util/util-app: scala 3 migration

**Problem**

Build util-app with Scala 3

**Solution**

# Change util-app project settings.
# Compile project + test with Scala 3 migration flags
# Remove flags
# Fix incompatibilities
# Run cross-building tests

**Result**

//Auto-rewrite fixes://
Scala 3 requires parentheses around the parameter of a lambda:
- ClassPath.scala
- Flaggable.scala

//Other fixes://
Scala3 requires explicit empty argument list to invoke method:
- ClassPath.scala
- AppTest.scala
- LoadServiceTest.scala

Manifest is no longer available in Scala 3, use ClassTag instead:
- Flags.scala
- GlobalFlag.scala

JavaConverters is deprecated, use CollectionConverters instead:
- CommandExecutor.scala
- RealCommandTest.scala:

JIRA Issues: CSL-11082, CSL-11091

Differential Revision: https://phabricator.twitter.biz/D714780

CHANGELOG.rst

@@ -7,6 +7,14 @@ Note that ``PHAB_ID=#`` and ``RB_ID=#`` correspond to associated messages in com
 Unreleased
 ----------
 
+New Features
+~~~~~~~~~~~~
+* util-app: Now builds with Scala 3.0! ``PHAB_ID=D714780``
+
+Breaking API Changes
+~~~~~~~~~~~~~~~~~~~~
+* util-app: Flags and GlobalFlag now use ClassTag instead of Manifest.  ``PHAB_ID=D714780``
+
 21.7.0
 ------
 

build.sbt

@@ -259,13 +259,22 @@ lazy val utilApp = Project(
   id = "util-app",
   base = file("util-app")
 ).settings(
-    sharedSettings
+   sharedScala3EnabledSettings
   ).settings(
     name := "util-app",
     libraryDependencies ++= Seq(
       "org.mockito" % "mockito-core" % mockitoVersion % "test",
-      "org.scalatestplus" %% "mockito-3-3" % "3.1.2.0" % "test"
-    )
+    ) ++ {
+      if (scalaVersion.value.startsWith("2")) {
+        Seq(
+          "org.scalatestplus" %% "mockito-3-3" % "3.1.2.0" % "test"
+        )
+      } else {
+        Seq(
+          "org.scalatestplus" %% "mockito-3-4" % "3.2.9.0" % "test"
+        )
+      }
+    },
   ).dependsOn(utilAppLifecycle, utilCore, utilRegistry)
 
 lazy val utilAppLifecycle = Project(

util-app/src/main/scala/com/twitter/app/ClassPath.scala

@@ -60,7 +60,7 @@ private[app] sealed abstract class ClassPath[CpInfo <: ClassPath.Info] {
       browseUri0(uri, loader, buf, seenUris)
       seenUris += uri
     }
-    buf.result
+    buf.result()
   }
 
   // Note: In JDK 9+ URLClassLoader is no longer the default ClassLoader.
@@ -71,7 +71,7 @@ private[app] sealed abstract class ClassPath[CpInfo <: ClassPath.Info] {
   // TODO - add suppport for the ModulePath after dropping JDK 8 support.
   private[this] def urlsFromClasspath(): Array[URL] = {
     val classpath: String = System.getProperty("java.class.path")
-    classpath.split(File.pathSeparator).map { pathEntry: String =>
+    classpath.split(File.pathSeparator).map { (pathEntry: String) =>
       Paths.get(pathEntry).toAbsolutePath().toUri().toURL
     }
   }

util-app/src/main/scala/com/twitter/app/Flaggable.scala

@@ -132,19 +132,19 @@ object Flaggable {
 
   implicit val ofInt: Flaggable[Int] = mandatory(_.toInt)
   implicit val ofJavaInteger: Flaggable[JInteger] =
-    mandatory { s: String => JInteger.valueOf(s.toInt) }
+    mandatory { (s: String) => JInteger.valueOf(s.toInt) }
 
   implicit val ofLong: Flaggable[Long] = mandatory(_.toLong)
   implicit val ofJavaLong: Flaggable[JLong] =
-    mandatory { s: String => JLong.valueOf(s.toLong) }
+    mandatory { (s: String) => JLong.valueOf(s.toLong) }
 
   implicit val ofFloat: Flaggable[Float] = mandatory(_.toFloat)
   implicit val ofJavaFloat: Flaggable[JFloat] =
-    mandatory { s: String => JFloat.valueOf(s.toFloat) }
+    mandatory { (s: String) => JFloat.valueOf(s.toFloat) }
 
   implicit val ofDouble: Flaggable[Double] = mandatory(_.toDouble)
   implicit val ofJavaDouble: Flaggable[JDouble] =
-    mandatory { s: String => JDouble.valueOf(s.toDouble) }
+    mandatory { (s: String) => JDouble.valueOf(s.toDouble) }
 
