diff --git a/middle_end/flambda2/bound_identifiers/bound_for_function.ml b/middle_end/flambda2/bound_identifiers/bound_for_function.ml
index b1da1458714..0bb2a6d9950 100644
--- a/middle_end/flambda2/bound_identifiers/bound_for_function.ml
+++ b/middle_end/flambda2/bound_identifiers/bound_for_function.ml
@@ -19,8 +19,8 @@ type t =
     exn_continuation : Continuation.t;
     params : Bound_parameters.t;
     my_closure : Variable.t;
-    my_region : Variable.t;
-    my_ghost_region : Variable.t;
+    my_region : Variable.t option;
+    my_ghost_region : Variable.t option;
     my_depth : Variable.t
   }
 
@@ -39,8 +39,8 @@ let[@ocamlformat "disable"] print ppf
     Continuation.print exn_continuation
     Bound_parameters.print params
     Variable.print my_closure
-    Variable.print my_region
-    Variable.print my_ghost_region
+    (Format.pp_print_option Variable.print) my_region
+    (Format.pp_print_option Variable.print) my_ghost_region
     Variable.print my_depth
 
 let create ~return_continuation ~exn_continuation ~params ~my_closure ~my_region
@@ -49,10 +49,19 @@ let create ~return_continuation ~exn_continuation ~params ~my_closure ~my_region
   (if Flambda_features.check_invariants ()
   then
     let params_set = Bound_parameters.var_set params in
-    let my_set =
-      Variable.Set.of_list [my_closure; my_region; my_ghost_region; my_depth]
+    let my_set, expected_size =
+      let regions, num_regions =
+        match my_region, my_ghost_region with
+        | None, None -> [], 0
+        | Some region, Some ghost_region -> [region; ghost_region], 2
+        | None, Some _ | Some _, None ->
+          Misc.fatal_errorf
+            "[my_region] and [my_ghost_region] must be both present or both \
+             absent"
+      in
+      Variable.Set.of_list (my_closure :: my_depth :: regions), 2 + num_regions
     in
-    if Variable.Set.cardinal my_set <> 4
+    if Variable.Set.cardinal my_set <> expected_size
        || not (Variable.Set.is_empty (Variable.Set.inter my_set params_set))
     then
       Misc.fatal_errorf
@@ -106,10 +115,17 @@ let free_names
     Name_occurrences.add_variable free_names my_closure Name_mode.normal
   in
   let free_names =
-    Name_occurrences.add_variable free_names my_region Name_mode.normal
+    Option.fold ~none:free_names
+      ~some:(fun my_region ->
+        Name_occurrences.add_variable free_names my_region Name_mode.normal)
+      my_region
   in
   let free_names =
-    Name_occurrences.add_variable free_names my_ghost_region Name_mode.normal
+    Option.fold ~none:free_names
+      ~some:(fun my_ghost_region ->
+        Name_occurrences.add_variable free_names my_ghost_region
+          Name_mode.normal)
+      my_ghost_region
   in
   Name_occurrences.add_variable free_names my_depth Name_mode.normal
 
@@ -130,8 +146,10 @@ let apply_renaming
   in
   let params = Bound_parameters.apply_renaming params renaming in
   let my_closure = Renaming.apply_variable renaming my_closure in
-  let my_region = Renaming.apply_variable renaming my_region in
-  let my_ghost_region = Renaming.apply_variable renaming my_ghost_region in
+  let my_region = Option.map (Renaming.apply_variable renaming) my_region in
+  let my_ghost_region =
+    Option.map (Renaming.apply_variable renaming) my_ghost_region
+  in
   let my_depth = Renaming.apply_variable renaming my_depth in
   (* CR mshinwell: this should have a phys-equal check *)
   { return_continuation;
@@ -158,8 +176,17 @@ let ids_for_export
   let ids = Ids_for_export.add_continuation ids exn_continuation in
   let ids = Ids_for_export.union ids (Bound_parameters.ids_for_export params) in
   let ids = Ids_for_export.add_variable ids my_closure in
-  let ids = Ids_for_export.add_variable ids my_region in
-  let ids = Ids_for_export.add_variable ids my_ghost_region in
+  let ids =
+    Option.fold ~none:ids
+      ~some:(fun my_region -> Ids_for_export.add_variable ids my_region)
+      my_region
+  in
+  let ids =
+    Option.fold ~none:ids
+      ~some:(fun my_ghost_region ->
+        Ids_for_export.add_variable ids my_ghost_region)
+      my_ghost_region
+  in
   Ids_for_export.add_variable ids my_depth
 
 let rename
@@ -175,8 +202,8 @@ let rename
     exn_continuation = Continuation.rename exn_continuation;
     params = Bound_parameters.rename params;
     my_closure = Variable.rename my_closure;
-    my_region = Variable.rename my_region;
-    my_ghost_region = Variable.rename my_ghost_region;
+    my_region = Option.map (Variable.rename ?append:None) my_region;
+    my_ghost_region = Option.map (Variable.rename ?append:None) my_ghost_region;
     my_depth = Variable.rename my_depth
   }
 
@@ -216,10 +243,21 @@ let renaming
       ~guaranteed_fresh:my_closure2
   in
   let renaming =
-    Renaming.add_fresh_variable renaming my_region1 ~guaranteed_fresh:my_region2
+    match my_region1, my_region2 with
+    | None, None -> renaming
+    | Some my_region1, Some my_region2 ->
+      Renaming.add_fresh_variable renaming my_region1
+        ~guaranteed_fresh:my_region2
+    | None, Some _ | Some _, None ->
+      Misc.fatal_error "Mismatched [my_region] field in renaming"
   in
   let renaming =
-    Renaming.add_fresh_variable renaming my_ghost_region1
-      ~guaranteed_fresh:my_ghost_region2
+    match my_ghost_region1, my_ghost_region2 with
+    | None, None -> renaming
+    | Some my_ghost_region1, Some my_ghost_region2 ->
+      Renaming.add_fresh_variable renaming my_ghost_region1
+        ~guaranteed_fresh:my_ghost_region2
+    | None, Some _ | Some _, None ->
+      Misc.fatal_error "Mismatched [my_ghost_region] field in renaming"
   in
   Renaming.add_fresh_variable renaming my_depth1 ~guaranteed_fresh:my_depth2
diff --git a/middle_end/flambda2/bound_identifiers/bound_for_function.mli b/middle_end/flambda2/bound_identifiers/bound_for_function.mli
index c3e52e04699..6be44d305a0 100644
--- a/middle_end/flambda2/bound_identifiers/bound_for_function.mli
+++ b/middle_end/flambda2/bound_identifiers/bound_for_function.mli
@@ -24,8 +24,8 @@ val create :
   exn_continuation:Continuation.t ->
   params:Bound_parameters.t ->
   my_closure:Variable.t ->
-  my_region:Variable.t ->
-  my_ghost_region:Variable.t ->
+  my_region:Variable.t option ->
+  my_ghost_region:Variable.t option ->
   my_depth:Variable.t ->
   t
 
@@ -37,9 +37,9 @@ val params : t -> Bound_parameters.t
 
 val my_closure : t -> Variable.t
 
-val my_region : t -> Variable.t
+val my_region : t -> Variable.t option
 
-val my_ghost_region : t -> Variable.t
+val my_ghost_region : t -> Variable.t option
 
