lrytz

Configuring and suppressing warnings in Scala

Scala 2.13.2 and 2.12.13 introduced the -Wconf compiler flag to globally configure reporting of warnings, and the @nowarn annotation to locally suppress them. Having more control over compiler warnings makes them a lot more valuable:

• In projects where the build log shows a lot of of warnings that are mostly ignored, new helpful warnings can easily go undetected. The new functionality can be used to clean up the build log with manageable efforts and potentially enable fatal warning (-Werror in 2.13, -Xfatal-warnings in 2.12). This has happened for example in Scala compiler and standard library projects in the past few months.
• Projects that already use fatal warnings get better utilities to work around corner cases where a warning cannot be avoided. This can allow further -Xlint checks to be enabled.

In this post we go through the mechanics of configuring warnigns and also look at the new @nowarn annotation.

Warnings from the Scala compil

lrytz

object A {
  def main(args: Array[String]): Unit = {
    val testVar = "hai"
    val res = B.foo(testVar)
    println(res)
  }
}

lrytz

import java.util.concurrent.*;

public class Test {
  static boolean triggerIssue = true;

  static ForkJoinPool fjp;

  static void hotSleep(int ms) {
    long until = System.nanoTime() + ms*1000_000;
    while (System.nanoTime() < until) { }

lrytz

@@ -33,16 +33,14 @@
     ASTORE 2
     ALOAD 0
     ALOAD 2
-    INVOKEVIRTUAL BI.f ()Ljava/lang/Object;
-    INVOKESTATIC scala/runtime/BoxesRunTime.unboxToInt (Ljava/lang/Object;)I
+    INVOKEVIRTUAL BI.f ()I
     BIPUSH 11
     IADD
     INVOKEVIRTUAL Test$.show (I)V

lrytz

diff --git a/src/library/scala/collection/Map.scala b/src/library/scala/collection/Map.scala
index 9187c7c041..704a0f7f3e 100644
--- a/src/library/scala/collection/Map.scala
+++ b/src/library/scala/collection/Map.scala
@@ -33,7 +33,7 @@ trait Map[K, +V]
     case that: Map[K, _] =>
       (this eq that) ||
       (that canEqual this) &&
-      (this.size == that.size) && {
+      (this.sizeCompare(that) == 0) && {

lrytz

sbt:root> partest --srcpath async test/async/jvm/live.scala
Partest version:     2.13.3-20200619-135756-79afa3f
Compiler under test: $baseDir/compiler
Scala version is:    Scala compiler version 2.13.3-20200619-135756-79afa3f -- Copyright 2002-2020, LAMP/EPFL and Lightbend, Inc.
Scalac options are:  -Wconf:cat=unchecked&msg=The outer reference in this type test cannot be checked at run time.:s -Wconf:cat=optimizer:is
Compilation Path:    /Users/luc/scala/scala13/target/test/it-classes:$baseDir/test:$baseDir/compiler:$baseDir/library:$baseDir/reflect:$baseDir/interactive:$baseDir/repl-frontend:$baseDir/repl:$baseDir/scalap:$baseDir/partest:$baseDir/scaladoc:$baseDir/testkit:/Users/luc/scala/scala13/target/partest-javaagent/scala-partest-javaagent-2.13.3-bin-SNAPSHOT.jar:/Users/luc/Library/Caches/Coursier/v1/http/127.0.0.1%3A8081/artifactory/scala-ci-virtual/org/hamcrest/hamcrest-core/1.3/hamcrest-core-1.3.jar:/Users/luc/Library/Caches/Coursier/v1/http/127.0.0.1%3A8081/artifactory/scala-ci-virtual/org/scala-sbt

lrytz

➜  scala-parallel-collections git:(master) unzip -l /Users/luc/.ivy2/local/org.scala-lang.modules/scala-parallel-collections_2.13/0.1.2+107-ca1a2942-SNAPSHOT/bundles/scala-parallel-collections_2.13.jar | c4 | sort



----
META-INF/
META-INF/MANIFEST.MF
Name
scala-parallel-collections.properties
scala/

lrytz

given object intOrd extends Ord[Int] {}
given object _ extends Ord[Int] {}
given object extends Ord[Int] {}

given val intOrd: Ord[Int] = ...
given val _ : Ord[Int] = ...
given val : Ord[Int] = ...

given def intListOrd(using Ord[Int]): Ord[List[Int]] = ...
given def _(using Ord[Int]): Ord[List[Int]] = ...

lrytz

➜  sandbox git:(2.13.x) ~/Applications/bin/scala-runners/scala -212x
Welcome to Scala 2.12.11-bin-d1e9ac0 (Java HotSpot(TM) 64-Bit GraalVM EE 19.2.1, Java 1.8.0_231).
Type in expressions for evaluation. Or try :help.

scala> case class K(var x: Int)
defined class K

scala> val k = K(1)
k: K = K(1)

lrytz

// class version 52.0 (52)
// access flags 0x601
public abstract interface A {

  // compiled from: Test.scala

  @Lscala/reflect/ScalaSignature;(bytes="\u0006\u0005a1qa\u0001\u0003\u0011\u0002\u0007\u0005q\u0001C\u0003\u000f\u0001\u0011\u0005q\u0002C\u0004\u0014\u0001\u0009\u0007I\u0011\u0001\u000b\u0003\u0003\u0005S\u0011!B\u0001\u0008y\u0015l\u0007\u000f^=?\u0007\u0001\u0019\"\u0001\u0001\u0005\u0011\u0005%aQ\"\u0001\u0006\u000b\u0003-\u0009Qa]2bY\u0006L!!\u0004\u0006\u0003\r\u0005s\u0017PU3g\u0003\u0019!\u0013N\\5uIQ\u0009\u0001\u0003\u0005\u0002\n#%\u0011!C\u0003\u0002\u0005+:LG/A\u0001y+\u0005)\u0002CA\u0005\u0017\u0013\u00099\"BA\u0002J]R\u0004")

  ATTRIBUTE ScalaSig : unknown