[CI] Use Mill for testing (#4466)

* Fix scalacOptions for Chisel to use plugin
* Improve error reporting to support and suggest -Xcheckinit

.github/workflows/test.yml

@@ -76,7 +76,10 @@ jobs:
           $CHISEL_FIRTOOL_PATH/firtool -version >> $GITHUB_STEP_SUMMARY
           echo \`\`\` >> $GITHUB_STEP_SUMMARY
       - name: Test
-        run: sbt ++${{ inputs.scala }} test
+        run: |
+          ./mill firrtl[_].test
+          ./mill svsim[_].test
+          ./mill chisel[_].test
       - name: Binary compatibility
         # TODO either make this also check the plugin or decide that we don't
         #      support binary compatibility for the plugin
@@ -155,7 +158,6 @@ jobs:
         with:
           distribution: 'adopt'
           java-version: '11'
-          cache: 'sbt'
       - name: Install CIRCT
         id: install-circt
         if: ${{ inputs.circt }}
@@ -170,7 +172,7 @@ jobs:
           dir=$(dirname $(which firtool))
           echo "CHISEL_FIRTOOL_PATH=$dir" >> "$GITHUB_ENV"
       - name: Integration Tests
-        run: sbt integrationTests/test
+        run: ./mill integrationTests[_].test
 
   # Currently just a sanity check that the benchmarking flow works
   benchmark:
@@ -190,7 +192,7 @@ jobs:
         run: ./mill benchmark.runJmh
 
   std:
-    name: Standard Library Tests
+    name: Standard Library
     runs-on: ubuntu-22.04
     strategy:
       matrix:
@@ -207,9 +209,8 @@ jobs:
         with:
           distribution: 'adopt'
           java-version: '11'
-          cache: 'sbt'
-      - name: Unit Tests
-        run: sbt ++${{ inputs.scala }} standardLibrary/test
+      - name: Compile
+        run: ./mill stdlib[_].compile
 
   website:
     name: Build Mdoc & Website

build.sc

@@ -317,7 +317,8 @@ object chisel extends Cross[Chisel](v.scalaCrossVersions)
 trait Chisel extends common.ChiselModule with CrossSbtModule with ScalafmtModule {
   override def millSourcePath = super.millSourcePath / os.up
 
-  override def scalacOptions = v.commonOptions
+  // Make sure to include super.scalacOptions for Chisel Plugin
+  override def scalacOptions = T(super.scalacOptions() ++ v.commonOptions)
 
   def svsimModule = svsim(crossScalaVersion)
 

core/src/main/scala/chisel3/ModuleImpl.scala

@@ -661,7 +661,12 @@ package experimental {
     def desiredName: String = simpleClassName(this.getClass)
 
     /** Legalized name of this module. */
-    final lazy val name =
+    final lazy val name = {
+      def err(msg: String, cause: Throwable = null) = {
+        val msg = s"Error: desiredName of ${this.getClass.getName} is null. "
+        s"Did you evaluate 'name' before all values needed by desiredName were available? $msg."
+        throwException(msg, cause)
+      }
       try {
         // PseudoModules are not "true modules" and thus should share
         // their original modules names without uniquification
@@ -671,12 +676,11 @@ package experimental {
         }
       } 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
-          )
+          err("Try adding -Xcheckinit to your scalacOptions to get a more useful stack trace", e)
+        case UninitializedFieldError(msg) => err(msg)
         case t: Throwable => throw t
       }
+    }
 
     /** Returns a FIRRTL ModuleName that references this object
       *

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

@@ -813,13 +813,23 @@ private[chisel3] object Builder extends LazyLogging {
 
   // Helper for reasonable errors when clock or reset value not yet initialized
   private def getDelayed[A](field: String, dc: Delayed[A]): A = {
-    val result = dc.value
-    if (result == null) {
-      // TODO add SourceInfo and change to Builder.exception
+    // TODO add SourceInfo and change to Builder.exception.
+    def err(extra: String) = {
       throwException(
-        s"The implicit $field is null which means the code that sets its definition has not yet executed."
+        s"The implicit $field is null which means the code that sets its definition has not yet executed. $extra."
       )
     }
+    val result =
+      try {
+        dc.value
+      } catch {
+        // If user uses -Xcheckinit there will be additional information.
+        case UninitializedFieldError(msg) => err(msg)
+      }
+    // If user is not using -Xcheckinit, suggest that they do.
+    if (result == null) {
+      err("Try adding -Xcheckinit to your scalacOptions to get a more useful stack trace")
+    }
     result
   }