diff --git a/compiler/bin-js_of_ocaml/cmd_arg.ml b/compiler/bin-js_of_ocaml/cmd_arg.ml
index ac0b2af78c..13367e20f0 100644
--- a/compiler/bin-js_of_ocaml/cmd_arg.ml
+++ b/compiler/bin-js_of_ocaml/cmd_arg.ml
@@ -80,6 +80,26 @@ type t =
   ; effects : Config.effects_backend
   }
 
+let set_param =
+  let doc = "Set compiler options." in
+  let all = List.map (Config.Param.all ()) ~f:(fun (x, _, _) -> x, x) in
+  let pair = Arg.(pair ~sep:'=' (enum all) string) in
+  let parser s =
+    match Arg.conv_parser pair s with
+    | Ok (k, v) -> (
+        match
+          List.find ~f:(fun (k', _, _) -> String.equal k k') (Config.Param.all ())
+        with
+        | _, _, valid -> (
+            match valid v with
+            | Ok () -> Ok (k, v)
+            | Error msg -> Error (`Msg ("Unexpected VALUE after [=], " ^ msg))))
+    | Error _ as e -> e
+  in
+  let printer = Arg.conv_printer pair in
+  let c = Arg.conv (parser, printer) in
+  Arg.(value & opt_all (list c) [] & info [ "set" ] ~docv:"PARAM=VALUE" ~doc)
+
 let wrap_with_fun_conv =
   let conv s =
     if String.equal s ""
@@ -180,14 +200,6 @@ let options =
     in
     Arg.(value & opt wrap_with_fun_conv `Iife & info [ "wrap-with-fun" ] ~doc)
   in
-  let set_param =
-    let doc = "Set compiler options." in
-    let all = List.map (Config.Param.all ()) ~f:(fun (x, _) -> x, x) in
-    Arg.(
-      value
-      & opt_all (list (pair ~sep:'=' (enum all) string)) []
-      & info [ "set" ] ~docv:"PARAM=VALUE" ~doc)
-  in
   let set_env =
     let doc = "Set environment variable statically." in
     Arg.(
@@ -502,14 +514,6 @@ let options_runtime_only =
     in
     Arg.(value & opt wrap_with_fun_conv `Iife & info [ "wrap-with-fun" ] ~doc)
   in
-  let set_param =
-    let doc = "Set compiler options." in
-    let all = List.map (Config.Param.all ()) ~f:(fun (x, _) -> x, x) in
-    Arg.(
-      value
-      & opt_all (list (pair ~sep:'=' (enum all) string)) []
-      & info [ "set" ] ~docv:"PARAM=VALUE" ~doc)
-  in
   let set_env =
     let doc = "Set environment variable statically." in
     Arg.(
diff --git a/compiler/bin-js_of_ocaml/compile.ml b/compiler/bin-js_of_ocaml/compile.ml
index 6364f88776..deb995b991 100644
--- a/compiler/bin-js_of_ocaml/compile.ml
+++ b/compiler/bin-js_of_ocaml/compile.ml
@@ -226,10 +226,11 @@ let run
   let check_debug (one : Parse_bytecode.one) =
     if Option.is_some source_map && Parse_bytecode.Debug.is_empty one.debug
     then
-      warn
-        "Warning: '--source-map' is enabled but the bytecode program was compiled with \
-         no debugging information.\n\
-         Warning: Consider passing '-g' option to ocamlc.\n\
+      Warning.warn
+        `Missing_debug_event
+        "'--source-map' is enabled but the bytecode program was compiled with no \
+         debugging information.\n\
+         Consider passing '-g' option to ocamlc.\n\
          %!"
   in
   let pseudo_fs_instr prim debug cmis =
diff --git a/compiler/bin-js_of_ocaml/js_of_ocaml.ml b/compiler/bin-js_of_ocaml/js_of_ocaml.ml
index 7d665d927a..f4203025e8 100644
--- a/compiler/bin-js_of_ocaml/js_of_ocaml.ml
+++ b/compiler/bin-js_of_ocaml/js_of_ocaml.ml
@@ -23,7 +23,7 @@ open Js_of_ocaml_compiler
 
 let () =
   Sys.catch_break true;
-  let argv = Jsoo_cmdline.normalize_argv ~warn:(warn "%s") Sys.argv in
+  let argv = Sys.argv in
   let argv =
     let like_arg x = String.length x > 0 && Char.equal x.[0] '-' in
     let like_command x =
@@ -59,11 +59,8 @@ let () =
            ])
     with
     | Ok (`Ok () | `Help | `Version) ->
-        if !warnings > 0 && !werror
-        then (
-          Format.eprintf "%s: all warnings being treated as errors@." Sys.argv.(0);
-          exit 1)
-        else exit 0
+        Warning.process_warnings ();
+        exit 0
     | Error `Term -> exit 1
     | Error `Parse -> exit Cmdliner.Cmd.Exit.cli_error
     | Error `Exn -> ()
diff --git a/compiler/bin-jsoo_minify/jsoo_minify.ml b/compiler/bin-jsoo_minify/jsoo_minify.ml
index f1640187fd..f636ab50e0 100644
--- a/compiler/bin-jsoo_minify/jsoo_minify.ml
+++ b/compiler/bin-jsoo_minify/jsoo_minify.ml
@@ -92,12 +92,7 @@ let main =
   Cmdliner.Cmd.v Cmd_arg.info t
 
 let (_ : int) =
-  try
-    Cmdliner.Cmd.eval
-      ~catch:false
-      ~argv:(Jsoo_cmdline.normalize_argv ~warn:(warn "%s") Sys.argv)
-      main
-  with
+  try Cmdliner.Cmd.eval ~catch:false ~argv:Sys.argv main with
   | (Match_failure _ | Assert_failure _ | Not_found) as exc ->
       let backtrace = Printexc.get_backtrace () in
       Format.eprintf
diff --git a/compiler/bin-wasm_of_ocaml/cmd_arg.ml b/compiler/bin-wasm_of_ocaml/cmd_arg.ml
index 1dcce5b33d..5911580f46 100644
--- a/compiler/bin-wasm_of_ocaml/cmd_arg.ml
+++ b/compiler/bin-wasm_of_ocaml/cmd_arg.ml
@@ -66,6 +66,26 @@ type t =
   ; shape_files : string list
   }
 
