[New feature] A rule to check about calling close_in/out after open_in/out

[david-maison]

A new idea for a rule: is a possible to check if the function close_in (resp. close_out) is always called for each opened in_channel (resp. out_channel) before the end of its lifetime?

We can assume that it should be done in the same scope as the open_in (resp. open_out).

Examples:

let f name =
  try
    let out = open_out name in
    let text = compute_something () in
    output_string out text;
    close_out out
  with _ ->
    Format.eprintf "Error@."

Should warn about not closing out if an exception is raised between the open and close.

let f name =
  let out = open_out name in
  try
    let text = compute_something () in
    output_string out text;
    close_out out
  with _ ->
    Format.eprintf "Error@.";
    close_out out

Should be ok.

let f name =
  try
    let out = open_out name in
    compute_something out
  with _ ->
    Format.eprintf "Error@."

Should warn even if out is always closed in compute_something.

What do you think about this?
Comments (even negative ones!) and suggestions are welcome.