 val my_depth : t -> Variable.t
 
diff --git a/middle_end/flambda2/from_lambda/closure_conversion.ml b/middle_end/flambda2/from_lambda/closure_conversion.ml
index 4af35523526..9e1a8ba6fea 100644
--- a/middle_end/flambda2/from_lambda/closure_conversion.ml
+++ b/middle_end/flambda2/from_lambda/closure_conversion.ml
@@ -571,9 +571,9 @@ let close_c_call acc env ~loc ~let_bound_ids_with_kinds
         prim_is_layout_poly
       } :
        Lambda.external_call_description) as prim_desc)
-    ~(args : Simple.t list list) exn_continuation dbg ~current_region
-    ~current_ghost_region (k : Acc.t -> Named.t list -> Expr_with_acc.t) :
-    Expr_with_acc.t =
+    ~(args : Simple.t list list) exn_continuation dbg
+    ~(current_region : Variable.t option) ~current_ghost_region
+    (k : Acc.t -> Named.t list -> Expr_with_acc.t) : Expr_with_acc.t =
   if prim_is_layout_poly
   then
     Misc.fatal_errorf
@@ -1098,11 +1098,16 @@ let close_named acc env ~let_bound_ids_with_kinds (named : IR.named)
       prim Debuginfo.none k
   | Begin_region { is_try_region; ghost; parent_region } ->
     let prim : Lambda_to_flambda_primitives_helpers.expr_primitive =
+      let arg : Lambda_to_flambda_primitives_helpers.simple_or_prim list =
+        match parent_region with
+        | None -> []
+        | Some parent_region -> [Simple (find_simple_from_id env parent_region)]
+      in
       Variadic
         ( (if is_try_region
           then Begin_try_region { ghost }
           else Begin_region { ghost }),
-          [Simple (find_simple_from_id env parent_region)] )
+          arg )
     in
     Lambda_to_flambda_primitives_helpers.bind_recs acc None ~register_const0
       prim Debuginfo.none k
@@ -1118,10 +1123,12 @@ let close_named acc env ~let_bound_ids_with_kinds (named : IR.named)
     Lambda_to_flambda_primitives_helpers.bind_recs acc None ~register_const0
       prim Debuginfo.none k
   | Prim { prim; args; loc; exn_continuation; region; ghost_region } ->
+    let get_region_ident region =
+      Option.map (fun region -> fst (Env.find_var env region)) region
+    in
     close_primitive acc env ~let_bound_ids_with_kinds named prim ~args loc
-      exn_continuation
-      ~current_region:(fst (Env.find_var env region))
-      ~current_ghost_region:(fst (Env.find_var env ghost_region))
+      exn_continuation ~current_region:(get_region_ident region)
+      ~current_ghost_region:(get_region_ident ghost_region)
       k
 
 type simplified_block_load =
@@ -1499,12 +1506,16 @@ let close_exact_or_unknown_apply acc env
      } :
       IR.apply) callee_approx ~replace_region : Expr_with_acc.t =
   let callee = find_simple_from_id env func in
-  let current_region, current_ghost_region =
-    match replace_region with
-    | None -> fst (Env.find_var env region), fst (Env.find_var env ghost_region)
-    | Some (region, ghost_region) -> region, ghost_region
-  in
   let mode =
+    let current_region, current_ghost_region =
+      match replace_region with
+      | None ->
+        let convert_region region =
+          Option.map (fun region -> fst (Env.find_var env region)) region
+        in
+        convert_region region, convert_region ghost_region
+      | Some (region, ghost_region) -> Some region, Some ghost_region
+    in
     Alloc_mode.For_applications.from_lambda mode ~current_region
       ~current_ghost_region
   in
@@ -1928,8 +1939,9 @@ let compute_body_of_unboxed_function acc my_region my_closure
 
 let make_unboxed_function_wrapper acc function_slot ~unarized_params:params
     params_arity ~unarized_param_modes:param_modes return result_arity_main_code
-    code_id main_code_id decl loc external_env recursive cost_metrics dbg
-    is_tupled inlining_decision absolute_history relative_history main_code
+    code_id main_code_id decl loc external_env recursive
+    contains_no_escaping_local_allocs cost_metrics dbg is_tupled
+    inlining_decision absolute_history relative_history main_code
     by_function_slot function_code_ids unboxed_function_slot unboxed_params
     unboxed_return =
   (* The outside caller gave us the function slot and code ID meant for the
@@ -1941,8 +1953,16 @@ let make_unboxed_function_wrapper acc function_slot ~unarized_params:params
   let return_continuation = Continuation.create () in
   let exn_continuation = Continuation.create () in
   let my_closure = Variable.create "my_closure" in
-  let my_region = Variable.create "my_region" in
-  let my_ghost_region = Variable.create "my_ghost_region" in
+  let my_region =
+    if contains_no_escaping_local_allocs
+    then None
+    else Some (Variable.create "my_region")
+  in
+  let my_ghost_region =
+    if contains_no_escaping_local_allocs
+    then None
+    else Some (Variable.create "my_ghost_region")
+  in
   let my_depth = Variable.create "my_depth" in
   let rec unbox_params params params_unboxing =
     match params, params_unboxing with
@@ -2115,8 +2135,8 @@ let make_unboxed_function_wrapper acc function_slot ~unarized_params:params
     Name_occurrences.remove_continuation ~continuation:return_continuation
       (Name_occurrences.remove_continuation ~continuation:exn_continuation
          (Name_occurrences.remove_var ~var:my_closure
-            (Name_occurrences.remove_var ~var:my_region
-               (Name_occurrences.remove_var ~var:my_ghost_region
+            (Name_occurrences.remove_var_opt ~var:my_region
+               (Name_occurrences.remove_var_opt ~var:my_ghost_region
                   (Name_occurrences.remove_var ~var:my_depth
                      (List.fold_left
                         (fun free_names param ->
@@ -2134,7 +2154,7 @@ let make_unboxed_function_wrapper acc function_slot ~unarized_params:params
       ~contains_no_escaping_local_allocs:
         (match Function_decl.result_mode decl with
         | Alloc_heap -> true
-        | Alloc_local -> true)
+        | Alloc_local -> false)
       ~stub:true ~inline:Inline_attribute.Default_inline
       ~poll_attribute:
         (Poll_attribute.from_lambda (Function_decl.poll_attribute decl))
@@ -2310,12 +2330,24 @@ let close_one_function acc ~code_id ~external_env ~by_function_slot
       unarized_params closure_env
   in
   let closure_env, my_region =
-    Env.add_var_like closure_env my_region Not_user_visible
-      K.With_subkind.region
+    match my_region with
+    | None -> closure_env, None
+    | Some my_region ->
+      let env, region =
+        Env.add_var_like closure_env my_region Not_user_visible
+          K.With_subkind.region
+      in
+      env, Some region
   in
   let closure_env, my_ghost_region =
-    Env.add_var_like closure_env my_ghost_region Not_user_visible
-      K.With_subkind.region
+    match my_ghost_region with
+    | None -> closure_env, None
+    | Some my_ghost_region ->
+      let env, region =
+        Env.add_var_like closure_env my_ghost_region Not_user_visible
+          K.With_subkind.region
+      in
+      env, Some region
   in
   let closure_env = Env.with_depth closure_env my_depth in
   let closure_env, absolute_history, relative_history =
@@ -2456,26 +2488,20 @@ let close_one_function acc ~code_id ~external_env ~by_function_slot
       ~my_depth ~free_names_of_body:(Known free_names_of_body)
   in
   let result_mode = Function_decl.result_mode decl in
-  if Name_occurrences.mem_var free_names_of_body my_region
-     && Lambda.is_heap_mode result_mode
-  then
-    Misc.fatal_errorf
-      "Unexpected free my_region in code (%a) with heap result mode:\n%a"
-      Code_id.print code_id Function_params_and_body.print params_and_body;
-  if Name_occurrences.mem_var free_names_of_body my_ghost_region
-     && Lambda.is_heap_mode result_mode
-  then
-    Misc.fatal_errorf
-      "Unexpected free my_ghost_region in code (%a) with heap result mode:\n%a"
-      Code_id.print code_id Function_params_and_body.print params_and_body;
+  (match my_region with
+  | Some _ -> assert (not (Lambda.is_heap_mode result_mode))
+  | None -> assert (Lambda.is_heap_mode result_mode));
+  (match my_ghost_region with
+  | Some _ -> assert (not (Lambda.is_heap_mode result_mode))
+  | None -> assert (Lambda.is_heap_mode result_mode));
   let acc =
     List.fold_left
       (fun acc param -> Acc.remove_var_from_free_names (BP.var param) acc)
       acc
       (Bound_parameters.to_list main_code_unarized_params)
     |> Acc.remove_var_from_free_names my_closure
-    |> Acc.remove_var_from_free_names my_region
-    |> Acc.remove_var_from_free_names my_ghost_region
+    |> Acc.remove_var_opt_from_free_names my_region
+    |> Acc.remove_var_opt_from_free_names my_ghost_region
     |> Acc.remove_var_from_free_names my_depth
     |> Acc.remove_continuation_from_free_names return_continuation
     |> Acc.remove_continuation_from_free_names
@@ -2506,6 +2532,9 @@ let close_one_function acc ~code_id ~external_env ~by_function_slot
     | Normal_calling_convention -> code_id
     | Unboxed_calling_convention _ -> Code_id.rename code_id
   in
+  let contains_no_escaping_local_allocs =
+    Function_decl.contains_no_escaping_local_allocs decl
+  in
   let main_code =
     Code.create main_code_id ~params_and_body
       ~free_names_of_params_and_body:(Acc.free_names acc)
@@ -2513,9 +2542,7 @@ let close_one_function acc ~code_id ~external_env ~by_function_slot
       ~param_modes:main_code_unarized_param_modes
       ~first_complex_local_param:first_complex_local_param_main_code
       ~result_arity:result_arity_main_code ~result_types:Unknown ~result_mode
-      ~contains_no_escaping_local_allocs:
-        (Function_decl.contains_no_escaping_local_allocs decl)
-      ~stub ~inline
+      ~contains_no_escaping_local_allocs ~stub ~inline
       ~poll_attribute:
         (Poll_attribute.from_lambda (Function_decl.poll_attribute decl))
       ~zero_alloc_attribute:
@@ -2539,7 +2566,8 @@ let close_one_function acc ~code_id ~external_env ~by_function_slot
         (unboxed_params, unboxed_return, unboxed_function_slot) ->
       make_unboxed_function_wrapper acc function_slot ~unarized_params
         params_arity ~unarized_param_modes return result_arity_main_code code_id
-        main_code_id decl loc external_env recursive cost_metrics dbg is_tupled
+        main_code_id decl loc external_env recursive
+        contains_no_escaping_local_allocs cost_metrics dbg is_tupled
         inlining_decision absolute_history relative_history main_code
         by_function_slot function_code_ids unboxed_function_slot unboxed_params
         unboxed_return
@@ -2799,7 +2827,9 @@ let close_functions acc external_env ~current_region function_declarations =
 