+let set_param =
+  let doc = "Set compiler options." in
+  let all = List.map (Config.Param.all ()) ~f:(fun (x, _, _) -> x, x) in
+  let pair = Arg.(pair ~sep:'=' (enum all) string) in
+  let parser s =
+    match Arg.conv_parser pair s with
+    | Ok (k, v) -> (
+        match
+          List.find ~f:(fun (k', _, _) -> String.equal k k') (Config.Param.all ())
+        with
+        | _, _, valid -> (
+            match valid v with
+            | Ok () -> Ok (k, v)
+            | Error msg -> Error (`Msg ("Unexpected VALUE after [=], " ^ msg))))
+    | Error _ as e -> e
+  in
+  let printer = Arg.conv_printer pair in
+  let c = Arg.conv (parser, printer) in
+  Arg.(value & opt_all (list c) [] & info [ "set" ] ~docv:"PARAM=VALUE" ~doc)
+
 let options () =
   let runtime_files =
     let doc = "Link JavaScript and WebAssembly files [$(docv)]. " in
@@ -110,14 +130,6 @@ let options () =
     let doc = "root dir for source map." in
     Arg.(value & opt (some string) None & info [ "source-map-root" ] ~doc)
   in
-  let set_param =
-    let doc = "Set compiler options." in
-    let all = List.map (Config.Param.all ()) ~f:(fun (x, _) -> x, x) in
-    Arg.(
-      value
-      & opt_all (list (pair ~sep:'=' (enum all) string)) []
-      & info [ "set" ] ~docv:"PARAM=VALUE" ~doc)
-  in
   let include_dirs =
     let doc = "Add [$(docv)] to the list of include directories." in
     Arg.(value & opt_all string [] & info [ "I" ] ~docv:"DIR" ~doc)
@@ -232,14 +244,6 @@ let options_runtime_only () =
     let doc = "Add [$(docv)] to the list of include directories." in
     Arg.(value & opt_all string [] & info [ "I" ] ~docv:"DIR" ~doc)
   in
-  let set_param =
-    let doc = "Set compiler options." in
-    let all = List.map (Config.Param.all ()) ~f:(fun (x, _) -> x, x) in
-    Arg.(
-      value
-      & opt_all (list (pair ~sep:'=' (enum all) string)) []
-      & info [ "set" ] ~docv:"PARAM=VALUE" ~doc)
-  in
   let effects =
     let doc =
       "Select an implementation of effect handlers. [$(docv)] should be one of $(b,jspi) \
diff --git a/compiler/bin-wasm_of_ocaml/compile.ml b/compiler/bin-wasm_of_ocaml/compile.ml
index 9a11c79802..08fcfda78d 100644
--- a/compiler/bin-wasm_of_ocaml/compile.ml
+++ b/compiler/bin-wasm_of_ocaml/compile.ml
@@ -414,9 +414,10 @@ let run
       && Parse_bytecode.Debug.is_empty one.debug
       && not (Code.is_empty one.code)
     then
-      warn
-        "Warning: '--source-map' is enabled but the bytecode program was compiled with \
-         no debugging information.\n\
+      Warning.warn
+        `Missing_debug_event
+        "'--source-map' is enabled but the bytecode program was compiled with no \
+         debugging information.\n\
          Warning: Consider passing '-g' option to ocamlc.\n\
          %!"
   in
diff --git a/compiler/bin-wasm_of_ocaml/gen/gen.ml b/compiler/bin-wasm_of_ocaml/gen/gen.ml
index 0310943921..2083f81cfd 100644
--- a/compiler/bin-wasm_of_ocaml/gen/gen.ml
+++ b/compiler/bin-wasm_of_ocaml/gen/gen.ml
@@ -17,7 +17,7 @@ let check_js_file fname =
   let freenames = StringSet.diff freenames Reserved.provided in
   if not (StringSet.is_empty freenames)
   then (
-    Format.eprintf "warning: free variables in %S@." fname;
+    Format.eprintf "Warning: free variables in %S@." fname;
     Format.eprintf "vars: %s@." (String.concat ~sep:", " (StringSet.elements freenames));
     exit 2);
   ()
diff --git a/compiler/bin-wasm_of_ocaml/wasm_of_ocaml.ml b/compiler/bin-wasm_of_ocaml/wasm_of_ocaml.ml
index 91fe026df7..bc87e9ba75 100644
--- a/compiler/bin-wasm_of_ocaml/wasm_of_ocaml.ml
+++ b/compiler/bin-wasm_of_ocaml/wasm_of_ocaml.ml
@@ -21,7 +21,7 @@ open Js_of_ocaml_compiler
 
 let () =
   Sys.catch_break true;
-  let argv = Jsoo_cmdline.normalize_argv ~warn:(warn "%s") Sys.argv in
+  let argv = Sys.argv in
   let argv =
     let like_arg x = String.length x > 0 && Char.equal x.[0] '-' in
     let like_command x =
@@ -57,11 +57,8 @@ let () =
            ])
     with
     | Ok (`Ok () | `Help | `Version) ->
-        if !warnings > 0 && !werror
-        then (
-          Format.eprintf "%s: all warnings being treated as errors@." Sys.argv.(0);
-          exit 1)
-        else exit 0
+        Warning.process_warnings ();
+        exit 0
     | Error `Term -> exit 1
     | Error `Parse -> exit Cmdliner.Cmd.Exit.cli_error
     | Error `Exn -> ()
diff --git a/compiler/bin-wasm_of_ocaml/wasmoo_link_wasm.ml b/compiler/bin-wasm_of_ocaml/wasmoo_link_wasm.ml
index d9f7a24766..1192fbe17d 100644
--- a/compiler/bin-wasm_of_ocaml/wasmoo_link_wasm.ml
+++ b/compiler/bin-wasm_of_ocaml/wasmoo_link_wasm.ml
@@ -20,12 +20,7 @@
 open Js_of_ocaml_compiler.Stdlib
 
 let (_ : int) =
-  try
-    Cmdliner.Cmd.eval
-      ~catch:false
-      ~argv:(Jsoo_cmdline.normalize_argv ~warn:(warn "%s") Sys.argv)
-      Link_wasm.command
-  with
+  try Cmdliner.Cmd.eval ~catch:false ~argv:Sys.argv Link_wasm.command with
   | (Match_failure _ | Assert_failure _ | Not_found) as exc ->
       let backtrace = Printexc.get_backtrace () in
       Format.eprintf
diff --git a/compiler/lib-cmdline/arg.ml b/compiler/lib-cmdline/arg.ml
index 2b30270cdf..bceec63034 100644
--- a/compiler/lib-cmdline/arg.ml
+++ b/compiler/lib-cmdline/arg.ml
@@ -31,6 +31,7 @@ type t =
   ; optim : string list on_off
   ; quiet : bool
   ; werror : bool
+  ; warnings : (bool * Warning.t) list
   ; custom_header : string option
   }
 
