Unbound placeholder parameter error in worksheets with Dotty

[ckipp01]

Describe the bug
Given the following code snippet in a worksheet with dotty...

def x = 1 -> 2
val (a, _) = x

...you'll see an error.

[mdoc] [E] scala.worksheet.sc:14 (mdoc generated code) Unbound placeholder
  parameter; incorrect use of _
     $doc.binder(_, 2, 8, 2, 9)

384

To Reproduce
Steps to reproduce the behavior:

1. Paste the code snippet above in a Dotty worksheet
2. Evaluate and see error

Expected behavior
This works fine in Scala 2 and is valid syntax afaik in Scala 3. This shouldn't give this error message.

Installation:

• Operating system: macOS
• Editor: Vim
• Metals version: 0.9.3+102-31fb9f1e-SNAPSHOT

Additional context
I'm still trying to figure out what exact placeholder syntax triggers this which doesn't some of the other types of placeholder syntaxes I've tried doesn't seem to be an issue like this one is.

cc @eloots

Search terms
placeholder syntax, dotty, worksheets

[tgodzik]

Thanks for finding this! The problem is most likely in mdoc, I am not super sure why this is, but it should be easily reproducible in the tests.

[ckipp01]

So from looking at the evaluated document in Scala 2.x we get:

EvaluatedDocument(Input.VirtualFile("placeholder.scala", "package repl
object MdocSession extends _root_.mdoc.internal.document.DocumentBuilder {
  def app(): _root_.scala.Unit = {val _ = new App()}
  class App {
$doc.startSection();
$doc.startStatement(0, 0, 0, 14);
def x = 1 -> 2
$doc.endStatement();
$doc.startStatement(1, 0, 1, 14);
val (a, _) = x; $doc.binder(a, 1, 5, 1, 6)
$doc.endStatement();
$doc.endSection();
  }
}

With Dotty it looks like so:

EvaluatedDocument(Input.VirtualFile("placeholder.scala", "package repl
object MdocSession extends _root_.mdoc.internal.document.DocumentBuilder {
  def app(): _root_.scala.Unit = {val _ = new App()}
  class App {
    $doc.startSection();
    $doc.startStatement(0, 0, 0, 14);
    def x = 1 -> 2
    $doc.endStatement();
    $doc.startStatement(1, 0, 1, 14);
    val (a, _) = x
    $doc.binder(a, 1, 5, 1, 6)
+   $doc.binder(_, 1, 8, 1, 9)
    $doc.endStatement();
    $doc.endSection();
  }
}

I'm going to make a wild guess and say that the $doc.binder(_, 1, 8, 1, 9) should just be skipped like it is in Scala 2.x. At least that's the route I'm heading and trying to figure out.

[tgodzik]

I think it should be simple to add a check here: https://github.com/scalameta/mdoc/blob/f103ee4db62a1cebcaf5265a6d771bceb842b378/mdoc/src/main/scala-3/mdoc/internal/markdown/Instrumenter.scala#L147

[tgodzik]

Do you want to take a look at it or should I try to fix it? @ckipp01

[ckipp01]

Do you want to take a look at it or should I try to fix it? @ckipp01

I'm actually looking at it right now. So I'll ping you if I get stuck 😆