 let close_let_rec acc env ~function_declarations
     ~(body : Acc.t -> Env.t -> Expr_with_acc.t) ~current_region =
-  let current_region = fst (Env.find_var env current_region) in
+  let current_region =
+    Option.map (fun region -> fst (Env.find_var env region)) current_region
+  in
   let env =
     List.fold_right
       (fun decl env ->
@@ -2956,6 +2986,16 @@ let wrap_partial_application acc env apply_continuation (apply : IR.apply)
   let all_args =
     provided @ List.map (fun (p : Function_decl.param) -> IR.Var p.name) params
   in
+  let my_region =
+    if contains_no_escaping_local_allocs
+    then None
+    else Some (Ident.create_local "my_region")
+  in
+  let my_ghost_region =
+    if contains_no_escaping_local_allocs
+    then None
+    else Some (Ident.create_local "my_ghost_region")
+  in
   let fbody acc env =
     close_exact_or_unknown_apply acc env
       { apply with
@@ -2966,7 +3006,9 @@ let wrap_partial_application acc env apply_continuation (apply : IR.apply)
         exn_continuation;
         inlined = Lambda.Default_inlined;
         mode = result_mode;
-        return_arity = result_arity
+        return_arity = result_arity;
+        region = my_region;
+        ghost_region = my_ghost_region
       }
       (Some approx) ~replace_region:None
   in
@@ -3017,11 +3059,10 @@ let wrap_partial_application acc env apply_continuation (apply : IR.apply)
                })
           ~params ~params_arity ~removed_params:Ident.Set.empty
           ~return:result_arity ~calling_convention:Normal_calling_convention
-          ~return_continuation ~exn_continuation ~my_region:apply.region
-          ~my_ghost_region:apply.ghost_region ~body:fbody ~attr ~loc:apply.loc
-          ~free_idents_of_body ~closure_alloc_mode ~first_complex_local_param
-          ~result_mode ~contains_no_escaping_local_allocs
-          Recursive.Non_recursive ]
+          ~return_continuation ~exn_continuation ~my_region ~my_ghost_region
+          ~body:fbody ~attr ~loc:apply.loc ~free_idents_of_body
+          ~closure_alloc_mode ~first_complex_local_param ~result_mode
+          ~contains_no_escaping_local_allocs Recursive.Non_recursive ]
     in
     let body acc env =
       let arg = find_simple_from_id env wrapper_id in
@@ -3057,9 +3098,11 @@ let wrap_over_application acc env full_call (apply : IR.apply) ~remaining
   let apply_region, apply_ghost_region =
     match needs_region with
     | None ->
-      ( fst (Env.find_var env apply.region),
-        fst (Env.find_var env apply.ghost_region) )
-    | Some (region, ghost_region, _) -> region, ghost_region
+      ( Option.map (fun region -> fst (Env.find_var env region)) apply.region,
+        Option.map
+          (fun region -> fst (Env.find_var env region))
+          apply.ghost_region )
+    | Some (region, ghost_region, _) -> Some region, Some ghost_region
   in
   let perform_over_application acc =
     let acc, apply_exn_continuation =
@@ -3302,6 +3345,12 @@ let close_apply acc env (apply : IR.apply) : Expr_with_acc.t =
     | Over_app { full; provided_arity; remaining; remaining_arity; result_mode }
       ->
       let full_args_call apply_continuation ~region ~ghost_region acc =
+        let replace_region =
+          match region, ghost_region with
+          | None, None -> None
+          | Some region, Some ghost_region -> Some (region, ghost_region)
+          | Some _, None | None, Some _ -> Misc.fatal_error "Mismatched regions"
+        in
         close_exact_or_unknown_apply acc env
           { apply with
             args = full;
@@ -3312,8 +3361,7 @@ let close_apply acc env (apply : IR.apply) : Expr_with_acc.t =
               Flambda_arity.create_singletons
                 [Flambda_kind.With_subkind.any_value]
           }
-          (Some approx)
-          ~replace_region:(Some (region, ghost_region))
+          (Some approx) ~replace_region
       in
       wrap_over_application acc env full_args_call apply ~remaining
         ~remaining_arity ~result_mode)
diff --git a/middle_end/flambda2/from_lambda/closure_conversion.mli b/middle_end/flambda2/from_lambda/closure_conversion.mli
index cc86bc6ac03..26eada90a02 100644
--- a/middle_end/flambda2/from_lambda/closure_conversion.mli
+++ b/middle_end/flambda2/from_lambda/closure_conversion.mli
@@ -36,7 +36,7 @@ val close_let_rec :
   Env.t ->
   function_declarations:Function_decl.t list ->
   body:(Acc.t -> Env.t -> Expr_with_acc.t) ->
-  current_region:Ident.t ->
+  current_region:Ident.t option ->
   Expr_with_acc.t
 
 val close_let_cont :
diff --git a/middle_end/flambda2/from_lambda/closure_conversion_aux.ml b/middle_end/flambda2/from_lambda/closure_conversion_aux.ml
index 9e380e6a038..c6268881df0 100644
--- a/middle_end/flambda2/from_lambda/closure_conversion_aux.ml
+++ b/middle_end/flambda2/from_lambda/closure_conversion_aux.ml
@@ -38,7 +38,7 @@ module IR = struct
     | Begin_region of
         { ghost : bool;
           is_try_region : bool;
-          parent_region : Ident.t
+          parent_region : Ident.t option
         }
     | End_region of
         { is_try_region : bool;
@@ -50,8 +50,8 @@ module IR = struct
           args : simple list list;
           loc : Lambda.scoped_location;
           exn_continuation : exn_continuation option;
-          region : Ident.t;
-          ghost_region : Ident.t
+          region : Ident.t option;
+          ghost_region : Ident.t option
         }
 
   type apply_kind =
@@ -72,8 +72,8 @@ module IR = struct
       inlined : Lambda.inlined_attribute;
       probe : Lambda.probe;
       mode : Lambda.locality_mode;
-      region : Ident.t;
-      ghost_region : Ident.t;
+      region : Ident.t option;
+      ghost_region : Ident.t option;
       args_arity : [`Complex] Flambda_arity.t;
       return_arity : [`Unarized] Flambda_arity.t
     }
@@ -619,6 +619,9 @@ module Acc = struct
   let remove_var_from_free_names var t =
     { t with free_names = Name_occurrences.remove_var t.free_names ~var }
 
