hint about when appropriate

Author: zth

compiler/ml/parmatch.ml

@@ -2013,7 +2013,7 @@ let ppat_of_type env ty =
     (Conv.mkpat Parsetree.Ppat_any, Hashtbl.create 0, Hashtbl.create 0)
   | pats -> Conv.conv (orify_many pats)
 
-let do_check_partial ?pred exhaust loc casel pss =
+let do_check_partial ?partial_match_warning_hint ?pred exhaust loc casel pss =
   match pss with
   | [] ->
     (*
@@ -2071,6 +2071,11 @@ let do_check_partial ?pred exhaust loc casel pss =
                     Matching over values of extensible variant types (the \
                     *extension* above)\n\
                     must include a wild card pattern in order to be exhaustive.";
+               (match partial_match_warning_hint with
+               | None -> ()
+               | Some h when String.length h > 0 ->
+                 Buffer.add_string buf ("\n\n  " ^ h)
+               | Some _ -> ());
                Buffer.contents buf
              with _ -> ""
            in
@@ -2083,8 +2088,8 @@ let do_check_partial_normal loc casel pss =
   do_check_partial exhaust loc casel pss
  *)
 
-let do_check_partial_gadt pred loc casel pss =
-  do_check_partial ~pred exhaust_gadt loc casel pss
+let do_check_partial_gadt ?partial_match_warning_hint pred loc casel pss =
+  do_check_partial ?partial_match_warning_hint ~pred exhaust_gadt loc casel pss
 
 (*****************)
 (* Fragile check *)
@@ -2265,9 +2270,9 @@ let check_partial_param do_check_partial do_check_fragile loc casel =
       do_check_partial_normal
       do_check_fragile_normal*)
 
-let check_partial_gadt pred loc casel =
+let check_partial_gadt ?partial_match_warning_hint pred loc casel =
   check_partial_param
-    (do_check_partial_gadt pred)
+    (do_check_partial_gadt ?partial_match_warning_hint pred)
     do_check_fragile_gadt loc casel
 
 (*************************************)

compiler/ml/parmatch.mli

@@ -69,6 +69,7 @@ val ppat_of_type :
 
 val pressure_variants : Env.t -> pattern list -> unit
 val check_partial_gadt :
+  ?partial_match_warning_hint:string ->
   ((string, constructor_description) Hashtbl.t ->
   (string, label_description) Hashtbl.t ->
   Parsetree.pattern ->

compiler/ml/typecore.ml

@@ -1722,13 +1722,14 @@ let partial_pred ~lev ?mode ?explode env expected_ty constrs labels p =
     set_state state env;
     None
 
-let check_partial ?(lev = get_current_level ()) env expected_ty loc cases =
+let check_partial ?(lev = get_current_level ()) ?partial_match_warning_hint env
+    expected_ty loc cases =
   let explode =
     match cases with
     | [_] -> 5
     | _ -> 0
   in
-  Parmatch.check_partial_gadt
+  Parmatch.check_partial_gadt ?partial_match_warning_hint
     (partial_pred ~lev ~explode env expected_ty)
     loc cases
 
@@ -4179,7 +4180,24 @@ and type_let ~context ?(check = fun s -> Warnings.Unused_var s)
   List.iter2
     (fun pat (attrs, exp) ->
       Builtin_attributes.warning_scope ~ppwarning:false attrs (fun () ->
-          ignore (check_partial env pat.pat_type pat.pat_loc [case pat exp])))
+          let partial_match_warning_hint =
+            if Experimental_features.is_enabled Experimental_features.LetUnwrap
+            then
+              let ty = repr (Ctype.expand_head env pat.pat_type) in
+              match ty.desc with
+              | Tconstr (path, _, _)
+                when Path.same path Predef.path_option
+                     || Path.same path Predef.path_result ->
+                Some
+                  "Hint: You can use `let?` to automatically unwrap this \
+                   expression."
+              | _ -> None
+            else None
+          in
+          ignore
+            (check_partial ?partial_match_warning_hint env pat.pat_type
+               pat.pat_loc
+               [case pat exp])))
     pat_list
     (List.map2 (fun (attrs, _) e -> (attrs, e)) spatl exp_list);
   end_def ();

compiler/ml/typecore.mli

@@ -35,6 +35,7 @@ val type_expression :
   Typedtree.expression
 val check_partial :
   ?lev:int ->
+  ?partial_match_warning_hint:string ->
   Env.t ->
   type_expr ->
   Location.t ->

tests/build_tests/super_errors/expected/let_without_unwrap_but_elgible.res.expected

@@ -0,0 +1,14 @@
+
+  Warning number 8
+  /.../fixtures/let_without_unwrap_but_elgible.res:6:7-11
+
+  4 │ 
+  5 │ let f = {
+  6 │   let Ok(_) = x
+  7 │   Ok(1)
+  8 │ }
+
+  You forgot to handle a possible case here, for example: 
+  | Error(_)
+
+  Hint: You can use `let?` to automatically unwrap this expression.

tests/build_tests/super_errors/fixtures/let_without_unwrap_but_elgible.res

@@ -0,0 +1,8 @@
+@@config({flags: ["-enable-experimental", "LetUnwrap"]})
+
+let x = Ok(1)
+
+let f = {
+  let Ok(_) = x
+  Ok(1)
+}