Crash when reporting initialization warning using -Ysafe-init-global flag

[q-ata]

Compiler version

3.4.0-RC1-bin-SNAPSHOT-git-48a7871 (commit 48a7871)

Minimized code

The crash occurs when compiling the following code with -Ysafe-init-global.

object A {
  def a: Int =
    B
      .s.length
}

object B {
  val s: String = A.a + "a"
}

This should report an initialization warning but instead crashes the compiler.

It seems this is caused by the method positionMarker in Trace.scala.

  private def positionMarker(pos: SourcePosition): String =
    val trimmed = pos.lineContent.takeWhile(c => c.isWhitespace).length
    val padding = pos.startColumnPadding.substring(trimmed).nn + "   "
    val carets =
      if (pos.startLine == pos.endLine)
        "^" * math.max(1, pos.endColumn - pos.startColumn)
      else "^"

    s"$padding$carets\n"

Since the line .s.length in the test case has 6 leading spaces, trimmed in positionMarker is 6, but pos.startColumnPadding is only of length 4 since it is computed from the previous line B. As such, trying to take the substring of pos.startColumnPadding starting from index 6 is an error.

Output (click arrow to expand)

The stack trace is as follows:

  An unhandled exception was thrown in the compiler.
  Please file a crash report here:
  https://github.com/lampepfl/dotty/issues/new/choose

     while compiling: <no file>
        during phase: parser
                mode: Mode(ImplicitsEnabled)
     library version: version 2.13.12
    compiler version: version 3.4.0-RC1-bin-SNAPSHOT-git-48a7871
            settings: -Ysafe-init-global true

                tree: EmptyTree
       tree position: :<unknown>
           tree type: <notype>
              symbol: val <none>
           call site: package <root> in module class <root>

  == Source file context for tree position ==


Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: -2
        at java.base/java.lang.String.substring(String.java:1841)
        at dotty.tools.dotc.transform.init.Trace$.positionMarker(Trace.scala:81)
        at dotty.tools.dotc.transform.init.Trace$.buildStacktrace$$anonfun$1(Trace.scala:60)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
        at scala.collection.immutable.Vector.foreach(Vector.scala:2124)
        at dotty.tools.dotc.transform.init.Trace$.buildStacktrace(Trace.scala:69)
        at dotty.tools.dotc.transform.init.Trace$.show(Trace.scala:30)
        at dotty.tools.dotc.transform.init.Objects$.select$$anonfun$3(Objects.scala:796)
        at dotty.tools.dotc.core.Decorators$.toMessage$$anonfun$1(Decorators.scala:70)
        at dotty.tools.dotc.reporting.NoExplanation.msg(Message.scala:431)
        at dotty.tools.dotc.reporting.Message.message$$anonfun$1(Message.scala:375)
        at dotty.tools.dotc.reporting.Message.inMessageContext(Message.scala:361)
        at dotty.tools.dotc.reporting.Message.message(Message.scala:375)
        at dotty.tools.dotc.reporting.Message.isNonSensical(Message.scala:352)
        at dotty.tools.dotc.reporting.HideNonSensicalMessages.isHidden(HideNonSensicalMessages.scala:16)
        at dotty.tools.dotc.reporting.HideNonSensicalMessages.isHidden$(HideNonSensicalMessages.scala:10)
        at dotty.tools.dotc.reporting.AbstractReporter.isHidden(AbstractReporter.scala:8)
        at dotty.tools.dotc.reporting.Reporter.issueUnconfigured(Reporter.scala:156)
        at dotty.tools.dotc.reporting.Reporter.go$1(Reporter.scala:177)
        at dotty.tools.dotc.reporting.Reporter.issueIfNotSuppressed(Reporter.scala:191)
        at dotty.tools.dotc.reporting.Reporter.report(Reporter.scala:203)
        at dotty.tools.dotc.transform.init.Objects$State$.doCheckObject$$anonfun$1(Objects.scala:277)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
        at scala.collection.immutable.List.foreach(List.scala:333)
        at dotty.tools.dotc.transform.init.Objects$State$.doCheckObject(Objects.scala:277)
        at dotty.tools.dotc.transform.init.Objects$State$.checkObjectAccess(Objects.scala:302)
        at dotty.tools.dotc.transform.init.Objects$.accessObject(Objects.scala:1003)
        at dotty.tools.dotc.transform.init.Objects$.checkClasses$$anonfun$2(Objects.scala:1016)
        at scala.collection.IterableOnceOps.foreach(IterableOnce.scala:576)
        at scala.collection.IterableOnceOps.foreach$(IterableOnce.scala:574)
        at scala.collection.AbstractIterable.foreach(Iterable.scala:933)
        at scala.collection.IterableOps$WithFilter.foreach(Iterable.scala:903)
        at dotty.tools.dotc.transform.init.Objects$.checkClasses(Objects.scala:1016)
        at dotty.tools.dotc.transform.init.Checker.runOn(Checker.scala:43)
        at dotty.tools.dotc.Run.runPhases$1$$anonfun$1(Run.scala:246)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:15)
        at scala.runtime.function.JProcedure1.apply(JProcedure1.java:10)
        at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1323)
        at dotty.tools.dotc.Run.runPhases$1(Run.scala:263)
        at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:271)
        at dotty.tools.dotc.Run.compileUnits$$anonfun$adapted$1(Run.scala:280)
        at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:71)
        at dotty.tools.dotc.Run.compileUnits(Run.scala:280)
        at dotty.tools.dotc.Run.compileSources(Run.scala:194)
        at dotty.tools.dotc.Run.compile(Run.scala:179)
        at dotty.tools.dotc.Driver.doCompile(Driver.scala:37)
        at dotty.tools.dotc.Driver.process(Driver.scala:197)
        at dotty.tools.dotc.Driver.process(Driver.scala:165)
        at dotty.tools.dotc.Driver.process(Driver.scala:177)
        at dotty.tools.dotc.Driver.main(Driver.scala:207)
        at dotty.tools.MainGenericCompiler$.run$1(MainGenericCompiler.scala:162)
        at dotty.tools.MainGenericCompiler$.main(MainGenericCompiler.scala:186)
        at dotty.tools.MainGenericCompiler.main(MainGenericCompiler.scala)

[olhotak]

Fixed by #18668