Support connecting Properties.

core/src/main/scala/chisel3/Data.scala

@@ -385,6 +385,22 @@ trait BaseType extends HasId with NamedComponent {
   private[chisel3] def earlyName: String = Arg.earlyLocalName(this)
 
   private[chisel3] def parentNameOpt: Option[String] = this._parent.map(_.name)
+
+  private[chisel3] def requireVisible(): Unit = {
+    val mod = topBindingOpt.flatMap(_.location)
+    topBindingOpt match {
+      case Some(tb: TopBinding) if (mod == Builder.currentModule) =>
+      case Some(pb: PortBinding)
+          if (mod.flatMap(Builder.retrieveParent(_, Builder.currentModule.get)) == Builder.currentModule) =>
+      case Some(pb: SecretPortBinding) => // Ignore secret to not require visibility
+      case Some(_: UnconstrainedBinding) =>
+      case _ =>
+        throwException(s"operand '$this' is not visible from the current module ${Builder.currentModule.get.name}")
+    }
+    if (!MonoConnect.checkWhenVisibility(this)) {
+      throwException(s"operand has escaped the scope of the when in which it was constructed")
+    }
+  }
 }
 
 /** This forms the root of the type system for wire data types. The data value
@@ -579,22 +595,6 @@ abstract class Data extends BaseType with SourceInfoDoc {
     rec(leftType, rightType)
   }
 
-  private[chisel3] def requireVisible(): Unit = {
-    val mod = topBindingOpt.flatMap(_.location)
-    topBindingOpt match {
-      case Some(tb: TopBinding) if (mod == Builder.currentModule) =>
-      case Some(pb: PortBinding)
-          if (mod.flatMap(Builder.retrieveParent(_, Builder.currentModule.get)) == Builder.currentModule) =>
-      case Some(pb: SecretPortBinding) => // Ignore secret to not require visibility
-      case Some(_: UnconstrainedBinding) =>
-      case _ =>
-        throwException(s"operand '$this' is not visible from the current module ${Builder.currentModule.get.name}")
-    }
-    if (!MonoConnect.checkWhenVisibility(this)) {
-      throwException(s"operand has escaped the scope of the when in which it was constructed")
-    }
-  }
-
   // Internal API: returns a ref that can be assigned to, if consistent with the binding
   private[chisel3] def lref: Node = {
     requireIsHardware(this)

core/src/main/scala/chisel3/internal/MonoConnect.scala

@@ -8,6 +8,7 @@ import chisel3.internal.containsProbe
 import chisel3.internal.Builder.pushCommand
 import chisel3.internal.firrtl.{Connect, Converter, DefInvalid}
 import chisel3.experimental.dataview.{isView, reify, reifyToAggregate}
+import chisel3.properties.Property
 
 import scala.language.experimental.macros
 import scala.annotation.tailrec
@@ -412,6 +413,15 @@ private[chisel3] object checkConnect {
     checkConnection(sourceInfo, sink, source, context_mod)
   }
 
+  def apply[T](
+    sourceInfo:  SourceInfo,
+    sink:        Property[T],
+    source:      Property[T],
+    context_mod: RawModule
+  ): Unit = {
+    checkConnection(sourceInfo, sink, source, context_mod)
+  }
+
   private def checkConnection(
     sourceInfo:  SourceInfo,
     sink:        BaseType,

core/src/main/scala/chisel3/internal/firrtl/Converter.scala

@@ -159,6 +159,8 @@ private[chisel3] object Converter {
       )
     case Connect(info, loc, exp) =>
       Some(fir.Connect(convert(info), convert(loc, ctx, info), convert(exp, ctx, info)))
+    case PropAssign(info, loc, exp) =>
+      Some(fir.PropAssign(convert(info), convert(loc, ctx, info), convert(exp, ctx, info)))
     case Attach(info, locs) =>
       Some(fir.Attach(convert(info), locs.map(l => convert(l, ctx, info))))
     case DefInvalid(info, arg) =>

core/src/main/scala/chisel3/internal/firrtl/IR.scala

@@ -338,6 +338,7 @@ case class AltBegin(sourceInfo: SourceInfo) extends Command
 case class OtherwiseEnd(sourceInfo: SourceInfo, firrtlDepth: Int) extends Command
 @deprecated(deprecatedPublicAPIMsg, "Chisel 3.6")
 case class Connect(sourceInfo: SourceInfo, loc: Node, exp: Arg) extends Command
+private[chisel3] case class PropAssign(sourceInfo: SourceInfo, loc: Node, exp: Arg) extends Command
 @deprecated(deprecatedPublicAPIMsg, "Chisel 3.6")
 case class Attach(sourceInfo: SourceInfo, locs: Seq[Node]) extends Command
 @deprecated(deprecatedPublicAPIMsg, "Chisel 3.6")

core/src/main/scala/chisel3/properties/Property.scala

@@ -2,9 +2,10 @@
 
 package chisel3.properties
 
-import chisel3.{ActualDirection, BaseType, SpecifiedDirection}
-import chisel3.internal.Binding
+import chisel3.{ActualDirection, BaseType, MonoConnectException, SpecifiedDirection}
+import chisel3.internal.{checkConnect, throwException, Binding, Builder, MonoConnect, ReadOnlyBinding, TopBinding}
 import chisel3.internal.{firrtl => ir}
+import chisel3.experimental.{prefix, requireIsHardware, SourceInfo}
 import scala.reflect.runtime.universe.{typeOf, TypeTag}
 import scala.annotation.implicitNotFound
 