+  let remove_var_opt_from_free_names var t =
+    { t with free_names = Name_occurrences.remove_var_opt t.free_names ~var }
+
   let add_continuation_application ~cont args_approx t =
     let continuation_application =
       match args_approx with
@@ -767,8 +770,8 @@ module Function_decls = struct
         calling_convention : calling_convention;
         return_continuation : Continuation.t;
         exn_continuation : IR.exn_continuation;
-        my_region : Ident.t;
-        my_ghost_region : Ident.t;
+        my_region : Ident.t option;
+        my_ghost_region : Ident.t option;
         body : Acc.t -> Env.t -> Acc.t * Flambda.Import.Expr.t;
         free_idents_of_body : Ident.Set.t;
         attr : Lambda.function_attribute;
@@ -791,6 +794,18 @@ module Function_decls = struct
         | None -> Ident.create_local "unnamed_function"
         | Some let_rec_ident -> let_rec_ident
       in
+      (match contains_no_escaping_local_allocs, my_region, my_ghost_region with
+      | true, None, None -> ()
+      | false, Some _, Some _ -> ()
+      | _, _, _ ->
+        Misc.fatal_errorf
+          "Function %a declared with contains_no_escaping_local_allocs:%b but \
+           my_region is %a and my_ghost_region is %a"
+          Ident.print let_rec_ident contains_no_escaping_local_allocs
+          (Format.pp_print_option Ident.print)
+          my_region
+          (Format.pp_print_option Ident.print)
+          my_ghost_region);
       { let_rec_ident;
         function_slot;
         kind;
diff --git a/middle_end/flambda2/from_lambda/closure_conversion_aux.mli b/middle_end/flambda2/from_lambda/closure_conversion_aux.mli
index 3b878333654..3516379e827 100644
--- a/middle_end/flambda2/from_lambda/closure_conversion_aux.mli
+++ b/middle_end/flambda2/from_lambda/closure_conversion_aux.mli
@@ -40,7 +40,7 @@ module IR : sig
     | Begin_region of
         { ghost : bool;
           is_try_region : bool;
-          parent_region : Ident.t
+          parent_region : Ident.t option
         }
     | End_region of
         { is_try_region : bool;
@@ -54,8 +54,8 @@ module IR : sig
           args : simple list list;
           loc : Lambda.scoped_location;
           exn_continuation : exn_continuation option;
-          region : Ident.t;
-          ghost_region : Ident.t
+          region : Ident.t option;
+          ghost_region : Ident.t option
         }
 
   type apply_kind =
@@ -76,8 +76,8 @@ module IR : sig
       inlined : Lambda.inlined_attribute;
       probe : Lambda.probe;
       mode : Lambda.locality_mode;
-      region : Ident.t;
-      ghost_region : Ident.t;
+      region : Ident.t option;
+      ghost_region : Ident.t option;
       args_arity : [`Complex] Flambda_arity.t;
       return_arity : [`Unarized] Flambda_arity.t
     }
@@ -263,6 +263,8 @@ module Acc : sig
 
   val remove_var_from_free_names : Variable.t -> t -> t
 
+  val remove_var_opt_from_free_names : Variable.t option -> t -> t
+
   val remove_continuation_from_free_names : Continuation.t -> t -> t
 
   val mark_continuation_as_untrackable : Continuation.t -> t -> t
@@ -354,8 +356,8 @@ module Function_decls : sig
       calling_convention:calling_convention ->
       return_continuation:Continuation.t ->
       exn_continuation:IR.exn_continuation ->
-      my_region:Ident.t ->
-      my_ghost_region:Ident.t ->
+      my_region:Ident.t option ->
+      my_ghost_region:Ident.t option ->
       body:(Acc.t -> Env.t -> Acc.t * Flambda.Import.Expr.t) ->
       attr:Lambda.function_attribute ->
       loc:Lambda.scoped_location ->
@@ -385,9 +387,9 @@ module Function_decls : sig
 
     val exn_continuation : t -> IR.exn_continuation
 
-    val my_region : t -> Ident.t
+    val my_region : t -> Ident.t option
 
-    val my_ghost_region : t -> Ident.t
+    val my_ghost_region : t -> Ident.t option
 
     val body : t -> Acc.t -> Env.t -> Acc.t * Flambda.Import.Expr.t
 
diff --git a/middle_end/flambda2/from_lambda/lambda_to_flambda.ml b/middle_end/flambda2/from_lambda/lambda_to_flambda.ml
index 5705ba8b11f..61316ac032f 100644
--- a/middle_end/flambda2/from_lambda/lambda_to_flambda.ml
+++ b/middle_end/flambda2/from_lambda/lambda_to_flambda.ml
@@ -262,19 +262,25 @@ let restore_continuation_context acc env ccenv cont ~close_current_region_early
   if close_current_region_early
   then
     let env, region_stack_elt = Env.pop_one_region env in
-    let region = Env.Region_stack_element.region region_stack_elt in
-    let ghost_region = Env.Region_stack_element.ghost_region region_stack_elt in
-    CC.close_let acc ccenv
-      [Ident.create_local "unit", Flambda_kind.With_subkind.tagged_immediate]
-      Not_user_visible
-      (End_region { is_try_region = false; region; ghost = false })
-      ~body:(fun acc ccenv ->
-        CC.close_let acc ccenv
-          [Ident.create_local "unit", Flambda_kind.With_subkind.tagged_immediate]
-          Not_user_visible
-          (End_region
-             { is_try_region = false; region = ghost_region; ghost = true })
-          ~body:(fun acc ccenv -> normal_case env acc ccenv))
+    match region_stack_elt with
+    | None -> normal_case env acc ccenv
+    | Some region_stack_elt ->
+      let region = Env.Region_stack_element.region region_stack_elt in
+      let ghost_region =
+        Env.Region_stack_element.ghost_region region_stack_elt
+      in
+      CC.close_let acc ccenv
+        [Ident.create_local "unit", Flambda_kind.With_subkind.tagged_immediate]
+        Not_user_visible
+        (End_region { is_try_region = false; region; ghost = false })
+        ~body:(fun acc ccenv ->
+          CC.close_let acc ccenv
+            [ ( Ident.create_local "unit",
+                Flambda_kind.With_subkind.tagged_immediate ) ]
+            Not_user_visible
+            (End_region
+               { is_try_region = false; region = ghost_region; ghost = true })
+            ~body:(fun acc ccenv -> normal_case env acc ccenv))
   else normal_case env acc ccenv
 
 let restore_continuation_context_for_switch_arm env cont =
@@ -306,13 +312,10 @@ let apply_cont_with_extra_args acc env ccenv ~dbg cont traps args =
 
