Fix fatal error for external with @as (#7978)

* Fix fatal error for external with @as

* Add a synthetic unit argument when all external parameters collapse to constants

* Simplify

* CHANGELOG

Author: cknitt

CHANGELOG.md

@@ -21,6 +21,7 @@
 #### :bug: Bug fix
 
 - Fix @directive on function level with async and multiple parameters. https://github.com/rescript-lang/rescript/pull/7977
+- Fix fatal error for external with @as. https://github.com/rescript-lang/rescript/pull/7978
 
 #### :memo: Documentation
 

compiler/frontend/ast_external_process.ml

@@ -997,6 +997,24 @@ let handle_attributes (loc : Bs_loc.t) (type_annotation : Parsetree.core_type)
             new_arg_types,
             if arg_type = Ignore then i else i + 1 ))
     in
+    (* If every original argument was erased (e.g. all `@as(json ...) _`),
+       keep the external binding callable by threading a final `unit`
+       parameter through the type and arg specs. *)
+    let args, arg_type_specs =
+      match (args, arg_type_specs_length) with
+      | [], n when n > 0 ->
+        let unit_type =
+          Ast_helper.Typ.constr ~loc
+            (Location.mkloc (Longident.Lident "unit") loc)
+            []
+        in
+        let unit_arg = {Parsetree.attrs = []; lbl = Nolabel; typ = unit_type} in
+        ( [unit_arg],
+          arg_type_specs
+          @ [{External_arg_spec.arg_label = Arg_empty; arg_type = Extern_unit}]
+        )
+      | _ -> (args, arg_type_specs)
+    in
     let ffi : External_ffi_types.external_spec =
       external_desc_of_non_obj loc external_desc prim_name_with_source
         arg_type_specs_length arg_types_ty arg_type_specs
@@ -1008,8 +1026,7 @@ let handle_attributes (loc : Bs_loc.t) (type_annotation : Parsetree.core_type)
     let return_wrapper =
       check_return_wrapper loc external_desc.return_wrapper result_type
     in
-    let fn_type = Ast_helper.Typ.arrows ~loc args result_type in
-    ( build_uncurried_type ~arity:(List.length args) fn_type,
+    ( Ast_helper.Typ.arrows ~loc args result_type,
       External_ffi_types.ffi_bs arg_type_specs return_wrapper ffi,
       unused_attrs,
       relative )

tests/tests/src/AsInUncurriedExternals.mjs

@@ -19,9 +19,12 @@ function shouldNotFail(objectMode, name) {
   return 3;
 }
 
+let x = somescope.somefn({foo:true});
+
 export {
   mo,
   options,
   shouldNotFail,
+  x,
 }
-/* No side effect */
+/* x Not a pure module */

tests/tests/src/AsInUncurriedExternals.res

@@ -13,3 +13,8 @@ let mo = makeOptions
 let options = mo(~name="foo", ())
 
 let shouldNotFail: (~objectMode: _, ~name: string) => int = (~objectMode, ~name) => 3
+
+@scope("somescope")
+external constantArgOnly: @as(json`{foo:true}`) _ => string = "somefn"
+
+let x = constantArgOnly()