Check BaseModule.name for NullPointerException

This wraps the evaluation of BaseModule.name in try/catch to look for a
NullPointerException that may result from trying to evaluate desiredName
before it's ready. This catches a test case of using a desiredName that
depends on a later defined eager subinstance.

h/t @jackkoenig

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

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

@@ -180,7 +180,13 @@ abstract class BaseModule extends HasId {
   def desiredName = this.getClass.getName.split('.').last
 
   /** Legalized name of this module. */
-  final lazy val name = Builder.globalNamespace.name(desiredName)
+  final lazy val name = try {
+    Builder.globalNamespace.name(desiredName)
+  } catch {
+    case e: NullPointerException => throwException(
+      s"Error: desiredName of ${this.getClass.getName} is null. Did you evaluate 'name' before all values needed by desiredName were available?", e)
+    case t: Throwable => throw t
+  }
 
   /** Returns a FIRRTL ModuleName that references this object
     * @note Should not be called until circuit elaboration is complete

src/test/scala/chiselTests/Module.scala

@@ -65,7 +65,14 @@ class ModuleRewrap extends Module {
 class ModuleWrapper(gen: => Module) extends Module {
   val io = IO(new Bundle{})
   val child = Module(gen)
+  override val desiredName = s"${child.desiredName}Wrapper"
+}
+
+class NullModuleWrapper extends Module {
+  val io = IO(new Bundle{})
   override lazy val desiredName = s"${child.desiredName}Wrapper"
+  println(s"My name is ${name}")
+  val child = Module(new ModuleWire)
 }
 
 class ModuleSpec extends ChiselPropSpec {
@@ -147,4 +154,8 @@ class ModuleSpec extends ChiselPropSpec {
   property("A desiredName parameterized by a submodule should work") {
     Driver.elaborate(() => new ModuleWrapper(new ModuleWire)).name should be ("ModuleWireWrapper")
   }
+  property("A name generating a null pointer exception should provide a good error message") {
+    (the [Exception] thrownBy (Driver.elaborate(() => new NullModuleWrapper)))
+      .getMessage should include ("desiredName of chiselTests.NullModuleWrapper is null")
+  }
 }