 let wrap_return_continuation acc env ccenv (apply : IR.apply) =
   let extra_args = Env.extra_args_for_continuation env apply.continuation in
-  let close_current_region_early, region_stack_elt =
+  let close_current_region_early, region, ghost_region =
     match apply.region_close with
-    | Rc_normal | Rc_nontail ->
-      ( false,
-        Env.Region_stack_element.create ~region:apply.region
-          ~ghost_region:apply.ghost_region )
-    | Rc_close_at_apply ->
+    | Rc_normal | Rc_nontail -> false, apply.region, apply.ghost_region
+    | Rc_close_at_apply -> (
       (* [Rc_close_at_apply] means that the application is in tail position with
          respect to the *current region*. Only that region should be closed
          early, prior to the application, meaning that the region for the
@@ -320,13 +323,17 @@ let wrap_return_continuation acc env ccenv (apply : IR.apply) =
          application, further regions should be closed if necessary in order to
          bring the current region stack in line with the return continuation's
          region stack. *)
-      true, Env.parent_region env
+      match Env.parent_region env with
+      | None -> true, None, None
+      | Some region_stack_elt ->
+        ( true,
+          Some (Env.Region_stack_element.region region_stack_elt),
+          Some (Env.Region_stack_element.ghost_region region_stack_elt) ))
   in
-  let region = Env.Region_stack_element.region region_stack_elt in
-  let ghost_region = Env.Region_stack_element.ghost_region region_stack_elt in
   let body acc ccenv continuation =
     match extra_args with
-    | [] -> CC.close_apply acc ccenv { apply with continuation; region }
+    | [] ->
+      CC.close_apply acc ccenv { apply with continuation; region; ghost_region }
     | _ :: _ ->
       let wrapper_cont = Continuation.create () in
       let return_kinds = Flambda_arity.unarized_components apply.return_arity in
@@ -488,7 +495,8 @@ let rec cps acc env ccenv (lam : L.lambda) (k : cps_continuation)
         (Singleton Flambda_kind.With_subkind.any_value)
     in
     CC.close_let_rec acc ccenv ~function_declarations:[func] ~body
-      ~current_region:(Env.current_region env |> Env.Region_stack_element.region)
+      ~current_region:
+        (Env.current_region env |> Option.map Env.Region_stack_element.region)
   | Lmutlet (value_kind, id, defining_expr, body) ->
     (* CR mshinwell: user-visibleness needs thinking about here *)
     let temp_id = Ident.create_local "let_mutable" in
@@ -515,7 +523,8 @@ let rec cps acc env ccenv (lam : L.lambda) (k : cps_continuation)
         (fun body func acc ccenv ->
           CC.close_let_rec acc ccenv ~function_declarations:[func] ~body
             ~current_region:
-              (Env.current_region env |> Env.Region_stack_element.region))
+              (Env.current_region env
+              |> Option.map Env.Region_stack_element.region))
         body bindings
     in
     let_expr acc ccenv
@@ -578,9 +587,11 @@ let rec cps acc env ccenv (lam : L.lambda) (k : cps_continuation)
           in
           let body acc ccenv = cps acc env ccenv body k k_exn in
           let current_region = Env.current_region env in
-          let region = Env.Region_stack_element.region current_region in
+          let region =
+            Option.map Env.Region_stack_element.region current_region
+          in
           let ghost_region =
-            Env.Region_stack_element.ghost_region current_region
+            Option.map Env.Region_stack_element.ghost_region current_region
           in
           CC.close_let acc ccenv ids_with_kinds (is_user_visible env id)
             (Prim { prim; args; loc; exn_continuation; region; ghost_region })
@@ -641,7 +652,8 @@ let rec cps acc env ccenv (lam : L.lambda) (k : cps_continuation)
     let function_declarations = cps_function_bindings env bindings in
     let body acc ccenv = cps acc env ccenv body k k_exn in
     CC.close_let_rec acc ccenv ~function_declarations ~body
-      ~current_region:(Env.current_region env |> Env.Region_stack_element.region)
+      ~current_region:
+        (Env.current_region env |> Option.map Env.Region_stack_element.region)
   | Lprim (prim, args, loc) -> (
     match[@ocaml.warning "-fragile-match"] prim with
     | Praise raise_kind -> (
@@ -794,9 +806,12 @@ let rec cps acc env ccenv (lam : L.lambda) (k : cps_continuation)
                         inlined = Default_inlined;
                         probe = None;
                         mode;
-                        region = Env.Region_stack_element.region current_region;
+                        region =
+                          Option.map Env.Region_stack_element.region
+                            current_region;
                         ghost_region =
-                          Env.Region_stack_element.ghost_region current_region;
+                          Option.map Env.Region_stack_element.ghost_region
+                            current_region;
                         args_arity = Flambda_arity.create args_arity;
                         return_arity =
                           Flambda_arity.unarize_t
@@ -853,7 +868,8 @@ let rec cps acc env ccenv (lam : L.lambda) (k : cps_continuation)
         (Begin_region
            { is_try_region = true;
              ghost = false;
-             parent_region = Env.Region_stack_element.region region_stack_elt
+             parent_region =
+               Option.map Env.Region_stack_element.region region_stack_elt
            })
         ~body:(fun acc ccenv ->
           CC.close_let acc ccenv
@@ -863,7 +879,8 @@ let rec cps acc env ccenv (lam : L.lambda) (k : cps_continuation)
                { is_try_region = true;
                  ghost = true;
                  parent_region =
-                   Env.Region_stack_element.ghost_region region_stack_elt
+                   Option.map Env.Region_stack_element.ghost_region
+                     region_stack_elt
                })
             ~body)
     in
@@ -952,7 +969,11 @@ let rec cps acc env ccenv (lam : L.lambda) (k : cps_continuation)
   | Lregion (body, _) when not (Flambda_features.stack_allocation_enabled ()) ->
     cps acc env ccenv body k k_exn
   | Lexclave body ->
-    let current_region = Env.current_region env in
+    let current_region =
+      match Env.current_region env with
+      | Some region -> region
+      | None -> Misc.fatal_error "Lexclave in a context with no current region"
+    in
     let region = Env.Region_stack_element.region current_region in
     let ghost_region = Env.Region_stack_element.ghost_region current_region in
     CC.close_let acc ccenv
@@ -985,7 +1006,8 @@ let rec cps acc env ccenv (lam : L.lambda) (k : cps_continuation)
       (Begin_region
          { is_try_region = false;
            ghost = false;
-           parent_region = Env.Region_stack_element.region parent_stack_elt
+           parent_region =
+             Option.map Env.Region_stack_element.region parent_stack_elt
          })
       ~body:(fun acc ccenv ->
         CC.close_let acc ccenv
@@ -995,7 +1017,8 @@ let rec cps acc env ccenv (lam : L.lambda) (k : cps_continuation)
              { is_try_region = false;
                ghost = true;
                parent_region =
-                 Env.Region_stack_element.ghost_region parent_stack_elt
+                 Option.map Env.Region_stack_element.ghost_region
+                   parent_stack_elt
              })
           ~body:(fun acc ccenv ->
             maybe_insert_let_cont "body_return" layout k acc env ccenv
@@ -1117,9 +1140,9 @@ and cps_tail_apply acc env ccenv ap_func ap_args ap_region_close ap_mode ap_loc
               inlined = ap_inlined;
               probe = ap_probe;
               mode = ap_mode;
-              region = Env.Region_stack_element.region current_region;
+              region = Option.map Env.Region_stack_element.region current_region;
               ghost_region =
-                Env.Region_stack_element.ghost_region current_region;
+                Option.map Env.Region_stack_element.ghost_region current_region;
               args_arity = Flambda_arity.create args_arity;
               return_arity =
                 Flambda_arity.unarize_t
@@ -1237,8 +1260,11 @@ and cps_function_bindings env (bindings : Lambda.rec_binding list) =
     bindings_with_wrappers
 
 and cps_function env ~fid ~(recursive : Recursive.t) ?precomputed_free_idents
-    ({ kind; params; return; body; attr; loc; mode; ret_mode; region } :
+    ({ kind; params; return; body; attr; loc; mode; ret_mode; region = _ } :
       L.lfunction) : Function_decl.t =
+  let contains_no_escaping_local_allocs =
+    match ret_mode with Alloc_heap -> true | Alloc_local -> false
+  in
   let first_complex_local_param =
     List.length params
     - match kind with Curried { nlocal } -> nlocal | Tupled -> 0
@@ -1365,11 +1391,17 @@ and cps_function env ~fid ~(recursive : Recursive.t) ?precomputed_free_idents
     | Some ids -> ids
     | None -> Lambda.free_variables body
   in
-  let my_region = Ident.create_local "my_region" in
-  let my_ghost_region = Ident.create_local "my_ghost_region" in
-  let my_region_stack_elt =
-    Env.Region_stack_element.create ~region:my_region
-      ~ghost_region:my_ghost_region
+  let my_region_stack_elt, my_region, my_ghost_region =
+    if contains_no_escaping_local_allocs
+    then None, None, None
+    else
+      let my_region = Ident.create_local "my_region" in
+      let my_ghost_region = Ident.create_local "my_ghost_region" in
+      ( Some
+          (Env.Region_stack_element.create ~region:my_region
+             ~ghost_region:my_ghost_region),
+        Some my_region,
+        Some my_ghost_region )
   in
   let new_env =
     Env.create ~current_unit:(Env.current_unit env)
@@ -1448,7 +1480,7 @@ and cps_function env ~fid ~(recursive : Recursive.t) ?precomputed_free_idents
     ~params_arity ~removed_params ~return ~calling_convention
     ~return_continuation:body_cont ~exn_continuation ~my_region ~my_ghost_region
     ~body ~attr ~loc ~free_idents_of_body recursive ~closure_alloc_mode:mode
-    ~first_complex_local_param ~contains_no_escaping_local_allocs:region
+    ~first_complex_local_param ~contains_no_escaping_local_allocs
     ~result_mode:ret_mode
 
 and cps_switch acc env ccenv (switch : L.lambda_switch) ~condition_dbg
@@ -1588,9 +1620,11 @@ and cps_switch acc env ccenv (switch : L.lambda_switch) ~condition_dbg
                 isint_switch
             in
             let current_region = Env.current_region env in
-            let region = Env.Region_stack_element.region current_region in
+            let region =
+              Option.map Env.Region_stack_element.region current_region
+            in
             let ghost_region =
-              Env.Region_stack_element.ghost_region current_region
+              Option.map Env.Region_stack_element.ghost_region current_region
             in
             CC.close_let acc ccenv
               [is_scrutinee_int, Flambda_kind.With_subkind.tagged_immediate]
@@ -1630,13 +1664,9 @@ let lambda_to_flambda ~mode ~big_endian ~cmx_loader ~compilation_unit
   let toplevel_my_ghost_region =
     Ident.create_local "toplevel_my_ghost_region"
   in
-  let toplevel_my_region_stack_elt =
-    Env.Region_stack_element.create ~region:toplevel_my_region
-      ~ghost_region:toplevel_my_ghost_region
-  in
   let env =
     Env.create ~current_unit:compilation_unit ~return_continuation
-      ~exn_continuation ~my_region:toplevel_my_region_stack_elt
+      ~exn_continuation ~my_region:None
   in
   let program acc ccenv =
     cps_tail acc env ccenv lam return_continuation exn_continuation
diff --git a/middle_end/flambda2/from_lambda/lambda_to_flambda_env.ml b/middle_end/flambda2/from_lambda/lambda_to_flambda_env.ml
index b7419de1373..c43cc71a3d0 100644
--- a/middle_end/flambda2/from_lambda/lambda_to_flambda_env.ml
+++ b/middle_end/flambda2/from_lambda/lambda_to_flambda_env.ml
@@ -80,9 +80,7 @@ type t =
     try_stack_at_handler : Continuation.t list Continuation.Map.t;
     static_exn_continuation : Continuation.t Numeric_types.Int.Map.t;
     recursive_static_catches : Numeric_types.Int.Set.t;
-    my_region : Region_stack_element.t;
-    (* CR-someday ncourant/mshinwell: replace this with [my_region:
-       Region_stack_element.t option] *)
+    my_region : Region_stack_element.t option;
     region_stack : Region_stack_element.t list;
     region_stack_in_cont_scope : Region_stack_element.t list Continuation.Map.t;
     region_closure_continuations :
@@ -309,7 +307,7 @@ let current_region t =
   else
     match t.region_stack with
     | [] -> t.my_region
-    | region_stack_elt :: _ -> region_stack_elt
+    | region_stack_elt :: _ -> Some region_stack_elt
 