@@ -61,6 +62,35 @@ let disable =
      in
      Term.(const List.flatten $ arg))
 
+let parse_warning s =
+  let err s = `Msg (Printf.sprintf "Unknown warning %s" s) in
+  if String.is_empty s
+  then Error (err s)
+  else
+    match Warning.parse s with
+    | Some n -> Ok (true, n)
+    | None -> (
+        match String.drop_prefix ~prefix:"no-" s with
+        | Some n -> (
+            match Warning.parse n with
+            | Some n -> Ok (false, n)
+            | None -> Error (err n))
+        | None -> Error (err s))
+
+let print_warning fmt (b, w) =
+  Format.fprintf
+    fmt
+    "%s%s"
+    (match b with
+    | true -> ""
+    | false -> "")
+    (Warning.name w)
+
+let warnings : (bool * Warning.t) list Term.t =
+  let doc = "Enable or disable the warnings specified by the argument [$(docv)]." in
+  let c : 'a Arg.conv = Arg.conv ~docv:"" (parse_warning, print_warning) in
+  Arg.(value & opt_all c [] & info [ "w" ] ~docv:"WARN" ~doc)
+
 let pretty =
   let doc = "Pretty print the output." in
   Arg.(value & flag & info [ "pretty" ] ~doc)
@@ -91,7 +121,19 @@ let custom_header =
 let t =
   lazy
     Term.(
-      const (fun debug enable disable pretty debuginfo noinline quiet werror c_header ->
+      const
+        (fun
+          debug
+          enable
+          disable
+          pretty
+          debuginfo
+          noinline
+          quiet
+          (warnings : (bool * Warning.t) list)
+          werror
+          c_header
+        ->
           let enable = if pretty then "pretty" :: enable else enable in
           let enable = if debuginfo then "debuginfo" :: enable else enable in
           let disable = if noinline then "inline" :: disable else disable in
@@ -104,6 +146,7 @@ let t =
           let disable = disable_if_pretty "share" disable in
           { debug = { enable = debug; disable = [] }
           ; optim = { enable; disable }
+          ; warnings
           ; quiet
           ; werror
           ; custom_header = c_header
@@ -115,6 +158,7 @@ let t =
       $ debuginfo
       $ noinline
       $ is_quiet
+      $ warnings
       $ is_werror
       $ custom_header)
 
@@ -125,5 +169,8 @@ let on_off on off t =
 let eval t =
   Config.Flag.(on_off enable disable t.optim);
   Debug.(on_off enable disable t.debug);
-  quiet := t.quiet;
-  werror := t.werror
+  List.iter t.warnings ~f:(function
+    | true, w -> Warning.enable w
+    | false, w -> Warning.disable w);
+  Warning.quiet := t.quiet;
+  Warning.werror := t.werror
diff --git a/compiler/lib-cmdline/arg.mli b/compiler/lib-cmdline/arg.mli
index 295f58ac72..16a0eb6841 100644
--- a/compiler/lib-cmdline/arg.mli
+++ b/compiler/lib-cmdline/arg.mli
@@ -27,6 +27,7 @@ type t =
   ; optim : string list on_off
   ; quiet : bool
   ; werror : bool
+  ; warnings : (bool * Js_of_ocaml_compiler.Warning.t) list
   ; custom_header : string option
   }
 
diff --git a/compiler/lib-cmdline/jsoo_cmdline.ml b/compiler/lib-cmdline/jsoo_cmdline.ml
index 9943936bc7..2acd3ddd18 100644
--- a/compiler/lib-cmdline/jsoo_cmdline.ml
+++ b/compiler/lib-cmdline/jsoo_cmdline.ml
@@ -17,33 +17,4 @@
  * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
  *)
 
-open Js_of_ocaml_compiler.Stdlib
 module Arg = Arg
-
-let normalize_argv ?(warn = fun _ -> ()) a =
-  let bad = ref [] in
-  let a =
-    Array.map
-      ~f:(fun s ->
-        let size = String.length s in
-        if size <= 2
-        then s
-        else if
-          Char.equal s.[0] '-'
-          && (not (Char.equal s.[1] '-'))
-          && not (Char.equal s.[2] '=')
-        then (
-          bad := s :: !bad;
-          (* long option with one dash lets double the dash *)
-          "-" ^ s)
-        else s)
-      a
-  in
-  if not (List.is_empty !bad)
-  then
-    warn
-      (Format.sprintf
-         "[Warning] long options with a single '-' are now deprecated. Please use '--' \
-          for the following options: %s@."
-         (String.concat ~sep:", " !bad));
-  a
diff --git a/compiler/lib-wasm/generate.ml b/compiler/lib-wasm/generate.ml
index a1dd5a8b26..7db9e077bb 100644
--- a/compiler/lib-wasm/generate.ml
+++ b/compiler/lib-wasm/generate.ml
@@ -1397,7 +1397,6 @@ module Generate (Target : Target_sig.S) = struct
       ~live_vars
       ~in_cps (*
     ~should_export
-    ~warn_on_unhandled_effect
 *)
       ~deadcode_sentinal
       ~global_flow_info
diff --git a/compiler/lib-wasm/link.ml b/compiler/lib-wasm/link.ml
index f2c527323b..3f905507f1 100644
--- a/compiler/lib-wasm/link.ml
+++ b/compiler/lib-wasm/link.ml
@@ -409,12 +409,15 @@ let output_js js =
 
 let report_missing_primitives missing =
   if not (List.is_empty missing)
-  then (
-    warn "There are some missing Wasm primitives@.";
-    warn "Dummy implementations (raising an exception) ";
-    warn "will be provided.@.";
-    warn "Missing primitives:@.";
-    List.iter ~f:(fun nm -> warn "  %s@." nm) missing)
+  then
+    Warning.warn
+      `Missing_primitive
+      "There are some missing Wasm primitives\n\
+       Dummy implementations (raising an exception) will be provided.\n\
+       Missing primitives:\n\
+       %a"
+      (Format.pp_print_list Format.pp_print_string)
+      missing
 
