Fix crash with @get on external of type unit => 'a (#7866)

* Fix crash with `@get` on external of type `unit => 'a` * fix format * CHANGELOG

Author: cknitt

CHANGELOG.md

@@ -23,6 +23,7 @@
 #### :bug: Bug fix
 
 - Fix code generation for emojis in polyvars and labels. https://github.com/rescript-lang/rescript/pull/7853
+- Fix crash with `@get` on external of type `unit => 'a`. https://github.com/rescript-lang/rescript/pull/7866
 
 #### :memo: Documentation
 

compiler/core/lam_compile_external_call.ml

@@ -421,7 +421,11 @@ let translate_ffi ?(transformed_jsx = false) (cxt : Lam_compile_context.t)
  | [obj] ->
  let obj = translate_scoped_access scopes obj in
  E.dot obj name
- | _ -> assert false
+ | _ ->
+ (* This should have been caught in the frontend validation. *)
+ invalid_arg
+ "Internal compiler error: @get external called with wrong number of \
+ arguments. Expected exactly one object argument."
  (* Note these assertion happens in call site *))
  | Js_set {js_set_name = name; js_set_scopes = scopes} -> (
  (* assert (js_splice = false) ; *)

compiler/frontend/ast_external_process.ml

@@ -899,7 +899,12 @@ let external_desc_of_non_obj (loc : Location.t) (st : external_desc)
  tagged_template = _;
  } ->
  if arg_type_specs_length = 1 then
- Js_get {js_get_name = name; js_get_scopes = scopes}
+ (* Check if the first argument is unit, which is invalid for @get *)
+ match arg_type_specs with
+ | [{arg_type = Extern_unit}] ->
+ Location.raise_errorf ~loc
+ "Ill defined attribute %@get (unit argument is not allowed)"
+ | _ -> Js_get {js_get_name = name; js_get_scopes = scopes}
  else
  Location.raise_errorf ~loc
  "Ill defined attribute %@get (only one argument)"

tests/build_tests/super_errors/expected/get_unit_arg.res.expected

@@ -0,0 +1,12 @@
+
+ We've found a bug for you!
+ /.../fixtures/get_unit_arg.res:2:1-3:36
+
+ 1 │ // Test case for issue #7676 - @get external with unit => 'a should give
+ │ proper error
+ 2 │ @get
+ 3 │ external foo: unit => string = "foo"
+ 4 │ 
+ 5 │ let x = foo()
+
+ Ill defined attribute @get (unit argument is not allowed)

tests/build_tests/super_errors/fixtures/get_unit_arg.res

@@ -0,0 +1,5 @@
+// Test case for issue #7676 - @get external with unit => 'a should give proper error
+@get
+external foo: unit => string = "foo"
+
+let x = foo()