 let parent_region t =
   if not (Flambda_features.stack_allocation_enabled ())
@@ -319,7 +317,7 @@ let parent_region t =
     | [] ->
       Misc.fatal_error "Cannot determine parent region, region stack is empty"
     | [_] -> t.my_region
-    | _ :: region :: _ -> region
+    | _ :: region :: _ -> Some region
 
 let my_region t = t.my_region
 
@@ -340,7 +338,7 @@ let pop_one_region t =
   else
     match t.region_stack with
     | [] -> Misc.fatal_error "No regions available to pop"
-    | region :: region_stack -> { t with region_stack }, region
+    | region :: region_stack -> { t with region_stack }, Some region
 
 let pop_regions_up_to_context t continuation =
   let initial_stack_context = region_stack_in_cont_scope t continuation in
diff --git a/middle_end/flambda2/from_lambda/lambda_to_flambda_env.mli b/middle_end/flambda2/from_lambda/lambda_to_flambda_env.mli
index 049e7c922e5..3d6ce56da30 100644
--- a/middle_end/flambda2/from_lambda/lambda_to_flambda_env.mli
+++ b/middle_end/flambda2/from_lambda/lambda_to_flambda_env.mli
@@ -32,7 +32,7 @@ val create :
   current_unit:Compilation_unit.t ->
   return_continuation:Continuation.t ->
   exn_continuation:Continuation.t ->
-  my_region:Region_stack_element.t ->
+  my_region:Region_stack_element.t option ->
   t
 
 val current_unit : t -> Compilation_unit.t
@@ -162,15 +162,16 @@ val entering_region :
 val leaving_region : t -> t
 
 (** The region stack element corresponding to the [my_region] parameter of
-    the current function, or the toplevel region stack element created by
-    simplify.ml. *)
-val my_region : t -> Region_stack_element.t
+    the current function, if relevant.
+    The toplevel expression doesn't have such a variable, and functions
+    that cannot allocate in the parent region may not have one either. *)
+val my_region : t -> Region_stack_element.t option
 
 (** The current region stack element, to be used for allocation etc. *)
-val current_region : t -> Region_stack_element.t
+val current_region : t -> Region_stack_element.t option
 
 (** The region stack element immediately outside [current_region]. *)
-val parent_region : t -> Region_stack_element.t
+val parent_region : t -> Region_stack_element.t option
 
 (** The innermost (newest) region is first in the list. *)
 val region_stack : t -> Region_stack_element.t list
@@ -178,7 +179,7 @@ val region_stack : t -> Region_stack_element.t list
 val region_stack_in_cont_scope :
   t -> Continuation.t -> Region_stack_element.t list
 
-val pop_one_region : t -> t * Region_stack_element.t
+val pop_one_region : t -> t * Region_stack_element.t option
 
 (** Hack for staticfail (which should eventually use
       [pop_regions_up_to_context]) *)
diff --git a/middle_end/flambda2/from_lambda/lambda_to_flambda_primitives.mli b/middle_end/flambda2/from_lambda/lambda_to_flambda_primitives.mli
index 70cbfc63835..3968ce3ab2f 100644
--- a/middle_end/flambda2/from_lambda/lambda_to_flambda_primitives.mli
+++ b/middle_end/flambda2/from_lambda/lambda_to_flambda_primitives.mli
@@ -27,7 +27,7 @@ val convert_and_bind :
   Lambda.primitive ->
   args:Simple.t list list ->
   Debuginfo.t ->
-  current_region:Variable.t ->
-  current_ghost_region:Variable.t ->
+  current_region:Variable.t option ->
+  current_ghost_region:Variable.t option ->
   (Acc.t -> Flambda.Named.t list -> Expr_with_acc.t) ->
   Expr_with_acc.t
diff --git a/middle_end/flambda2/nominal/name_occurrences.ml b/middle_end/flambda2/nominal/name_occurrences.ml
index 37c53aad463..63d95bbc38c 100644
--- a/middle_end/flambda2/nominal/name_occurrences.ml
+++ b/middle_end/flambda2/nominal/name_occurrences.ml
@@ -831,6 +831,9 @@ let remove_var t ~var =
     let names = For_names.remove t.names (Name.var var) in
     { t with names }
 
+let remove_var_opt t ~var =
+  match var with None -> t | Some var -> remove_var t ~var
+
 let remove_symbol t ~symbol =
   if For_names.is_empty t.names
   then t
diff --git a/middle_end/flambda2/nominal/name_occurrences.mli b/middle_end/flambda2/nominal/name_occurrences.mli
index 9a445fdffbc..cc854f1a6cb 100644
--- a/middle_end/flambda2/nominal/name_occurrences.mli
+++ b/middle_end/flambda2/nominal/name_occurrences.mli
@@ -173,6 +173,8 @@ val value_slot_is_used_or_imported : t -> Value_slot.t -> bool
 
 val remove_var : t -> var:Variable.t -> t
 
+val remove_var_opt : t -> var:Variable.t option -> t
+
 val remove_code_id_or_symbol : t -> code_id_or_symbol:Code_id_or_symbol.t -> t
 
 val remove_continuation : t -> continuation:Continuation.t -> t
diff --git a/middle_end/flambda2/parser/fexpr_to_flambda.ml b/middle_end/flambda2/parser/fexpr_to_flambda.ml
index a2b30ee5271..f509ee8f680 100644
--- a/middle_end/flambda2/parser/fexpr_to_flambda.ml
+++ b/middle_end/flambda2/parser/fexpr_to_flambda.ml
@@ -917,7 +917,8 @@ let rec expr env (e : Fexpr.expr) : Flambda.Expr.t =
             Flambda.Function_params_and_body.create ~return_continuation
               ~exn_continuation:(Exn_continuation.exn_handler exn_continuation)
               (Bound_parameters.create params)
-              ~body ~my_closure ~my_region ~my_ghost_region ~my_depth
+              ~body ~my_closure ~my_region:(Some my_region)
+              ~my_ghost_region:(Some my_ghost_region) ~my_depth
               ~free_names_of_body:Unknown
           in
           let free_names =
diff --git a/middle_end/flambda2/parser/flambda_to_fexpr.ml b/middle_end/flambda2/parser/flambda_to_fexpr.ml
index af4a00318b5..7e88ba8bd86 100644
--- a/middle_end/flambda2/parser/flambda_to_fexpr.ml
+++ b/middle_end/flambda2/parser/flambda_to_fexpr.ml
@@ -918,8 +918,16 @@ and static_let_expr env bound_static defining_expr body : Fexpr.expr =
                 (Bound_parameters.to_list params)
             in
             let closure_var, env = Env.bind_var env my_closure in
-            let region_var, env = Env.bind_var env my_region in
-            let ghost_region_var, env = Env.bind_var env my_ghost_region in
+            let region_var, env =
+              match my_region with
+              | None -> nowhere "_region", env
+              | Some my_region -> Env.bind_var env my_region
+            in
+            let ghost_region_var, env =
+              match my_ghost_region with
+              | None -> nowhere "_ghost_region", env
+              | Some my_ghost_region -> Env.bind_var env my_ghost_region
+            in
             let depth_var, env = Env.bind_var env my_depth in
             let body = expr env body in
             (* CR-someday lmaurer: Omit exn_cont, closure_var if not used *)
diff --git a/middle_end/flambda2/reaper/rev_expr.ml b/middle_end/flambda2/reaper/rev_expr.ml
index 0dac11a7b80..2c3f4f14954 100644
--- a/middle_end/flambda2/reaper/rev_expr.ml
+++ b/middle_end/flambda2/reaper/rev_expr.ml
@@ -59,8 +59,8 @@ and rev_params_and_body =
     params : Bound_parameters.t;
     body : rev_expr;
     my_closure : Variable.t;
-    my_region : Variable.t;
-    my_ghost_region : Variable.t;
+    my_region : Variable.t option;
+    my_ghost_region : Variable.t option;
     my_depth : Variable.t
   }
 
