extends RawModule {
+      val in = IO(Input(Bool()))
+      val out = IO(Output(Bool()))
+      val wire = Wire(Bool())
+      val child = Module(new RawModule {})
+
+      val ports0 = DataMirror.internal.currentModulePorts(this)
+
+      val other = IO(Input(Bool()))
+
+      val ports1 = DataMirror.internal.currentModulePorts(this)
+    }
+
+    ChiselStage.emitCHIRRTL(new RawModule {
+      val foo = Module(new Foo)
+      foo.ports0 should be(Seq(foo.in, foo.out))
+      foo.ports1 should be(Seq(foo.in, foo.out, foo.other))
+    })
+  }
 }