`rustc` warns when finding an unknown jobserver style

[ojeda]

rustc warns when finding an unknown jobserver style, but the Make manual seems to suggest to ignore it (since new kinds may be introduced in the future), or at least that is one possible interpretation:

If your tool does not recognize the format of the --jobserver-auth string, it should assume the jobserver is using a different style and it cannot connect.

One could also read that as "... and then warn about it", like it asks to do for the simple pipe case. But if that is the case, I am not sure why it would say "it cannot connect" since obviously one would not be able to do so, i.e. I take it to mean "ignore it".

Thus, should CannotParse from jobserver be taken as an OK case, or should we keep the current behavior?

$ echo 'fn main() {}' | MAKEFLAGS=--jobserver-auth=new rustc -
warning: failed to connect to jobserver from environment variable `MAKEFLAGS="--jobserver-auth=new"`: cannot parse jobserver environment variable value: expected `fifo:PATH` or `R,W`, found `new`
  |
  = note: the build environment is likely misconfigured

From #120515.

[petrochenkov]

I'd prefer to keep the current behavior, if some specific --jobserver-auth format actually exists and is used, then jobserver-rs should support it, and the we won't report a warning in that specific case.

[ojeda]

Yeah, though users may have updated GNU Make and not rustc. Now, that may be unlikely, and it requires a new jobserver style to begin with, but it could happen. But, in that case, I would expect users to force their GNU Make to use an older jobserver style if they are really the kind of project that cannot upgrade rustc. So I think it is OK.

Perhaps what we should do here, for this case in particular, is adding a note: or similar to the diagnostic hinting users to do precisely that (upgrade or pass an older --jobserver-style to Make).