diff --git a/middle_end/flambda2/reaper/rev_expr.mli b/middle_end/flambda2/reaper/rev_expr.mli
index 0dac11a7b80..2c3f4f14954 100644
--- a/middle_end/flambda2/reaper/rev_expr.mli
+++ b/middle_end/flambda2/reaper/rev_expr.mli
@@ -59,8 +59,8 @@ and rev_params_and_body =
     params : Bound_parameters.t;
     body : rev_expr;
     my_closure : Variable.t;
-    my_region : Variable.t;
-    my_ghost_region : Variable.t;
+    my_region : Variable.t option;
+    my_ghost_region : Variable.t option;
     my_depth : Variable.t
   }
 
diff --git a/middle_end/flambda2/reaper/traverse.ml b/middle_end/flambda2/reaper/traverse.ml
index 3d4503e2b56..363291101f6 100644
--- a/middle_end/flambda2/reaper/traverse.ml
+++ b/middle_end/flambda2/reaper/traverse.ml
@@ -664,8 +664,12 @@ and traverse_function_params_and_body acc code_id code ~return_continuation
   then List.iter (fun v -> Acc.used ~denv (Simple.var v) acc) (exn :: return);
   Bound_parameters.iter (fun bp -> Acc.bound_parameter_kind bp acc) params;
   Acc.kind (Name.var my_closure) Flambda_kind.value acc;
-  Acc.kind (Name.var my_region) Flambda_kind.region acc;
-  Acc.kind (Name.var my_ghost_region) Flambda_kind.region acc;
+  Option.iter
+    (fun region -> Acc.kind (Name.var region) Flambda_kind.region acc)
+    my_region;
+  Option.iter
+    (fun region -> Acc.kind (Name.var region) Flambda_kind.region acc)
+    my_ghost_region;
   Acc.kind (Name.var my_depth) Flambda_kind.rec_info acc;
   if is_opaque
   then
diff --git a/middle_end/flambda2/simplify/rebuilt_expr.mli b/middle_end/flambda2/simplify/rebuilt_expr.mli
index 127a1686dfc..7871ae062ce 100644
--- a/middle_end/flambda2/simplify/rebuilt_expr.mli
+++ b/middle_end/flambda2/simplify/rebuilt_expr.mli
@@ -66,8 +66,8 @@ module Function_params_and_body : sig
     body:rebuilt_expr ->
     free_names_of_body:Name_occurrences.t ->
     my_closure:Variable.t ->
-    my_region:Variable.t ->
-    my_ghost_region:Variable.t ->
+    my_region:Variable.t option ->
+    my_ghost_region:Variable.t option ->
     my_depth:Variable.t ->
     t
 
diff --git a/middle_end/flambda2/simplify/rebuilt_static_const.ml b/middle_end/flambda2/simplify/rebuilt_static_const.ml
index 5558f0304ae..db2ba4b853e 100644
--- a/middle_end/flambda2/simplify/rebuilt_static_const.ml
+++ b/middle_end/flambda2/simplify/rebuilt_static_const.ml
@@ -339,8 +339,7 @@ module Group = struct
          ~body:(Expr.create_invalid Code_not_rebuilt)
          ~free_names_of_body:Unknown
          ~my_closure:(Variable.create "my_closure")
-         ~my_region:(Variable.create "my_region")
-         ~my_ghost_region:(Variable.create "my_ghost_region")
+         ~my_region:None ~my_ghost_region:None
          ~my_depth:(Variable.create "my_depth"))
 
   let pieces_of_code_including_those_not_rebuilt t =
diff --git a/middle_end/flambda2/simplify/simplify_apply_expr.ml b/middle_end/flambda2/simplify/simplify_apply_expr.ml
index f4cc96bd669..b09cd1c2a1f 100644
--- a/middle_end/flambda2/simplify/simplify_apply_expr.ml
+++ b/middle_end/flambda2/simplify/simplify_apply_expr.ml
@@ -521,9 +521,20 @@ let simplify_direct_partial_application ~simplify_expr dacc apply
           | None -> applied_unarized_args
           | Some applied_callee -> applied_callee :: applied_unarized_args
         in
+        let contains_no_escaping_local_allocs =
+          Code_metadata.contains_no_escaping_local_allocs callee's_code_metadata
+        in
         let my_closure = Variable.create "my_closure" in
-        let my_region = Variable.create "my_region" in
-        let my_ghost_region = Variable.create "my_ghost_region" in
+        let my_region =
+          if contains_no_escaping_local_allocs
+          then None
+          else Some (Variable.create "my_region")
+        in
+        let my_ghost_region =
+          if contains_no_escaping_local_allocs
+          then None
+          else Some (Variable.create "my_ghost_region")
+        in
         let my_depth = Variable.create "my_depth" in
         let exn_continuation =
           Apply.exn_continuation apply |> Exn_continuation.without_extra_args
@@ -627,11 +638,8 @@ let simplify_direct_partial_application ~simplify_expr dacc apply
               ~params_arity:remaining_param_arity
               ~param_modes:remaining_params_alloc_modes
               ~first_complex_local_param ~result_arity ~result_types:Unknown
-              ~result_mode
-              ~contains_no_escaping_local_allocs:
-                (Code_metadata.contains_no_escaping_local_allocs
-                   callee's_code_metadata)
-              ~stub:true ~inline:Default_inline ~poll_attribute:Default
+              ~result_mode ~contains_no_escaping_local_allocs ~stub:true
+              ~inline:Default_inline ~poll_attribute:Default
               ~zero_alloc_attribute:Zero_alloc_attribute.Default_zero_alloc
               ~is_a_functor:false ~is_opaque:false ~recursive
               ~cost_metrics:cost_metrics_of_body
diff --git a/middle_end/flambda2/simplify/simplify_set_of_closures.ml b/middle_end/flambda2/simplify/simplify_set_of_closures.ml
index f012ea3fa15..0cf7cf8ca0a 100644
--- a/middle_end/flambda2/simplify/simplify_set_of_closures.ml
+++ b/middle_end/flambda2/simplify/simplify_set_of_closures.ml
@@ -65,12 +65,18 @@ let dacc_inside_function context ~outer_dacc ~params ~my_closure ~my_region
           (T.alias_type_of K.value (Simple.name name)))
   in
   let denv =
-    let my_region = Bound_var.create my_region Name_mode.normal in
-    DE.add_variable denv my_region (T.unknown K.region)
+    match my_region with
+    | None -> denv
+    | Some my_region ->
+      let my_region = Bound_var.create my_region Name_mode.normal in
+      DE.add_variable denv my_region (T.unknown K.region)
   in
   let denv =
-    let my_ghost_region = Bound_var.create my_ghost_region Name_mode.normal in
-    DE.add_variable denv my_ghost_region (T.unknown K.region)
+    match my_ghost_region with
+    | None -> denv
+    | Some my_ghost_region ->
+      let my_ghost_region = Bound_var.create my_ghost_region Name_mode.normal in
+      DE.add_variable denv my_ghost_region (T.unknown K.region)
   in
   let denv =
     let my_depth = Bound_var.create my_depth Name_mode.normal in
@@ -177,18 +183,25 @@ let simplify_function_body context ~outer_dacc function_slot_opt
     Misc.fatal_errorf "Did not expect lifted constants in [dacc]:@ %a" DA.print
       dacc;
   assert (not (DE.at_unit_toplevel (DA.denv dacc)));
+  let region_params =
+    let region_param region =
+      match region with
+      | None -> []
+      | Some region ->
+        [Bound_parameter.create region Flambda_kind.With_subkind.region]
+    in
+    region_param my_region @ region_param my_ghost_region
+  in
   match
     C.simplify_function_body context dacc body ~return_continuation
       ~exn_continuation ~return_arity:(Code.result_arity code)
       ~implicit_params:
         (Bound_parameters.create
-           [ Bound_parameter.create my_closure
-               Flambda_kind.With_subkind.any_value;
-             Bound_parameter.create my_region Flambda_kind.With_subkind.region;
-             Bound_parameter.create my_ghost_region
-               Flambda_kind.With_subkind.region;
-             Bound_parameter.create my_depth Flambda_kind.With_subkind.rec_info
-           ])
+           ([ Bound_parameter.create my_closure
+                Flambda_kind.With_subkind.any_value;
+              Bound_parameter.create my_depth Flambda_kind.With_subkind.rec_info
+            ]
+           @ region_params))
       ~loopify_state ~params
   with
   | body, uacc ->
@@ -213,27 +226,13 @@ let simplify_function_body context ~outer_dacc function_slot_opt
       then Recursive
       else Non_recursive
     in
-    if NO.mem_var free_names_of_body my_region
-       && Lambda.is_heap_mode (Code.result_mode code)
-    then
-      Misc.fatal_errorf
-        "Unexpected free my_region in code with heap result mode:\n%a"
-        (RE.print (UA.are_rebuilding_terms uacc))
-        body;
-    if NO.mem_var free_names_of_body my_ghost_region
-       && Lambda.is_heap_mode (Code.result_mode code)
-    then
-      Misc.fatal_errorf
-        "Unexpected free my_ghost_region in code with heap result mode:\n%a"
-        (RE.print (UA.are_rebuilding_terms uacc))
-        body;
     let free_names_of_code =
       free_names_of_body
       |> NO.remove_continuation ~continuation:return_continuation
       |> NO.remove_continuation ~continuation:exn_continuation
       |> NO.remove_var ~var:my_closure
-      |> NO.remove_var ~var:my_region
-      |> NO.remove_var ~var:my_ghost_region
+      |> NO.remove_var_opt ~var:my_region
+      |> NO.remove_var_opt ~var:my_ghost_region
       |> NO.remove_var ~var:my_depth
       |> NO.diff ~without:(Bound_parameters.free_names params)
       |> NO.diff ~without:previously_free_depth_variables
@@ -247,8 +246,11 @@ let simplify_function_body context ~outer_dacc function_slot_opt
          %a@ \n\
          Simplified version:@ fun %a %a %a %a %a ->@ \n\
         \  %a" NO.print free_names_of_code Code_id.print code_id
-        Bound_parameters.print params Variable.print my_closure Variable.print
-        my_region Variable.print my_ghost_region Variable.print my_depth
+        Bound_parameters.print params Variable.print my_closure
+        (Format.pp_print_option Variable.print)
+        my_region
+        (Format.pp_print_option Variable.print)
+        my_ghost_region Variable.print my_depth
         (RE.print (UA.are_rebuilding_terms uacc))
         body;
     { params;
diff --git a/middle_end/flambda2/simplify_shared/inlining_helpers.ml b/middle_end/flambda2/simplify_shared/inlining_helpers.ml
index a0c20243aa0..c3d5ad7f7e1 100644
--- a/middle_end/flambda2/simplify_shared/inlining_helpers.ml
+++ b/middle_end/flambda2/simplify_shared/inlining_helpers.ml
@@ -17,8 +17,8 @@
 open! Flambda.Import
 module RC = Apply.Result_continuation
 
