map begin fun on one line, second attempt

[EmileTrotignon]

Same goal as #2675 , but with a cleaner implementation that uses pro instead.

The box structure is not ideal, because fmt_function cannot open a box in the middle of pro, but the only case where this is an issue is the following :

let _ =
  lazy begin fun
         xxxxxxxxxxxxxxxxxxxxxxx
         yyyyyyyyyyyyyyyyyyy
         zzzzzzzzzzzzzzzzzzzzzzzz
       ->
    print_endline xxxxxxxxx ;
    f xxxxxxxxxx yyyyyyyyyyyyy zzzzzzzzzzzzzzzzzzzzz
  end

[EmileTrotignon]

I will merge to check interaction with #2694

[Julow]

I'm a bit surprised that the -> and the body don't have the same indentation baseline. The -> seems indented relative to the column where the begin keyword starts and not relative to a naturally incrementing indent.
Is it intended ?

I'd expect the -> and the argument list to be indented 2 and 4 relative to the body of the fun:

  lazy begin fun
        argument list
      ->
    body
  end

[EmileTrotignon]

This is not intended. What I would have liked is the following:

lazy begin fun
    argument list
  ->
    body
  end

This seemed like a small issue in a very rare situation though, and it might be difficult to fix because everything is in pro, and context does not tell you you were lazy when formatting the function

[Julow]

The pro includes the lazy keyword ? This seems to be the right way to make the formatting you want. The box around begin ... -> is unwanted and the forced break after the fun was not the right thing to do.