[SAOC 2025] Improving Error Messages and LSP Integration Weekly Update #5

[Iskaban10]

## [SAOC 2025] Improve error messages and LSP integration Weekly 
Update #5

This week I continued working on the LSP-style printing of 
diagnostics in dmd. I also made certain changes to the [#PR 
21979](https://github.com/dlang/dmd/pull/21979).

## What I worked on

- **Added compiler flag "-verror-style=lsp" in dmd**
     Made changes in mars.d and cli.d to add a new compiler flag 
'lsp' to -verror-style.

- **Removed the ErrosinkLSP and shifted printLSPdiagnostic() in 
errors.d**
     Creating a new ErrorSink for LSP diagnostics was causing 
unneccessary complications. A simpler approach was to create a 
printLSPDiagnostic() function in errors.d and call it evrytime 
instead of printDiagnostic() if the lsp flag was activated (i.e. 
global.params.v.messageStyle == MessageStyle.lsp).

## Challenges

  - While the diagnostics are being printed in this format
  ```json
  {
     "severity":"Error",
     "filename":"file.d",
     "line":37,
     "column": 10,
     "message": "missing semicolon ;"
  }
  ```
  The challenge I am facing is dmd currently calls the verror and 
verrorSupplemental functions separately i.e. the supplemental 
message for any diagnostic is called separately after and not 
along with the diagnostic. Since several error messages do not 
call for a supplemental message, understanding whether to close 
the JSON array and print the next diagnostic, or to wait for the 
printing of the supplemental message, is difficult.
```json
  {
     "severity":"Error",
     "filename":"file.d",
     "line":37,
     "column": 10,
     "message": "missing semicolon ;",
     "note":"may or may not be printed"
  }
  ```

## Next Steps
- Create a fail-compilation test for lsp.
- Work on the issue of printing of supplemental messages.