   // Conversions for common non-primitive types and collections.
   implicit val ofDuration: Flaggable[Duration] = mandatory(Duration.parse(_))

util-app/src/main/scala/com/twitter/app/Flags.scala

@@ -4,6 +4,7 @@ import scala.collection.immutable.TreeSet
 import scala.collection.mutable
 import scala.collection.mutable.ArrayBuffer
 import scala.jdk.CollectionConverters._
+import scala.reflect.ClassTag
 import scala.util.control.NonFatal
 
 /**
@@ -277,7 +278,7 @@ class Flags(argv0: String, includeGlobal: Boolean, failFastUntilParsed: Boolean)
    * @param name The name of the flag.
    * @param help The help string of the flag.
    */
-  def apply[T](name: String, help: String)(implicit _f: Flaggable[T], m: Manifest[T]): Flag[T] = {
+  def apply[T](name: String, help: String)(implicit _f: Flaggable[T], m: ClassTag[T]): Flag[T] = {
     val f = new Flag[T](name, help, m.toString, failFastUntilParsed)
     add(f)
     f

util-app/src/main/scala/com/twitter/app/GlobalFlag.scala

@@ -2,6 +2,7 @@ package com.twitter.app
 
 import java.lang.reflect.Modifier
 import scala.util.control.NonFatal
+import scala.reflect.ClassTag
 
 /**
  * Subclasses of GlobalFlag (that are defined in libraries) are "global" in the
@@ -85,7 +86,7 @@ abstract class GlobalFlag[T] private[app] (
    *
    * @param help documentation regarding usage of this [[Flag]].
    */
-  def this(help: String)(implicit _f: Flaggable[T], m: Manifest[T]) =
+  def this(help: String)(implicit _f: Flaggable[T], m: ClassTag[T]) =
     this(Right(m.toString), help)
 
   /**

util-app/src/main/scala/com/twitter/app/command/CommandExecutor.scala

@@ -1,7 +1,7 @@
 package com.twitter.app.command
 
 import java.io.File
-import scala.collection.JavaConverters._
+import scala.jdk.CollectionConverters._
 
 /**
  * CommandExecutor is a private trait used for testing so that the actual forking of the

util-app/src/test/scala/com/twitter/app/AppTest.scala

@@ -205,7 +205,7 @@ class AppTest extends AnyFunSuite {
     assert(n2 == 0)
     assert(!f.isDefined)
 
-    p.setDone
+    p.setDone()
     assert(n2 == 1)
     assert(f.isDefined)
   }

util-app/src/test/scala/com/twitter/app/LoadServiceTest.scala

@@ -109,7 +109,7 @@ class LoadServiceTest extends AnyFunSuite with MockitoSugar {
       f.setReadable(false)
 
       new LoadServiceClassPath().browseUri(f.toURI, loader, buf)
-      assert(buf.result.isEmpty)
+      assert(buf.result().isEmpty)
       f.delete()
     }
   }
@@ -128,7 +128,7 @@ class LoadServiceTest extends AnyFunSuite with MockitoSugar {
       subDir.setReadable(false)
 
       new LoadServiceClassPath().browseUri(f.toURI, loader, buf)
-      assert(buf.result.isEmpty)
+      assert(buf.result().isEmpty)
 
       subDir.delete()
       f.delete()

util-app/src/test/scala/com/twitter/app/command/RealCommandTest.scala

@@ -8,7 +8,7 @@ import java.nio.file.attribute.PosixFilePermission
 import java.nio.file.Files
 import org.scalatest.funsuite.AnyFunSuite
 import org.scalatest.matchers.should.Matchers
-import scala.collection.JavaConverters._
+import scala.jdk.CollectionConverters._
 
 object RealCommandTest {
   // Bazel provides the internal dependency as a JAR, and consequently,