@@ -80,6 +81,61 @@ class Property[T: PropertyType] extends BaseType {
   private[chisel3] def getPropertyType: ir.PropertyType = {
     implicitly[PropertyType[T]].getPropertyType
   }
+
+  /** Connect a source Property[T] to this sink Property[T]
+    */
+  def :=(source: => Property[T])(implicit sourceInfo: SourceInfo): Unit = {
+    prefix(this) {
+      this.connect(source)(sourceInfo)
+    }
+  }
+
+  /** Internal implementation of connecting a source Property[T] to this sink Property[T].
+    */
+  private def connect(source: Property[T])(implicit sourceInfo: SourceInfo): Unit = {
+    requireIsHardware(this, "property to be connected to")
+    requireIsHardware(source, "property to be connected from")
+    this.topBinding match {
+      case _: ReadOnlyBinding => throwException(s"Cannot reassign to read-only $this")
+      case _ => // fine
+    }
+
+    try {
+      checkConnect(sourceInfo, this, source, Builder.referenceUserModule)
+    } catch {
+      case MonoConnectException(message) =>
+        throwException(
+          s"Connection between sink ($this) and source ($source) failed @: $message"
+        )
+    }
+
+    Builder.pushCommand(ir.PropAssign(sourceInfo, this.lref, source.ref))
+  }
+
+  /** Internal API: returns a ref that can be assigned to, if consistent with the binding.
+    */
+  private[chisel3] def lref: ir.Node = {
+    requireIsHardware(this)
+    requireVisible()
+    topBindingOpt match {
+      case Some(binding: ReadOnlyBinding) =>
+        throwException(s"internal error: attempted to generate LHS ref to ReadOnlyBinding $binding")
+      case Some(binding: TopBinding) => ir.Node(this)
+      case opt => throwException(s"internal error: unknown binding $opt in generating LHS ref")
+    }
+  }
+
+  /** Internal API: returns a ref, if bound.
+    */
+  private[chisel3] final def ref: ir.Arg = {
+    requireIsHardware(this)
+    requireVisible()
+    topBindingOpt match {
+      case Some(binding: TopBinding) => ir.Node(this)
+      case opt => throwException(s"internal error: unknown binding $opt in generating RHS ref")
+    }
+  }
+
 }
 
 /** Companion object for Property.

docs/src/explanations/properties.md

@@ -52,8 +52,26 @@ constructs.
 The legal `Property` types may be used in ports. For example:
 
 ```scala mdoc:silent
-class Example extends RawModule {
+class PortsExample extends RawModule {
   // An Int Property type port.
   val myPort = IO(Input(Property[Int]()))
 }
 ```
+
+### Property Connections
+
+The legal `Property` types may be connected using the `:=` operator. For
+example, an input `Property` type port may be connected to an output `Property`
+type port:
+
+```scala mdoc:silent
+class ConnectExample extends RawModule {
+  val inPort = IO(Input(Property[Int]()))
+  val outPort = IO(Output(Property[Int]()))
+  outPort := inPort
+}
+```
+
+Connections are only supported between the same `Property` type. For example, a
+`Property[Int]` may only be connected to a `Property[Int]`. This is enforced by
+the Scala compiler.

firrtl/src/main/scala/firrtl/ir/IR.scala

@@ -302,6 +302,7 @@ object Block {
 
 case class Block(stmts: Seq[Statement]) extends Statement with UseSerializer
 case class Connect(info: Info, loc: Expression, expr: Expression) extends Statement with HasInfo with UseSerializer
+case class PropAssign(info: Info, loc: Expression, expr: Expression) extends Statement with HasInfo with UseSerializer
 case class IsInvalid(info: Info, expr: Expression) extends Statement with HasInfo with UseSerializer
 case class Attach(info: Info, exprs: Seq[Expression]) extends Statement with HasInfo with UseSerializer
 

firrtl/src/main/scala/firrtl/ir/Serializer.scala

@@ -237,8 +237,9 @@ object Serializer {
   }
 
   private def s(node: Statement)(implicit b: StringBuilder, indent: Int): Unit = node match {
-    case DefNode(info, name, value) => b ++= "node "; b ++= legalize(name); b ++= " = "; s(value); s(info)
-    case Connect(info, loc, expr)   => b ++= "connect "; s(loc); b ++= ", "; s(expr); s(info)
+    case DefNode(info, name, value)  => b ++= "node "; b ++= legalize(name); b ++= " = "; s(value); s(info)
+    case Connect(info, loc, expr)    => b ++= "connect "; s(loc); b ++= ", "; s(expr); s(info)
+    case PropAssign(info, loc, expr) => b ++= "propassign "; s(loc); b ++= ", "; s(expr); s(info)
     case c: Conditionally => b ++= sIt(c).mkString
     case EmptyStmt => b ++= "skip"
     case bb: Block => b ++= sIt(bb).mkString

src/test/scala/chiselTests/properties/PropertySpec.scala

@@ -46,4 +46,26 @@ class PropertySpec extends ChiselFlatSpec with MatchesAndOmits {
       "input bigIntProp : Integer"
     )()
   }
+
+  it should "support connecting Property types of the same type" in {
+    val chirrtl = ChiselStage.emitCHIRRTL(new RawModule {
+      val propIn = IO(Input(Property[Int]()))
+      val propOut = IO(Output(Property[Int]()))
+      propOut := propIn
+    })
+
+    matchesAndOmits(chirrtl)(
+      "propassign propOut, propIn"
+    )()
+  }
+
+  it should "fail to compile when connecting Property types of different types" in {
+    assertTypeError("""
+      new RawModule {
+        val propIn = IO(Input(Property[Int]()))
+        val propOut = IO(Output(Property[BigInt]()))
+        propOut := propIn
+      }
+    """)
+  }
 }