Improve error when using '@deriving(accessors)` on a variant with a record arg (rescript-lang#6712)

* Improve error when using '@deriving(accessors)` on a variant with a record arg

* Refactor raise_unsuppored_variant_record_arg function

Author: JonoPrest

CHANGELOG.md

@@ -12,6 +12,10 @@
 
 # 11.1.0-rc.8 (Unreleased)
 
+#### :bug: Bug Fix
+
+- Improve error when using '@deriving(accessors)' on a variant with record arguments. https://github.com/rescript-lang/rescript-compiler/pull/6712
+
 # 11.1.0-rc.7
 
 #### :bug: Bug Fix

jscomp/build_tests/super_errors/expected/DerivingAccessorsRecordParam.res.expected

@@ -0,0 +1,9 @@
+
+  We've found a bug for you!
+  /.../fixtures/DerivingAccessorsRecordParam.res:2:10-25
+
+  1 │ @deriving(accessors)
+  2 │ type t = Struct({a: int})
+  3 │ 
+
+  @deriving(accessors) from a variant record argument is unsupported. Either define the record type separately from the variant type or use a positional argument.

jscomp/build_tests/super_errors/fixtures/DerivingAccessorsRecordParam.res

@@ -0,0 +1,2 @@
+@deriving(accessors)
+type t = Struct({a: int})

jscomp/frontend/ast_derive_projector.ml

@@ -4,6 +4,12 @@ let invalid_config (config : Parsetree.expression) =
   Location.raise_errorf ~loc:config.pexp_loc
     "such configuration is not supported"
 
+let raise_unsupported_vaiant_record_arg loc =
+  Location.raise_errorf ~loc
+    "@deriving(accessors) from a variant record argument is unsupported. \
+     Either define the record type separately from the variant type or use a \
+     positional argument."
+
 type tdcls = Parsetree.type_declaration list
 
 let derivingName = "accessors"
@@ -55,15 +61,16 @@ let init () =
                     {
                       pcd_name = {loc; txt = con_name};
                       pcd_args;
-                      pcd_loc = _;
+                      pcd_loc;
                       pcd_res;
                     }
                   ->
                     (* TODO: add type annotations *)
                     let pcd_args =
                       match pcd_args with
                       | Pcstr_tuple pcd_args -> pcd_args
-                      | Pcstr_record _ -> assert false
+                      | Pcstr_record _ ->
+                        raise_unsupported_vaiant_record_arg pcd_loc
                     in
                     let little_con_name =
                       Ext_string.uncapitalize_ascii con_name
@@ -146,14 +153,15 @@ let init () =
                     {
                       pcd_name = {loc; txt = con_name};
                       pcd_args;
-                      pcd_loc = _;
+                      pcd_loc;
                       pcd_res;
                     }
                   ->
                     let pcd_args =
                       match pcd_args with
                       | Pcstr_tuple pcd_args -> pcd_args
-                      | Pcstr_record _ -> assert false
+                      | Pcstr_record _ ->
+                        raise_unsupported_vaiant_record_arg pcd_loc
                     in
                     let arity = pcd_args |> List.length in
                     let annotate_type =