Wrong line number information in bytecode

[scabug]

Positions of translated patterns (and consequently line numbers in bytecode) are sometimes wrong. While things are significantly better than in 2.9, there are still issues with unapplies. In this example, notice that in the last case some temporary variable loads are positioned at the match position (and the return statement as well).

This causes the debugger to jump between the current line and the beginning of the match for those steps and it is very confusing. I think this is a low-hanging fruit that can improve the user experience a lot.

abstract class Super
case class Case1(x: Int) extends Super
case class Case2(str: String) extends Super
object Case3 {
  def apply(x: Int) = new Super {}
  def unapply(z: Super): Option[Int] = {
    Some(-1)
  }
}

object Test extends App {

  def test(x: Super) {
    x match {
      case Case1(n) =>
        println(n)
      case Case2(str) =>
        println(str)
      case Case3(nr) =>
        println(nr)
    }
  }
  
  test(Case3(-1))
}

the output:

public void test(Super);
  Code:
   0: aload_1
   1: astore_2
   2: aload_2
   3: instanceof  #59; //class Case1
   6: ifeq  43
   9: aload_2
   10:   checkcast   #59; //class Case1
   13:   astore_3
   14:   aload_3
   15:   ifnull   43
   18:   aload_3
   19:   invokevirtual  #63; //Method Case1.x:()I
   22:   istore   4
   24:   getstatic   #68; //Field scala/Predef$.MODULE$:Lscala/Predef$;
   27:   iload 4
   29:   invokestatic   #74; //Method scala/runtime/BoxesRunTime.boxToInteger:(I)Ljava/lang/Integer;
   32:   invokevirtual  #78; //Method scala/Predef$.println:(Ljava/lang/Object;)V
   35:   getstatic   #84; //Field scala/runtime/BoxedUnit.UNIT:Lscala/runtime/BoxedUnit;
   38:   astore   5
   40:   goto  136
   43:   aload_2
   44:   instanceof  #86; //class Case2
   47:   ifeq  84
   50:   aload_2
   51:   checkcast   #86; //class Case2
   54:   astore   6
   56:   aload 6
   58:   ifnull   84
   61:   aload 6
   63:   invokevirtual  #90; //Method Case2.str:()Ljava/lang/String;
   66:   astore   7
   68:   getstatic   #68; //Field scala/Predef$.MODULE$:Lscala/Predef$;
   71:   aload 7
   73:   invokevirtual  #78; //Method scala/Predef$.println:(Ljava/lang/Object;)V
   76:   getstatic   #84; //Field scala/runtime/BoxedUnit.UNIT:Lscala/runtime/BoxedUnit;
   79:   astore   5
   81:   goto  136
   84:   getstatic   #95; //Field Case3$.MODULE$:LCase3$;
   87:   aload_2
   88:   invokevirtual  #99; //Method Case3$.unapply:(LSuper;)Lscala/Option;
   91:   astore   8
   93:   aload 8
   95:   invokevirtual  #105; //Method scala/Option.isEmpty:()Z
   98:   ifeq  110
   101:  new   #107; //class scala/MatchError
   104:  dup
   105:  aload_2
   106:  invokespecial  #109; //Method scala/MatchError."<init>":(Ljava/lang/Object;)V
   109:  athrow
   110:  aload 8
   112:  invokevirtual  #113; //Method scala/Option.get:()Ljava/lang/Object;
   115:  invokestatic   #117; //Method scala/runtime/BoxesRunTime.unboxToInt:(Ljava/lang/Object;)I
   118:  istore   9
   120:  getstatic   #68; //Field scala/Predef$.MODULE$:Lscala/Predef$;
   123:  iload 9
   125:  invokestatic   #74; //Method scala/runtime/BoxesRunTime.boxToInteger:(I)Ljava/lang/Integer;
   128:  invokevirtual  #78; //Method scala/Predef$.println:(Ljava/lang/Object;)V
   131:  getstatic   #84; //Field scala/runtime/BoxedUnit.UNIT:Lscala/runtime/BoxedUnit;
   134:  astore   5
   136:  return

  LineNumberTable: 
   line 14: 0
   line 15: 2
   line 16: 24
   line 20: 40
   line 17: 43
   line 18: 68
   line 20: 81
   line 19: 84
   line 14: 87
   line 19: 88
   line 14: 101
   line 19: 118
   line 20: 120
   line 14: 136

The interesting bits are at offset 84 and 87 and further down.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-6288?orig=1
Reporter: @dragos
Affected Versions: 2.10.0

[scabug]

@dragos said:
Related IDE ticket:

https://www.assembla.com/spaces/scala-ide/tickets/1001300

[scabug]

@retronym said:
I've also seen the debugger jump to the default case after the chosen case finished executing, which is really confusing. I'll take a look at both issues.

[scabug]

@retronym said (edited on Dec 11, 2012 11:02:13 AM UTC):

cat sandbox/t6288.scala && squalac -Xprint-pos -Xprint:patmat sandbox/t6288.scala 
object Case3 {
  def unapply(z: Any): Option[Int] = Some(-1)
}

object Test extends App {
  def test(x: Any) {
    x match {
      case Case3(nr) => ()
    }
  }

  test(0)
}
[[syntax trees at end of                    patmat]] // t6288.scala
[7]package [7]<empty> {
  [7]object Case3 extends [13][64]scala.AnyRef {
    [64]def <init>(): [13]Case3.type = [64]{
      [64][64][64]Case3.super.<init>();
      [13]()
    };
    [21]def unapply([29]z: [32]<type: [32]scala.Any>): [21]Option[Int] = [56][52][52]scala.Some.apply[[52]Int]([58]-1)
  };
  [71]object Test extends [76][84]Object with [84]<type: [84]scala.App> {
    [84]def <init>(): [76]Test.type = [84]{
      [84][84][84]Test.super.<init>();
      [76]()
    };
    [96]def test([101]x: [104]<type: [104]scala.Any>): [96]Unit = [115]{
      [115]case <synthetic> val x1: [115]Any = [115]x;
      [115]case5()[136]{
        [136]<synthetic> val o7: [136]Option[Int] = [136][136]Case3.unapply([115]x1);
        [136]if ([136]o7.isEmpty.unary_!)
          [136]{
            [142]val nr: [142]Int = [115]o7.get;
            [149][149]matchEnd4([149]())
          }
        else
          [136][136]case6()
      };
      [115]case6(){
        [115][115]matchEnd4([115]throw [115][115][115]new [115]MatchError([115]x1))
      };
      [115]matchEnd4(x: [NoPosition]Unit){
        [115]x
      }
    };
    [169][165][165]Test.this.test([170]0)
  }
}

Looks like dereferencing the scrutinee is wrongly positioned: {noformat}[136][136]Case3.unapply([115]x1);{noformat}

[scabug]

@retronym said:
This fixes the reported problem: retronym/scala@scala:2.10.x...retronym:ticket/6288

I'll see if I can find any others.

[scabug]

@retronym said:
scala/scala#1754