languageserver/editor become unresponsive when .Rprofile prints to stdout

I recently got a report from the Sublime Text extension LSP-R about a user cannot get the server running.

After some investigation, it is caused by a line in his .Rprofile which writes to stdout. The exact line is not important, a reproducible example could be just

# put in .Rprofile
cat("Welcome to R\n")

If stdio interface is used (which is the case of Vim, Emacs and Sublime Text users), this unexpected line breaks the communication between R and the editor. Vscode users are luckier because socket connection is used in default.

Possible solution: we could add the --vanilla flag in our documentation, but it will break some users if they rely on anything in .Rprofile.

Start by tracing how the R language server is launched over stdio when .Rprofile emits output, and review the documentation around --vanilla. Confirm the expected behavior for Vim, Emacs, and Sublime Text clients, then verify that startup output no longer breaks editor communication without assuming users can lose .Rprofile settings.