Local `lazy val`s wrapper variables are not named in the bytecode

Compiler version

3.2.0

Comparing to Scala 2.13.10

object main {
  def main(args: Array[String]): Unit = {
    lazy val str = "abc"
    println(str)
  }
}

The main method compiles to the following bytecode:

  public void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=2, locals=3, args_size=2
         0: new           #19                 // class scala/runtime/LazyRef
         3: dup
         4: invokespecial #20                 // Method scala/runtime/LazyRef."<init>":()V
         7: astore_2
         8: getstatic     #25                 // Field scala/Predef$.MODULE$:Lscala/Predef$;
        11: aload_2
        12: invokestatic  #29                 // Method str$1:(Lscala/runtime/LazyRef;)Ljava/lang/String;
        15: invokevirtual #33                 // Method scala/Predef$.println:(Ljava/lang/Object;)V
        18: return
      LineNumberTable:
        line 3: 0
        line 4: 8
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            8      11     2 str$lzy   Lscala/runtime/LazyRef;
            0      19     0  this   Lmain$;
            0      19     1  args   [Ljava/lang/String;
    MethodParameters:
      Name                           Flags
      args                           final

str$lzy is a scala.runtime.LazyRef local variable. This variable can be passed to the synthetic getter str$1 (signature: java.lang.String str$1(scala.runtime.LazyRef)) and the lazy value initialized.

Scala 3.2.0 bytecode

The exact same code is compiled in the following way:

  public void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: (0x0001) ACC_PUBLIC
    Code:
      stack=3, locals=3, args_size=2
         0: new           #30                 // class scala/runtime/LazyRef
         3: dup
         4: invokespecial #31                 // Method scala/runtime/LazyRef."<init>":()V
         7: astore_2
         8: getstatic     #36                 // Field scala/Predef$.MODULE$:Lscala/Predef$;
        11: aload_0
        12: aload_2
        13: invokespecial #40                 // Method str$1:(Lscala/runtime/LazyRef;)Ljava/lang/String;
        16: invokevirtual #44                 // Method scala/Predef$.println:(Ljava/lang/Object;)V
        19: return
      LineNumberTable:
        line 2: 0
        line 3: 0
        line 4: 8
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      20     0  this   Lmain$;
            0      20     1  args   [Ljava/lang/String;
    Signature: #27                          // ([Ljava/lang/String;)V
    MethodParameters:
      Name                           Flags
      args                           final

where there is no entry for the scala.runtime.LazyRef wrapper variable.

If this variable is surfaced (like in Scala 2.13), additional debugger functionality can be built on top of it. For example, IDEA is able to present lazy values like the following:

And after initializing the lazy value on demand by clicking on the initialize prompt:

Thanks.

[unkarjedy]

Related issues in IntelliJ Scala Plugin YouTrack:

• non-initialized lazy vals shows default values in variable view
• Debugger can not evaluate given instances

[vasilmkd]

The ghost account that opened this ticket is actually me, from a separate work account I had created when I started working at JetBrains. I was advised to close it and merge it with my original account.

I already said it in my original post above, but this change would be very beneficial for the Scala debugging experience. A nice UI/UX can be built on top of it, allowing users to manually initialize lazy values ahead of time. We have already built this functionality for Scala 2. Thanks in advance.