 let build_runtime_arguments
     ~link_spec
diff --git a/compiler/lib/builtins.ml b/compiler/lib/builtins.ml
index 1319030375..91b244d080 100644
--- a/compiler/lib/builtins.ml
+++ b/compiler/lib/builtins.ml
@@ -40,7 +40,9 @@ let register ~name ~content ~fragments =
   let name = "+" ^ name in
   let t = { File.name; content; fragments } in
   if String.Hashtbl.mem tbl name
-  then warn "The builtin runtime file %S was registered multiple time" name;
+  then
+    failwith
+      (Printf.sprintf "The builtin runtime file %S was registered multiple time" name);
   String.Hashtbl.add tbl name t;
   t
 
diff --git a/compiler/lib/config.ml b/compiler/lib/config.ml
index c5cab8ffe7..65c45d39ca 100644
--- a/compiler/lib/config.ml
+++ b/compiler/lib/config.ml
@@ -89,8 +89,6 @@ module Flag = struct
 
   let improved_stacktrace = o ~name:"with-js-error" ~default:false
 
-  let warn_unused = o ~name:"warn-unused" ~default:false
-
   let inline_callgen = o ~name:"callgen" ~default:false
 
   let safe_string = o ~name:"safestring" ~default:true
@@ -111,36 +109,51 @@ module Flag = struct
 end
 
 module Param = struct
-  let int default = default, int_of_string
+  let int default =
+    ( default
+    , int_of_string
+    , fun s ->
+        try
+          ignore (int_of_string s : int);
+          Ok ()
+        with _ -> Error "expecting an integer" )
 
   let enum : (string * 'a) list -> _ = function
-    | (_, v) :: _ as l -> (
+    | (_, v) :: _ as l ->
         ( v
-        , fun x ->
+        , (fun x ->
             match List.string_assoc x l with
             | Some x -> x
-            | None -> assert false ))
+            | None -> assert false)
+        , fun x ->
+            if List.exists ~f:(fun (y, _) -> String.equal x y) l
+            then Ok ()
+            else
+              Error
+                (Printf.sprintf
+                   "expecting one of %s"
+                   (String.concat ~sep:", " (List.map l ~f:fst))) )
     | _ -> assert false
 
   let params : (string * _) list ref = ref []
 
-  let p ~name ~desc (default, convert) =
+  let p ~name ~desc (default, convert, valid) =
     assert (Option.is_none (List.string_assoc name !params));
     let state = ref default in
     let set : string -> unit =
      fun v ->
       try state := convert v
-      with _ -> warn "Warning: malformed option %s=%s. IGNORE@." name v
+      with _ -> failwith (Printf.sprintf "malformed option %s=%s." name v)
     in
-    params := (name, (set, desc)) :: !params;
+    params := (name, (set, desc, valid)) :: !params;
     fun () -> !state
 
   let set s v =
     match List.string_assoc s !params with
-    | Some (f, _) -> f v
+    | Some (f, _, _) -> f v
     | None -> failwith (Printf.sprintf "The option named %S doesn't exist" s)
 
-  let all () = List.map !params ~f:(fun (n, (_, d)) -> n, d)
+  let all () = List.map !params ~f:(fun (n, (_, d, valid)) -> n, d, valid)
 
   (* V8 "optimize" switches with less than 128 case.
      60 seams to perform well. *)
diff --git a/compiler/lib/config.mli b/compiler/lib/config.mli
index 09c8b6ab1a..fc545a3fc4 100644
--- a/compiler/lib/config.mli
+++ b/compiler/lib/config.mli
@@ -60,8 +60,6 @@ module Flag : sig
 
   val improved_stacktrace : unit -> bool
 
-  val warn_unused : unit -> bool
-
   val inline_callgen : unit -> bool
 
   val safe_string : unit -> bool
@@ -87,7 +85,7 @@ end
 module Param : sig
   val set : string -> string -> unit
 
-  val all : unit -> (string * string) list
+  val all : unit -> (string * string * (string -> (unit, string) Result.t)) list
 
   val switch_max_case : unit -> int
 
diff --git a/compiler/lib/debug.ml b/compiler/lib/debug.ml
index 248eedab56..20f03f40fd 100644
--- a/compiler/lib/debug.ml
+++ b/compiler/lib/debug.ml
@@ -57,7 +57,7 @@ let find ?(even_if_quiet = false) s =
   in
   fun () ->
     if String.equal s "times" then take_snapshot ();
-    (even_if_quiet || not !quiet) && !state
+    (even_if_quiet || not !Warning.quiet) && !state
 
 let enable s =
   match List.string_assoc s !debugs with
diff --git a/compiler/lib/driver.ml b/compiler/lib/driver.ml
index b5bb3c1211..b5cd1aa938 100644
--- a/compiler/lib/driver.ml
+++ b/compiler/lib/driver.ml
@@ -267,7 +267,7 @@ let extra_js_files =
            (name, ss) :: acc
          with _ -> acc))
 
-let report_missing_primitives missing =
+let report_missing_primitives fmt missing =
   let missing =
     List.fold_left
       (Lazy.force extra_js_files)
@@ -276,15 +276,15 @@ let report_missing_primitives missing =
         let d = StringSet.inter missing pro in
         if not (StringSet.is_empty d)
         then (
-          warn "Missing primitives provided by %s:@." file;
-          StringSet.iter (fun nm -> warn "  %s@." nm) d;
+          Format.fprintf fmt "Missing primitives provided by %s:@." file;
+          StringSet.iter (fun nm -> Format.fprintf fmt "  %s@." nm) d;
           StringSet.diff missing pro)
         else missing)
   in
   if not (StringSet.is_empty missing)
   then (
-    warn "Missing primitives:@.";
-    StringSet.iter (fun nm -> warn "  %s@." nm) missing)
+    Format.fprintf fmt "Missing primitives:@.";
+    StringSet.iter (fun nm -> Format.fprintf fmt "  %s@." nm) missing)
 
 let gen_missing js missing =
   let open Javascript in
@@ -324,13 +324,17 @@ let gen_missing js missing =
       []
   in
   if not (StringSet.is_empty missing)
-  then (
-    warn "There are some missing primitives@.";
-    warn "Dummy implementations (raising 'Failure' exception) ";
-    warn "will be used if they are not available at runtime.@.";
-    warn "You can prevent the generation of dummy implementations with ";
-    warn "the commandline option '--disable genprim'@.";
-    report_missing_primitives missing);
+  then
+    Warning.warn
+      `Missing_primitive
+      "There are some missing primitives.\n\
+       Dummy implementations (raising 'Failure' exception) will be used if they are not \
+       available at runtime.\n\
+       You can prevent the generation of dummy implementations with the commandline \
+       option '--disable genprim'\n\
+       %a"
+      report_missing_primitives
+      missing;
   (variable_declaration miss, N) :: js
 
