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

Author: JonoPrest

CHANGELOG.md

@@ -15,6 +15,7 @@
 #### :bug: Bug Fix
 
 - Fix variance setting for builtin `dict` type. Fixes issues around inference. https://github.com/rescript-lang/rescript-compiler/pull/6707 
+- Improve error when using '@deriving(accessors)' on a variant with record arguments
 
 # 11.1.0-rc.6
 

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

@@ -55,15 +55,20 @@ 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 _ ->
+                        Location.raise_errorf ~loc:pcd_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."
                     in
                     let little_con_name =
                       Ext_string.uncapitalize_ascii con_name
@@ -146,14 +151,19 @@ 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 _ ->
+                        Location.raise_errorf ~loc:pcd_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."
                     in
                     let arity = pcd_args |> List.length in
                     let annotate_type =