Warn about an @as that does not name a record field

Every @as on a record field was marked used, which made sense when the
attribute was what the compiler read. Now the one that names the field
is taken out of the attributes when the field is built, so an @as still
there is either a second one, which the type checker rejects, or a
payload that is not a name at all.

@as(42) on a record field was accepted in silence: it renamed nothing
and nothing reported it. Only the attribute the compiler acts on is
marked now, so that case warns and a duplicate still reports just its
own error rather than both.

Signed-off-by: Cristiano Calcagno <ccrisccris@gmail.com>

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W8g8qwBARAcvW9MyuKQq8H

Author: cristianoc

CHANGELOG.md

@@ -39,6 +39,7 @@
 - Fix the side-effect analysis treating bigint exponentiation and bounds-checked array and string reads as pure, which let dead-code elimination drop an unused one that throws: `let _ = 2n ** -1n` no longer raised. https://github.com/rescript-lang/rescript/pull/8617
 - Fix excessive parentheses and indentation in function assignments to refs, align record and array assignment formatting across refs and fields, and preserve function return-type parentheses and consistent JSX fragment layout in callbacks. https://github.com/rescript-lang/rescript/pull/8611
 - Report an error instead of crashing when an integer in a variant constructor's `@as` annotation exceeds the compiler's integer range. https://github.com/rescript-lang/rescript/pull/8619
+- Warn about an `@as` on a record field whose payload does not name the field, such as `@as(42)`. It renamed nothing and was silently accepted. https://github.com/rescript-lang/rescript/pull/8619
 - Fix a recursive module with an empty signature discarding its right-hand side. Lambda-to-Lam conversion rewrote `Pupdate_mod` to unit when the module's shape had no fields, dropping the primitive's arguments - one of which is the right-hand side - so `module rec M: {} = { let () = Console.log("effect") }` emitted nothing for `M`. The elision now happens where the bindings are produced, with the right-hand side still in hand. https://github.com/rescript-lang/rescript/pull/8608
 - Fix a compiler crash on a polymorphic variant whose numeric name exceeds the `int32` range. `#99999999999("a")` and the same name in a pattern failed with `Failure("Int32.of_string")` and no location, because the range check ran in the frontend AST pass and matched only payload-free expressions. It now runs in `Typecore`, next to the integer literal decoding whose overflow error it mirrors, and covers both label positions. A bare `type t = [#99999999999]` still compiles, since nothing decodes a row field name. https://github.com/rescript-lang/rescript/pull/8608
 - Object typing errors now describe fields directly: assigning to a field without `@set` reports that the field is not settable and suggests the annotation, and missing-property errors name the field instead of a phantom `"x#="` member. https://github.com/rescript-lang/rescript/pull/8597

compiler/frontend/bs_ast_invariant.ml

@@ -94,10 +94,17 @@ let emit_external_warnings : iterator =
         | _ -> super.expr self a);
     label_declaration =
       (fun self lbl ->
-        Ext_list.iter lbl.pld_attributes (fun attr ->
-            match attr with
-            | {txt = "as"}, _ -> Used_attributes.mark_used_attribute attr
-            | _ -> ());
+        (* A field's [@as] is taken out of its attributes when it names the
+           field, so one still here is either a second one, which the type
+           checker rejects, or a payload that is not a name. The first is
+           reported already; the second is reported by nothing else, so let it
+           warn as the unused attribute it is. *)
+        Ext_list.iter lbl.pld_attributes
+          (fun (({txt}, payload) as attr : Parsetree.attribute) ->
+            if
+              txt = "as"
+              && Ast_payload.string_literal_of_payload payload <> None
+            then Used_attributes.mark_used_attribute attr);
         super.label_declaration self lbl);
     constructor_declaration =
       (fun self ({pcd_name = {txt; loc}} as ctr) ->

tests/build_tests/super_errors/expected/record_field_as_not_a_name.res.expected

@@ -0,0 +1,13 @@
+
+  Warning number 101 (configured as error) 
+  /.../fixtures/record_field_as_not_a_name.res:3:11-13
+
+  1 │ /* An @as that does not name the field renames nothing. Nothing else rep
+    │ orts
+  2 │  it, so it warns as the unused attribute it is. */
+  3 │ type t = {@as(42) a: int}
+  4 │ 
+
+  Unused attribute: @as
+This attribute has no effect here.
+For example, some attributes are only meaningful in externals.

tests/build_tests/super_errors/fixtures/record_field_as_not_a_name.res

@@ -0,0 +1,3 @@
+/* An @as that does not name the field renames nothing. Nothing else reports
+ it, so it warns as the unused attribute it is. */
+type t = {@as(42) a: int}