flambda-backend: Change error message for non-value class lets (#1953)

The old way of producing this error required of_sort_for_error,
which causes trouble in the modal kinds world. This kills off
that terrible function.

Author: goldfirere

testsuite/tests/typing-layouts/basics_alpha.ml

@@ -788,8 +788,8 @@ end
 Line 3, characters 11-12:
 3 |     let VV v = v in
                ^
-Error: Variables bound in a class must have layout value.
-       v has layout void, which is not a sublayout of value.
+Error: The types of variables bound by a 'let' in a class function
+       must have layout value. Instead, v's type has layout void.
 |}];;
 
 (* Hits the Cfk_concrete case of Pcf_val *)

testsuite/tests/typing-layouts/basics_beta.ml

@@ -702,8 +702,8 @@ end
 Line 4, characters 8-9:
 4 |     let d = f u in
             ^
-Error: Variables bound in a class must have layout value.
-       d has layout float64, which is not a sublayout of value.
+Error: The types of variables bound by a 'let' in a class function
+       must have layout value. Instead, d's type has layout float64.
 |}];;
 
 (* Hits the Cfk_concrete case of Pcf_val *)

typing/jkind.ml

@@ -490,8 +490,6 @@ let of_new_sort_var ~why =
 
 let of_new_sort ~why = fst (of_new_sort_var ~why)
 
-let of_sort_for_error ~why s = fresh_jkind (Sort s) ~why:(Concrete_creation why)
-
 let of_const ~why : const -> t = function
   | Any -> fresh_jkind Any ~why
   | Immediate -> fresh_jkind Immediate ~why

typing/jkind.mli

@@ -362,11 +362,6 @@ val of_new_sort_var : why:concrete_jkind_reason -> t * sort
 (** Create a fresh sort variable, packed into a jkind. *)
 val of_new_sort : why:concrete_jkind_reason -> t
 
-(** There should not be a need to convert a sort to a jkind, but this is
-    occasionally useful for formatting error messages. Do not use in actual
-    type-checking. *)
-val of_sort_for_error : why:concrete_jkind_reason -> sort -> t
-
 val of_const : why:creation_reason -> const -> t
 
 (* CR layouts v1.5: remove legacy_immediate when the old attributes mechanism

typing/typeclass.ml

@@ -109,6 +109,7 @@ type error =
   | Closing_self_type of class_signature
   | Polymorphic_class_parameter
   | Non_value_binding of string * Jkind.Violation.t
+  | Non_value_let_binding of string * Jkind.sort
 
 exception Error of Location.t * Env.t * error
 exception Error_forward of Location.error
@@ -1397,12 +1398,9 @@ and class_expr_aux cl_num val_env met_env virt self_scope scl =
                (fun (loc, mode, sort) ->
                   Typecore.escape ~loc ~env:val_env ~reason:Other mode;
                   if not (Jkind.Sort.(equate sort value))
-                  then let viol = Jkind.Violation.of_ (Not_a_subjkind(
-                    Jkind.of_sort_for_error ~why:Let_binding sort,
-                    Jkind.value ~why:Class_let_binding))
-                    in
+                  then
                     raise (Error(loc, met_env,
-                                 Non_value_binding (Ident.name id, viol)))
+                                 Non_value_let_binding (Ident.name id, sort)))
                )
                modes_and_sorts;
              let path = Pident id in
@@ -2248,6 +2246,11 @@ let report_error env ppf = function
     fprintf ppf
       "@[Variables bound in a class must have layout value.@ %a@]"
       (Jkind.Violation.report_with_name ~name:nm) err
+  | Non_value_let_binding (nm, sort) ->
+    fprintf ppf
+      "@[The types of variables bound by a 'let' in a class function@ \
+       must have layout value. Instead, %s's type has layout %a.@]"
+      nm Jkind.Sort.format sort
 
 let report_error env ppf err =
   Printtyp.wrap_printing_env ~error:true

typing/typeclass.mli

@@ -125,6 +125,7 @@ type error =
   | Closing_self_type of class_signature
   | Polymorphic_class_parameter
   | Non_value_binding of string * Jkind.Violation.t
+  | Non_value_let_binding of string * Jkind.sort
 
 exception Error of Location.t * Env.t * error
 exception Error_forward of Location.error