feature: Add the possibility to discover the main class to run

[tgodzik]

Previously, the only possibility to discover the main class to run was to try and run it. Now, we can also ask for it so that client editor can use that shell command.

Needed for scalameta/metals-vscode#1298

[ckipp01]

Maybe I'm missing the context, but reading over the VS Code pr it wasn't clear. Isn't this essentially the same as

[Trace - 08:58:37 am] Received request 'workspace/executeCommand - (21)'
Params: {
  "command": "metals.debug-adapter-start",
  "arguments": [
    {
      "path": "file:///Users/ckipp/Documents/scala-workspace/minimal/minimal3/src/Main.scala",
      "runType": "runOrTestFile"
    }
  ]
}

Which already works to discover mains int he current file. I find this a bit confusing because now we have multiple different ways to discover main classes. Couldn't the existing runtType here with debug-adapter-start be re-utilized to do this same thing? If I'm understanding the usecase wouldn't adding a new runType of runTarget accomplish this?

[tgodzik]

The issue is that the way we previously did this is by starting the session itself within Metals. Here we just want to get the command to run on the client. So we could potentially add a parameter (returnShellCommand: true or something along these lines), but then this flag would make the behaviour widely different. One would start app and return a address to connect to and the other would return the session object. I think we shouldn't to it in the same command

[tgodzik]

Also, this only adds a single new command, while reusing all the same mechanism, getting to work it with a flag might end up with considerable more effort.

[ckipp01]

Ahh ok ok, make sense. Took some more time to really understand it. Thanks for explaining!

[ckipp01]

Alright so just one more nit. Do you think we can also mention this in https://scalameta.org/metals/docs/integrations/debug-adapter-protocol and talk a bit about the intended flow. Mainly, in what situation would a client want to use the way that I questions about before using runType vs when would they use this? Basically outline when and why a client would want to implement this command.

[tgodzik]

Alright so just one more nit. Do you think we can also mention this in https://scalameta.org/metals/docs/integrations/debug-adapter-protocol and talk a bit about the intended flow. Mainly, in what situation would a client want to use the way that I questions about before using runType vs when would they use this? Basically outline when and why a client would want to implement this command.

Makes sense, I only mentioned shellCommand before, which is just very basic information.

[tgodzik]

Alright so just one more nit. Do you think we can also mention this in https://scalameta.org/metals/docs/integrations/debug-adapter-protocol and talk a bit about the intended flow. Mainly, in what situation would a client want to use the way that I questions about before using runType vs when would they use this? Basically outline when and why a client would want to implement this command.

I added some more explanation, let me know what you think! It's mostly an additional command that you can run from the client if you want to query metals. It's hopefully not very complicated.

[ckipp01]

Thanks for the extra docs! LGTM!