 let mark_start_of_generated_code = Debug.find ~even_if_quiet:true "mark-runtime-gen"
@@ -356,18 +360,25 @@ let link' ~export_runtime ~standalone ~link (js : Javascript.statement_list) :
     in
     let used =
       let all_provided = Linker.list_all () in
+      let free =
+        lazy
+          (let free = ref StringSet.empty in
+           let o =
+             new Js_traverse.fast_freevar (fun s -> free := StringSet.add s !free)
+           in
+           o#program js;
+           !free)
+      in
       match link with
-      | `All -> all_provided
+      | `All ->
+          let prim = Primitive.get_external () in
+          StringSet.union (StringSet.inter prim (Lazy.force free)) all_provided
       | `All_from from -> Linker.list_all ~from ()
       | `No -> StringSet.empty
       | `Needed ->
-          let free = ref StringSet.empty in
-          let o = new Js_traverse.fast_freevar (fun s -> free := StringSet.add s !free) in
-          o#program js;
-          let free = !free in
           let prim = Primitive.get_external () in
           let all_external = StringSet.union prim all_provided in
-          StringSet.inter free all_external
+          StringSet.inter (Lazy.force free) all_external
     in
     let linkinfos =
       let from =
@@ -473,17 +484,23 @@ let check_js js =
   let missing = StringSet.inter free all_external in
   let missing = StringSet.diff missing Reserved.provided in
   let other = StringSet.diff free missing in
-  if not (StringSet.is_empty missing) then report_missing_primitives missing;
+  if not (StringSet.is_empty missing)
+  then
+    Warning.warn
+      `Missing_primitive
+      "There are some missing primitives.\n%a"
+      report_missing_primitives
+      missing;
   let probably_prov = StringSet.inter other Reserved.provided in
   let other = StringSet.diff other probably_prov in
   if (not (StringSet.is_empty other)) && debug_linker ()
   then (
-    warn "Missing variables:@.";
-    StringSet.iter (fun nm -> warn "  %s@." nm) other);
+    Format.eprintf "Missing variables:@.";
+    StringSet.iter (fun nm -> Format.eprintf "  %s@." nm) other);
   if (not (StringSet.is_empty probably_prov)) && debug_linker ()
   then (
-    warn "Variables provided by the browser:@.";
-    StringSet.iter (fun nm -> warn "  %s@." nm) probably_prov);
+    Format.eprintf "Variables provided by the browser:@.";
+    StringSet.iter (fun nm -> Format.eprintf "  %s@." nm) probably_prov);
   if times () then Format.eprintf "  checks: %a@." Timer.print t;
   js
 
diff --git a/compiler/lib/generate.ml b/compiler/lib/generate.ml
index 6a6f729c72..4647e27d3f 100644
--- a/compiler/lib/generate.ml
+++ b/compiler/lib/generate.ml
@@ -1522,8 +1522,9 @@ let rec translate_expr ctx loc x e level : (_ * J.statement_list) Expr_builder.t
             assert (not (cps_transform ()));
             if not !(ctx.effect_warning)
             then (
-              warn
-                "Warning: your program contains effect handlers; you should probably run \
+              Warning.warn
+                `Effect_handlers_without_effect_backend
+                "your program contains effect handlers; you should probably run \
                  js_of_ocaml with option '--effects=cps'@.";
               ctx.effect_warning := true);
             let name = "jsoo_effect_not_supported" in
diff --git a/compiler/lib/link_js.ml b/compiler/lib/link_js.ml
index e2ea0cc902..82ab0ce928 100644
--- a/compiler/lib/link_js.ml
+++ b/compiler/lib/link_js.ml
@@ -345,9 +345,10 @@ let link
                   if u.effects_without_cps && not !warn_effects
                   then (
                     warn_effects := true;
-                    warn
-                      "Warning: your program contains effect handlers; you should \
-                       probably run js_of_ocaml with option '--effects=cps'@.");
+                    Warning.warn
+                      `Effect_handlers_without_effect_backend
+                      "your program contains effect handlers; you should probably run \
+                       js_of_ocaml with option '--effects=cps'@.");
                   (if mklib
                    then
                      let u = if linkall then { u with force_link = true } else u in
diff --git a/compiler/lib/linker.ml b/compiler/lib/linker.ml
index f0985384bf..f66b511c13 100644
--- a/compiler/lib/linker.ml
+++ b/compiler/lib/linker.ml
@@ -112,8 +112,9 @@ module Check = struct
         (match diff with
         | [] -> ()
         | l ->
-            warn
-              "WARN unused for primitive %s at %s:@. %s@."
+            Warning.warn
+              `Unused_js_variable
+              "unused variable for primitive %s at %s:@. %s@."
               name
               (loc pi)
               (String.concat ~sep:", " l));
@@ -122,7 +123,7 @@ module Check = struct
 
   let primitive ~name pi ~code ~requires ~has_flags =
     let freename =
-      if Config.Flag.warn_unused ()
+      if Warning.enabled `Unused_js_variable
       then
         let o = new check_and_warn name pi in
         let _code = o#program code in
@@ -147,21 +148,29 @@ module Check = struct
     in
     if StringSet.mem Global_constant.old_global_object freename
     then
-      warn
-        "warning: %s: 'joo_global_object' is being deprecated, please use `globalThis` \
-         instead@."
+      Warning.warn
+        `Deprecated_joo_global_object
+        "%s: 'joo_global_object' is being deprecated, please use `globalThis` instead@."
         (loc pi);
     let freename = StringSet.remove Global_constant.old_global_object freename in
     if not (StringSet.mem name (Js_traverse.declared_names code))
     then
-      warn
-        "warning: primitive code does not define value with the expected name: %s (%s)@."
+      Warning.warn
+        `Missing_define
+        "primitive code does not define value with the expected name: %s (%s)@."
         name
         (loc pi);
     if not (StringSet.is_empty freename)
-    then (
-      warn "warning: free variables in primitive code %S (%s)@." name (loc pi);
-      warn "vars: %s@." (String.concat ~sep:", " (StringSet.elements freename)))
+    then
+      Warning.warn
+        `Free_variables_in_primitive
+        "free variables in primitive code %S (%s)@.vars: %a@."
+        name
+        (loc pi)
+        (Format.pp_print_list
+           ~pp_sep:(fun fmt () -> Format.pp_print_string fmt ", ")
+           Format.pp_print_string)
+        (StringSet.elements freename)
 end
 
