Properly handle the return value of promote_files_registered_in_last_run

Signed-off-by: Ambre Austen Suhamy <[email protected]>

Author: ElectreAAS

src/dune_rpc_impl/server.ml

@@ -369,10 +369,15 @@ let handler (t : _ t Fdecl.t) handle : 'build_arg Dune_rpc_server.Handler.t =
       (* A 'successful' formatting means there is nothing to promote. *)
       | Success -> ()
       | Failure ->
-        let _is_necessarily_empty =
+        let missing =
           Promote.Diff_promotion.promote_files_registered_in_last_run
             Dune_rpc.Files_to_promote.All
         in
+        if List.is_non_empty missing
+        then
+          Code_error.raise
+            "promote_files_registered_in_last_run All should always return an empty list"
+            [];
         ()
     in
     Handler.implement_request rpc Procedures.Public.format f
@@ -447,7 +452,7 @@ let handler (t : _ t Fdecl.t) handle : 'build_arg Dune_rpc_server.Handler.t =
   let () =
     let f _ path =
       let files = For_handlers.source_path_of_string path in
-      let _ignored =
+      let _ignored : Path.Source.t list =
         Promote.Diff_promotion.promote_files_registered_in_last_run (These [ files ])
       in
       Fiber.return ()