Request feature to disable coverage check of the pattern part of or-patterns

[mbarbin]

I have noticed somewhat of a tension between a specific style of spelling out every cases in a pattern matching (let's call it loosely "defensive programming" for a minute) on one hand and reaching exhaustive coverage on the other.

Imagine you have a type

type t = A | B | C

And a particular function that distinguishes the A case:

let is_A : t -> bool = function
| A -> true
| _ -> false

Under the "defensive programming" style, you'd almost never make use of the patany constructor and write this instead:

let is_A = function
| A -> true
| (B | C) -> false

with the aim to be called upon to reconsider at that call site if t ever changes.

Now, currently with bisect_ppx this forces you to exercise all the cases B and C in your test suite if you want to fully cover this function. Sometimes, this is exactly what you want. But sometimes, not. In particular I have cases where I am happy to test for the A case, and 1 of the remaining one being enough.

Note however that I do not want to disable the coverage test for the right-hand side!

let is_A = function
| A -> true
| (B|C) -> false [@coverage off]

Nope!

In the test suite unary.t you can see an example of instrumentation for or-patterns:

   let _ =
     match Some `A with
     | Some (`A | `B) -> print_endline "foo"
     | None -> ()

Instrumented as:

  let _ =
    match Some `A with
    | Some (`A | `B) as ___bisect_matched_value___ ->
        (match[@ocaml.warning "-4-8-9-11-26-27-28-33"]
           ___bisect_matched_value___
         with
        | Some `A ->
            ___bisect_visit___ 1;
            ()
        | Some `B ->
            ___bisect_visit___ 2;
            ()
        | _ -> ());
        ___bisect_post_visit___ 0 (print_endline "foo")
    | None ->
        ___bisect_visit___ 3;
        ()

I would like to request something like a special directive that would cause it to be that instead:

  let _ =
    match Some `A with
    | Some (`A | `B) ->
        ___bisect_post_visit___ 0 (print_endline "foo")
    | None ->
        ___bisect_visit___ 3;
        ()

(Essentially, not generate the extra piece of code that makes sure individual patterns are visited).

I am opening this issue to get a feel for whether that desire is shared by others, and if there are ideas on what the directive or ways to support this should look like.

Thanks!