 module Fragment = struct
@@ -559,8 +568,9 @@ let load_fragment ~target_env ~filename (f : Fragment.t) =
                   if p.weakdef
                   then true
                   else (
-                    warn
-                      "warning: overriding primitive %S\n  old: %s\n  new: %s@."
+                    Warning.warn
+                      `Overriding_primitive
+                      "overriding primitive %S\n  old: %s\n  new: %s@."
                       name
                       (loc p.pi)
                       (loc pi);
@@ -603,7 +613,8 @@ let check_deps () =
           then
             try
               let name, ploc = Int.Hashtbl.find provided_rev id in
-              warn
+              Warning.warn
+                `Missing_deps
                 "code providing %s (%s) may miss dependencies: %s\n"
                 name
                 (loc ploc)
@@ -732,7 +743,11 @@ let link ?(check_missing = true) program (state : state) =
           if false
           then
             let name = fst (Int.Hashtbl.find provided_rev x) in
-            warn "The runtime primitive [%s] is deprecated. %s\n" name txt
+            Warning.warn
+              `Deprecated_primitive
+              "The runtime primitive [%s] is deprecated. %s\n"
+              name
+              txt
       | x :: path ->
           let name = fst (Int.Hashtbl.find provided_rev x) in
           let path =
@@ -742,7 +757,8 @@ let link ?(check_missing = true) program (state : state) =
                    let nm, loc = Int.Hashtbl.find provided_rev id in
                    Printf.sprintf "-> %s:%s" nm (Parse_info.to_string loc)))
           in
-          warn
+          Warning.warn
+            `Deprecated_primitive
             "The runtime primitive [%s] is deprecated. %s.  Used by:\n%s\n"
             name
             txt
diff --git a/compiler/lib/parse_bytecode.ml b/compiler/lib/parse_bytecode.ml
index 56d870704e..4f089e7f61 100644
--- a/compiler/lib/parse_bytecode.ml
+++ b/compiler/lib/parse_bytecode.ml
@@ -2714,9 +2714,10 @@ let from_exe
      with Not_found ->
        if Debug.enabled debug_data || include_cmis
        then
-         warn
-           "Warning: Program not linked with -g, original variable names and locations \
-            not available.@.");
+         Warning.warn
+           `Missing_debug_event
+           "Program not linked with -g, original variable names and locations not \
+            available.@.");
   if times () then Format.eprintf "    read debug events: %a@." Timer.print t;
 
   let globals = make_globals (Array.length init_data) init_data primitive_table in
diff --git a/compiler/lib/primitive.ml b/compiler/lib/primitive.ml
index 29ac474999..f8bd4bbebc 100644
--- a/compiler/lib/primitive.ml
+++ b/compiler/lib/primitive.ml
@@ -98,8 +98,9 @@ let register p k kargs arity =
   | exception Not_found -> ()
   | k' when kind_equal k k' -> ()
   | k' ->
-      warn
-        "Warning: overriding the purity of the primitive %s: %s -> %s@."
+      Warning.warn
+        `Overriding_primitive_purity
+        "overriding the purity of the primitive %s: %s -> %s@."
         p
         (string_of_kind k')
         (string_of_kind k));
diff --git a/compiler/lib/pseudo_fs.ml b/compiler/lib/pseudo_fs.ml
index 4e5dfe7209..b9f5ac442f 100644
--- a/compiler/lib/pseudo_fs.ml
+++ b/compiler/lib/pseudo_fs.ml
@@ -27,19 +27,15 @@ let expand_path exts real virt =
       List.fold_left l ~init:acc ~f:(fun acc s ->
           loop (Filename.concat realfile s) (Filename.concat virtfile s) acc)
     else
-      try
-        let exmatch =
-          try
-            let b = Filename.basename realfile in
-            let i = String.rindex b '.' in
-            let e = String.sub b ~pos:(i + 1) ~len:(String.length b - i - 1) in
-            List.mem ~eq:String.equal e exts
-          with Not_found -> List.mem ~eq:String.equal "" exts
-        in
-        if List.is_empty exts || exmatch then (virtfile, realfile) :: acc else acc
-      with exc ->
-        warn "ignoring %s: %s@." realfile (Printexc.to_string exc);
-        acc
+      let exmatch =
+        try
+          let b = Filename.basename realfile in
+          let i = String.rindex b '.' in
+          let e = String.sub b ~pos:(i + 1) ~len:(String.length b - i - 1) in
+          List.mem ~eq:String.equal e exts
+        with Not_found -> List.mem ~eq:String.equal "" exts
+      in
+      if List.is_empty exts || exmatch then (virtfile, realfile) :: acc else acc
   in
   loop real virt []
 
@@ -124,11 +120,15 @@ let f ~prim ~cmis ~files ~paths =
       ([], [])
   in
   if not (List.is_empty missing_cmis)
-  then (
-    warn "Some OCaml interface files were not found.@.";
-    warn "Use [-I dir_of_cmis] option to bring them into scope@.";
-    (* [`ocamlc -where`/expunge in.byte out.byte moduleA moduleB ... moduleN] *)
-    List.iter missing_cmis ~f:(fun nm -> warn "  %s@." nm));
+  then
+    Warning.warn
+      `Missing_cmi
+      "Some OCaml interface files were not found.\n\
+       Use [-I dir_of_cmis] option to bring them into scope\n\
+       %a"
+      (Format.pp_print_list Format.pp_print_string)
+      missing_cmis;
+  (* [`ocamlc -where`/expunge in.byte out.byte moduleA moduleB ... moduleN] *)
   let other_files =
     List.map files ~f:(fun f ->
         List.map (list_files f paths) ~f:(fun (name, filename) ->
diff --git a/compiler/lib/stdlib.ml b/compiler/lib/stdlib.ml
index bc2742ea93..8dfcfd63f1 100644
--- a/compiler/lib/stdlib.ml
+++ b/compiler/lib/stdlib.ml
@@ -70,19 +70,6 @@ let ( != ) = `use_phys_equal
 
 include Int_replace_polymorphic_compare
 
-let quiet = ref false
-
-let werror = ref false
-
-let warnings = ref 0
-
-let warn fmt =
-  Format.ksprintf
-    (fun s ->
-      incr warnings;
-      if not !quiet then Format.eprintf "%s%!" s)
-    fmt
-
 let fail = ref true
 
 let failwith_ fmt =
@@ -276,9 +263,9 @@ module Int32 = struct
   external ( >= ) : int32 -> int32 -> bool = "%greaterequal"
 
