error message: private extension without body (#6175)

Author: aspeddro

CHANGELOG.md

@@ -26,6 +26,7 @@
 - Fixed subtype checking for record types with "@as" attributes: The subtype relationship now takes into account the compatibility of "@as" attributes between corresponding fields, ensuring correctness in runtime representation.
  https://github.com/rescript-lang/rescript-compiler/issues/6158
 - Emit directive above header comment. https://github.com/rescript-lang/rescript-compiler/pull/6172
+- Add error message to private extension. https://github.com/rescript-lang/rescript-compiler/pull/6175
 
 #### :nail_care: Polish
 

jscomp/build_tests/super_errors/expected/private_without_body.res.expected

@@ -0,0 +1,9 @@
+
+  We've found a bug for you!
+  /.../fixtures/private_without_body.res:1:1-9
+
+  1 │ %%private
+  2 │ let a = 2
+  3 │ 
+
+  %%private extension expects a definition as its argument. Example: %%private(let a = "Hello")

jscomp/build_tests/super_errors/fixtures/private_without_body.res

@@ -0,0 +1,2 @@
+%%private
+let a = 2

jscomp/frontend/bs_builtin_ppx.ml

@@ -474,6 +474,9 @@ let rec structure_mapper (self : mapper) (stru : Ast_structure.t) =
           :: next -> (
           match payload with
           | PStr work ->
+            if List.length work = 0 then
+              Location.raise_errorf ~loc
+                {|%%%%private extension expects a definition as its argument. Example: %%%%private(let a = "Hello")|};
             aux
               (Ext_list.rev_map_append work acc (fun x ->
                    self.structure_item self x))