Assignment as the final statement in a block emits an additional debug line/breakpoint

[vasilmkd]

This ticket is the Scala 3 equivalent of scala/bug#12835.

Compiler version

Scala 3.3.0 (and also in Scala 2.12.18 and Scala 2.13.11)

Minimized code

@main
def main(): Unit = {
  val x = 123
}

Output

Compiled from "main.scala"
public final class main$package$ implements java.io.Serializable {
  public static final main$package$ MODULE$;

  private main$package$();
    Code:
       0: aload_0
       1: invokespecial #13                 // Method java/lang/Object."<init>":()V
       4: return
    LineNumberTable:
      line 1: 0
    LocalVariableTable:
      Start  Length  Slot  Name   Signature
          0       5     0  this   Lmain$package$;

  public static {};
    Code:
       0: new           #2                  // class main$package$
       3: dup
       4: invokespecial #16                 // Method "<init>":()V
       7: putstatic     #18                 // Field MODULE$:Lmain$package$;
      10: return
    LineNumberTable:
      line 1: 0

  private java.lang.Object writeReplace();
    Code:
       0: new           #22                 // class scala/runtime/ModuleSerializationProxy
       3: dup
       4: ldc           #2                  // class main$package$
       6: invokespecial #25                 // Method scala/runtime/ModuleSerializationProxy."<init>":(Ljava/lang/Class;)V
       9: areturn
    LineNumberTable:
      line 1: 0
    LocalVariableTable:
      Start  Length  Slot  Name   Signature
          0      10     0  this   Lmain$package$;

  public void main();
    Code:
       0: bipush        123
       2: istore_1
       3: return
    LineNumberTable:
      line 2: 0                <------------ this line is also possibly not necessary (not visible in the next bytecode dump below)
      line 3: 0
      line 4: 3                 <----------- this line is not necessary, line 4 is a closing bracket
    LocalVariableTable:
      Start  Length  Slot  Name   Signature
          3       1     1     x   I
          0       4     0  this   Lmain$package$;
}

Here's how this line manifests in the debugger. The behavior is the same in Metals.

Screen.Recording.2023-08-01.at.12.12.47.mov

Expectation

The debug information line 4 is not generated. For comparison, if we omit the curly braces and use the braceless syntax:

@main
def main(): Unit =
  val x = 123

the same debug line is not generated.

Compiled from "main.scala"
public final class main$package$ implements java.io.Serializable {
  public static final main$package$ MODULE$;

  private main$package$();
    Code:
       0: aload_0
       1: invokespecial #13                 // Method java/lang/Object."<init>":()V
       4: return
    LineNumberTable:
      line 1: 0
    LocalVariableTable:
      Start  Length  Slot  Name   Signature
          0       5     0  this   Lmain$package$;

  public static {};
    Code:
       0: new           #2                  // class main$package$
       3: dup
       4: invokespecial #16                 // Method "<init>":()V
       7: putstatic     #18                 // Field MODULE$:Lmain$package$;
      10: return
    LineNumberTable:
      line 1: 0

  private java.lang.Object writeReplace();
    Code:
       0: new           #22                 // class scala/runtime/ModuleSerializationProxy
       3: dup
       4: ldc           #2                  // class main$package$
       6: invokespecial #25                 // Method scala/runtime/ModuleSerializationProxy."<init>":(Ljava/lang/Class;)V
       9: areturn
    LineNumberTable:
      line 1: 0
    LocalVariableTable:
      Start  Length  Slot  Name   Signature
          0      10     0  this   Lmain$package$;

  public void main();
    Code:
       0: bipush        123
       2: istore_1
       3: return
    LineNumberTable:
      line 3: 0                             <--------------- only 1 debug line, both the line above and below were not generated
    LocalVariableTable:
      Start  Length  Slot  Name   Signature
          3       1     1     x   I
          0       4     0  this   Lmain$package$;
}

[som-snytt]

Relatedly #17425