   let warn_overflow name ~to_dec ~to_hex i i32 =
-    warn
-      "Warning: integer overflow: %s 0x%s (%s) truncated to 0x%lx (%ld); the generated \
-       code might be incorrect.@."
+    Warning.warn
+      `Integer_overflow
+      "%s 0x%s (%s) truncated to 0x%lx (%ld); the generated code might be incorrect.@."
       name
       (to_hex i)
       (to_dec i)
diff --git a/compiler/lib/warning.ml b/compiler/lib/warning.ml
new file mode 100644
index 0000000000..4e87834ebd
--- /dev/null
+++ b/compiler/lib/warning.ml
@@ -0,0 +1,125 @@
+open StdLabels
+
+type t =
+  [ (* Parsing bytecode *)
+    `Integer_overflow
+  | `Missing_debug_event
+  | `Missing_cmi
+  | `Effect_handlers_without_effect_backend
+  | (* runtime *)
+    `Missing_primitive
+  | `Missing_define
+  | `Missing_deps
+  | `Deprecated_joo_global_object
+  | `Overriding_primitive
+  | `Overriding_primitive_purity
+  | `Deprecated_primitive
+  | `Unused_js_variable
+  | `Free_variables_in_primitive
+  ]
+
+module StringTable = Hashtbl.Make (struct
+  type t = string
+
+  let equal = String.equal
+
+  let hash = Hashtbl.hash
+end)
+
+module Table = Hashtbl.Make (struct
+  type nonrec t = t
+
+  let hash = Hashtbl.hash
+
+  let equal (a : t) b = a = b
+end)
+
+let state = Table.create 0
+
+let enable t = Table.add state t true
+
+let disable t = Table.add state t false
+
+let default = function
+  (* Parsing bytecode *)
+  | `Integer_overflow | `Missing_debug_event | `Missing_cmi -> true
+  (* effects *)
+  | `Effect_handlers_without_effect_backend -> true
+  (* runtime *)
+  | `Missing_primitive | `Missing_define | `Missing_deps | `Free_variables_in_primitive ->
+      true
+  | `Deprecated_joo_global_object -> true
+  | `Overriding_primitive | `Overriding_primitive_purity -> true
+  | `Deprecated_primitive -> true
+  | `Unused_js_variable -> false
+
+let all =
+  [ (* Parsing bytecode *)
+    `Integer_overflow
+  ; `Missing_debug_event
+  ; `Missing_cmi
+  ; `Effect_handlers_without_effect_backend
+  ; (* runtime *)
+    `Missing_primitive
+  ; `Missing_define
+  ; `Missing_deps
+  ; `Deprecated_joo_global_object
+  ; `Overriding_primitive
+  ; `Overriding_primitive_purity
+  ; `Deprecated_primitive
+  ; `Unused_js_variable
+  ; `Free_variables_in_primitive
+  ]
+
+let name = function
+  (* Parsing bytecode *)
+  | `Integer_overflow -> "integer-overflow"
+  | `Missing_debug_event -> "missing-debug-event"
+  | `Missing_cmi -> "missing-cmi"
+  (* effects *)
+  | `Effect_handlers_without_effect_backend -> "missing-effects-backend"
+  (* runtime *)
+  | `Missing_primitive -> "missing-primitive"
+  | `Missing_define -> "missing-define"
+  | `Missing_deps -> "missing-deps"
+  | `Free_variables_in_primitive -> "free-variables"
+  | `Deprecated_joo_global_object -> "deprecated-joo-global-object"
+  | `Overriding_primitive -> "overriding-primitive"
+  | `Overriding_primitive_purity -> "overriding-primitive-purity"
+  | `Deprecated_primitive -> "deprecated-primitive"
+  | `Unused_js_variable -> "unused-js-vars"
+
+let parse : string -> t option =
+  let h = StringTable.create 18 in
+  List.iter all ~f:(fun t ->
+      let name = name t in
+      (* We use the no- prefix to disable warnings *)
+      assert (not (String.starts_with ~prefix:"no-" name));
+      StringTable.add h name t);
+  fun s -> StringTable.find_opt h s
+
+let enabled t =
+  match Table.find_opt state t with
+  | Some b -> b
+  | None -> default t
+
+let quiet = ref false
+
+let werror = ref false
+
+let warnings = ref 0
+
+let warn (t : t) fmt =
+  Format.kasprintf
+    (fun s ->
+      if enabled t && not !quiet
+      then (
+        incr warnings;
+        Format.eprintf "Warning%s: %s%!" (Printf.sprintf " [%s]" (name t)) s))
+    fmt
+
+let process_warnings () =
+  if !warnings > 0 && !werror
+  then (
+    Format.eprintf "%s: all warnings being treated as errors@." Sys.argv.(0);
+    exit 1)
diff --git a/compiler/lib/warning.mli b/compiler/lib/warning.mli
new file mode 100644
index 0000000000..2b1eb2dc55
--- /dev/null
+++ b/compiler/lib/warning.mli
@@ -0,0 +1,37 @@
+type t =
+  [ (* Parsing bytecode *)
+    `Integer_overflow
+  | `Missing_debug_event
+  | `Missing_cmi
+  | `Effect_handlers_without_effect_backend
+  | (* runtime *)
+    `Missing_primitive
+  | `Missing_define
+  | `Missing_deps
+  | `Free_variables_in_primitive
+  | `Deprecated_joo_global_object
+  | `Overriding_primitive
+  | `Overriding_primitive_purity
+  | `Deprecated_primitive
+  | `Unused_js_variable
+  ]
+
+val all : t list
+
+val name : t -> string
+
+val parse : string -> t option
+
+val enable : t -> unit
+
+val disable : t -> unit
+
+val enabled : t -> bool
+
+val quiet : bool ref
+
+val werror : bool ref
+
+val warn : t -> ('a, Format.formatter, unit, unit) format4 -> 'a
+
+val process_warnings : unit -> unit
diff --git a/compiler/tests-compiler/gh1051.ml b/compiler/tests-compiler/gh1051.ml
index 53cebd9e7e..ad0b17834d 100644
--- a/compiler/tests-compiler/gh1051.ml
+++ b/compiler/tests-compiler/gh1051.ml
@@ -25,7 +25,7 @@ let%expect_test _ =
   Util.compile_and_run ~werror:false prog;
   [%expect
     {|
-    Warning: integer overflow: native integer 0xffffffff (4294967295) truncated to 0xffffffff (-1); the generated code might be incorrect.
+    Warning [integer-overflow]: native integer 0xffffffff (4294967295) truncated to 0xffffffff (-1); the generated code might be incorrect.
     ffffffff
     |}];
   ()
@@ -34,7 +34,7 @@ let%expect_test _ =
   Util.print_fun_decl (Util.compile_and_parse ~werror:false prog) None;
   [%expect
     {|
-    Warning: integer overflow: native integer 0xffffffff (4294967295) truncated to 0xffffffff (-1); the generated code might be incorrect.
+    Warning [integer-overflow]: native integer 0xffffffff (4294967295) truncated to 0xffffffff (-1); the generated code might be incorrect.
     function caml_call2(f, a0, a1){
      return (f.l >= 0 ? f.l : f.l = f.length) === 2
              ? f(a0, a1)
diff --git a/compiler/tests-compiler/pbt/test_int31.ml b/compiler/tests-compiler/pbt/test_int31.ml
index 37f72d354c..5d346907da 100644
--- a/compiler/tests-compiler/pbt/test_int31.ml
+++ b/compiler/tests-compiler/pbt/test_int31.ml
@@ -68,7 +68,7 @@ let%expect_test _ =
   let output = [%expect.output] in
   let expected =
     Format.sprintf
-      "Warning: integer overflow: int32 0x%lx (%ld) truncated to 0x%lx (%ld); the \
+      "Warning [integer-overflow]: int32 0x%lx (%ld) truncated to 0x%lx (%ld); the \
        generated code might be incorrect.@."
       i
       i
@@ -86,7 +86,7 @@ let%expect_test _ =
   let output = [%expect.output] in
   let expected =
     Format.sprintf
-      "Warning: integer overflow: integer 0x%x (%d) truncated to 0x%lx (%ld); the \
+      "Warning [integer-overflow]: integer 0x%x (%d) truncated to 0x%lx (%ld); the \
        generated code might be incorrect.@."
       i
       i
@@ -104,7 +104,7 @@ let%expect_test _ =
   let output = [%expect.output] in
   let expected =
     Format.sprintf
-      "Warning: integer overflow: native integer 0x%nx (%nd) truncated to 0x%lx (%ld); \
+      "Warning [integer-overflow]: native integer 0x%nx (%nd) truncated to 0x%lx (%ld); \
        the generated code might be incorrect.@."
       i
       i
diff --git a/compiler/tests-dynlink-js/dune b/compiler/tests-dynlink-js/dune
index 0e1f614bb4..cef978f92d 100644
--- a/compiler/tests-dynlink-js/dune
+++ b/compiler/tests-dynlink-js/dune
@@ -24,6 +24,8 @@
    %{bin:js_of_ocaml}
    --linkall
    %{read-strings:effects_flags.txt}
+   -w
+   no-missing-effects-backend
    -o
    %{target}
    %{dep:main.bc})))
diff --git a/compiler/tests-dynlink/dune b/compiler/tests-dynlink/dune
index cd9b8a03fb..aa5ed08c50 100644
--- a/compiler/tests-dynlink/dune
+++ b/compiler/tests-dynlink/dune
@@ -14,6 +14,8 @@
    --export
    export
    --pretty
+   -w
+   no-missing-effects-backend
    -o
    %{target}
    %{dep:main.bc})))
diff --git a/compiler/tests-toplevel/dune b/compiler/tests-toplevel/dune
index f3eb01f5fb..b9c698ebb1 100644
--- a/compiler/tests-toplevel/dune
+++ b/compiler/tests-toplevel/dune
@@ -10,7 +10,14 @@
 (rule
  (targets test_toplevel.js)
  (action
-  (run %{bin:js_of_ocaml} --toplevel %{dep:test_toplevel.bc} -o %{targets})))
+  (run
+   %{bin:js_of_ocaml}
+   --toplevel
+   -w
+   no-missing-effects-backend
+   %{dep:test_toplevel.bc}
+   -o
+   %{targets})))
 
