`//> using option "--explain"` does nothing

[Kordyjan]

Version(s)
0.1.12

Describe the bug
//> using option "--explain" doesn't change the compilation output. Probably it has the same root cause as #1023. The problem goes away if we run scala-cli with --server=false.

To Reproduce
explain.scala:

//> using option "--explain"

class A

val i: Int = A()

Run scala-cli compile explain.scala. The output will be:

[error] ./explain.scala:5:14: Found:    A
[error] Required: Int
[error] val i: Int = A()
[error]              ^^^
Error compiling project (Scala 3.1.3, JVM)
Compilation failed

Expected behaviour

The output should look like this:

-- [E007] Type Mismatch Error: /Users/pmarks/projs/rep/explain.scala:5:14 ------
5 |val i: Int = A()
  |             ^^^
  |             Found:    A
  |             Required: Int
  |-----------------------------------------------------------------------------
  | Explanation (enabled by `-explain`)
  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
  |
  | Tree: new A()
  | I tried to show that
  |   A
  | conforms to
  |   Int
  | but the comparison trace ended with `false`:
  |
  |   ==> A  <:  Int
  |   <== A  <:  Int = false
  |
  | The tests were made under the empty constraint
   -----------------------------------------------------------------------------

[lwronski]

@Kordyjan Thanks for reporting. I want to summarize this issue.

First of all, the explain message which you mention is generated here in scalac and set to a field rendered in Problem class.

From the bloop side, the rendered message are available and ready to use but it is skipped. In BSP there is no reasonable field in Diagnostic class to send rendered message. The rendered content isn't so important to the IDE and is probably for this reason the message isn't sent anywhere from bloop.

In scala-cli which uses bloop under the hood, the rendered message isn't available because bloop doesn't send this message.

There are at least two solutions of this problem:

• modification of the BSP protocol to send rendered message to scala-cli
• if the user passes the explain option to scala-cli, the scalac compiler will be automatically used, i.e. the --server=false flag will be set

[tgodzik]

It's also a bit weird that explain message is only available in the rendered message and not in for example usual message field. This doesn't allow tools to sensibly use that additional message and render in their own style. It's either you use render or you don't get it.

I would be better if this was added to some other field or as additional diagnostics. Not sure currently what would be the best solution, but it fits into the whole structured errors and actionable diagnostics debacle. Ideally explain message would be a field that is always available without running a second compilation or modifying the whole build definition.