 (rule
  (target test_toplevel.bc.js.actual)
diff --git a/dune-workspace b/dune-workspace
index 42991f68f8..46e434ee35 100644
--- a/dune-workspace
+++ b/dune-workspace
@@ -8,4 +8,6 @@
   (js_of_ocaml
 ;; enable for debugging
 ;; (flags (:standard --debug stats-debug --debug invariant))
+   (flags (:standard -w "no-missing-effects-backend"))
+   (link_flags (:standard -w "no-missing-effects-backend"))
    (runtest_alias runtest-js))))
\ No newline at end of file
diff --git a/toplevel/bin/jsoo_common.ml b/toplevel/bin/jsoo_common.ml
index 2b3a75de36..257ea1f772 100644
--- a/toplevel/bin/jsoo_common.ml
+++ b/toplevel/bin/jsoo_common.ml
@@ -52,12 +52,12 @@ let read_cmi ~dir cmi =
     with Not_found -> (
       match cmi with
       (* HACK: here a list of known "hidden" cmi from the OCaml distribution. *)
-      | "dynlink_config.cmi"
-      | "dynlink_types.cmi"
-      | "dynlink_platform_intf.cmi"
-      | "dynlink_common.cmi"
-      | "dynlink_symtable.cmi"
-      | "dynlink_compilerlibs.cmi" -> raise Not_found
+      | "Dynlink_config.cmi"
+      | "Dynlink_types.cmi"
+      | "Dynlink_platform_intf.cmi"
+      | "Dynlink_common.cmi"
+      | "Dynlink_symtable.cmi"
+      | "Dynlink_compilerlibs.cmi" -> raise Not_found
       | cmi ->
           Format.eprintf
             "Could not find cmi %s or %s in %s@."
diff --git a/toplevel/examples/eval/dune b/toplevel/examples/eval/dune
index a9f4c6827d..d21bb5628f 100644
--- a/toplevel/examples/eval/dune
+++ b/toplevel/examples/eval/dune
@@ -22,6 +22,8 @@
    %{dep:export.txt}
    --toplevel
    --pretty
+   -w
+   no-missing-effects-backend
    %{dep:eval.bc}
    -o
    %{targets})))
diff --git a/toplevel/test/dune b/toplevel/test/dune
index 13fa263adf..ece87b1f3e 100644
--- a/toplevel/test/dune
+++ b/toplevel/test/dune
@@ -27,6 +27,8 @@
    --toplevel
    --disable
    shortvar
+   -w
+   no-missing-effects-backend
    %{dep:test_toplevel1.bc}
    -o
    %{targets})))
@@ -40,6 +42,8 @@
    --no-cmis
    --disable
    shortvar
+   -w
+   no-missing-effects-backend
    %{dep:test_toplevel2.bc}
    -o
    %{targets})))