-let make_inlined_body ~callee ~called_code_id:_ ~region_inlined_into ~params
-    ~args ~my_closure ~my_region ~my_ghost_region ~my_depth ~rec_info ~body
+let make_inlined_body ~callee ~called_code_id ~region_inlined_into ~params ~args
+    ~my_closure ~my_region ~my_ghost_region ~my_depth ~rec_info ~body
     ~exn_continuation ~return_continuation ~apply_exn_continuation
     ~apply_return_continuation ~bind_params ~bind_depth ~apply_renaming =
   let renaming = Renaming.empty in
@@ -36,14 +36,20 @@ let make_inlined_body ~callee ~called_code_id:_ ~region_inlined_into ~params
        [my_region] should be unused in the body. *)
     match (region_inlined_into : Alloc_mode.For_applications.t) with
     | Heap -> renaming
-    | Local { region; ghost_region } ->
+    | Local { region; ghost_region } -> (
       (* Unlike for parameters, we know that the argument for the [my_region]
          parameter is fresh for [body], so we can use a permutation without fear
          of swapping out existing occurrences of such argument within [body].
          Similarly for [ghost_region]. *)
-      Renaming.add_variable
-        (Renaming.add_variable renaming my_region region)
-        my_ghost_region ghost_region
+      match my_region, my_ghost_region with
+      | Some my_region, Some my_ghost_region ->
+        Renaming.add_variable
+          (Renaming.add_variable renaming my_region region)
+          my_ghost_region ghost_region
+      | None, None -> renaming
+      | None, Some _ | Some _, None ->
+        Misc.fatal_errorf "When inlining %a: Mismatched regions" Code_id.print
+          called_code_id)
   in
   let body =
     match callee with
diff --git a/middle_end/flambda2/simplify_shared/inlining_helpers.mli b/middle_end/flambda2/simplify_shared/inlining_helpers.mli
index 04e83ce5a82..e6788c9b37c 100644
--- a/middle_end/flambda2/simplify_shared/inlining_helpers.mli
+++ b/middle_end/flambda2/simplify_shared/inlining_helpers.mli
@@ -21,8 +21,8 @@ val make_inlined_body :
   params:'param list ->
   args:Simple.List.t ->
   my_closure:'param ->
-  my_region:Variable.t ->
-  my_ghost_region:Variable.t ->
+  my_region:Variable.t option ->
+  my_ghost_region:Variable.t option ->
   my_depth:Variable.t ->
   rec_info:Rec_info_expr.t ->
   body:'expr_with_acc ->
diff --git a/middle_end/flambda2/term_basics/alloc_mode.ml b/middle_end/flambda2/term_basics/alloc_mode.ml
index f1652fb44b7..0679d9d2fb0 100644
--- a/middle_end/flambda2/term_basics/alloc_mode.ml
+++ b/middle_end/flambda2/term_basics/alloc_mode.ml
@@ -102,8 +102,12 @@ module For_applications = struct
     else
       match mode with
       | Alloc_heap -> Heap
-      | Alloc_local ->
-        Local { region = current_region; ghost_region = current_ghost_region }
+      | Alloc_local -> (
+        match current_region, current_ghost_region with
+        | Some current_region, Some current_ghost_region ->
+          Local { region = current_region; ghost_region = current_ghost_region }
+        | None, _ | _, None ->
+          Misc.fatal_error "Local application without a region")
 
   let free_names t =
     match t with
@@ -167,7 +171,10 @@ module For_allocations = struct
     else
       match mode with
       | Alloc_heap -> Heap
-      | Alloc_local -> Local { region = current_region }
+      | Alloc_local -> (
+        match current_region with
+        | Some region -> Local { region }
+        | None -> Misc.fatal_error "Local allocation without a region")
 
   let free_names t =
     match t with
diff --git a/middle_end/flambda2/term_basics/alloc_mode.mli b/middle_end/flambda2/term_basics/alloc_mode.mli
index 54a3a6d439f..7151b8a0807 100644
--- a/middle_end/flambda2/term_basics/alloc_mode.mli
+++ b/middle_end/flambda2/term_basics/alloc_mode.mli
@@ -63,8 +63,8 @@ module For_applications : sig
 
   val from_lambda :
     Lambda.locality_mode ->
-    current_region:Variable.t ->
-    current_ghost_region:Variable.t ->
+    current_region:Variable.t option ->
+    current_ghost_region:Variable.t option ->
     t
 
   include Contains_names.S with type t := t
@@ -90,7 +90,8 @@ module For_allocations : sig
 
   val as_type : t -> For_types.t
 
-  val from_lambda : Lambda.locality_mode -> current_region:Variable.t -> t
+  val from_lambda :
+    Lambda.locality_mode -> current_region:Variable.t option -> t
 
   include Contains_names.S with type t := t
 
diff --git a/middle_end/flambda2/terms/flambda.ml b/middle_end/flambda2/terms/flambda.ml
index 28baf96bc38..1b1ed7e95b6 100644
--- a/middle_end/flambda2/terms/flambda.ml
+++ b/middle_end/flambda2/terms/flambda.ml
@@ -589,11 +589,14 @@ and print_function_params_and_body ppf t =
        \u{27c5}%t%a%t\u{27c6}@ %a %a %t%a%t %t.%t@]@ %a))@]"
       Flambda_colours.lambda Flambda_colours.pop Continuation.print
       return_continuation Continuation.print exn_continuation
-      Flambda_colours.parameter Variable.print my_region Flambda_colours.pop
-      Flambda_colours.parameter Variable.print my_ghost_region
-      Flambda_colours.pop Bound_parameters.print params Bound_parameter.print
-      my_closure Flambda_colours.depth_variable Variable.print my_depth
-      Flambda_colours.pop Flambda_colours.elide Flambda_colours.pop print body
+      Flambda_colours.parameter
+      (Format.pp_print_option Variable.print)
+      my_region Flambda_colours.pop Flambda_colours.parameter
+      (Format.pp_print_option Variable.print)
+      my_ghost_region Flambda_colours.pop Bound_parameters.print params
+      Bound_parameter.print my_closure Flambda_colours.depth_variable
+      Variable.print my_depth Flambda_colours.pop Flambda_colours.elide
+      Flambda_colours.pop print body
   in
   let module BFF = Bound_for_function in
   Name_abstraction.pattern_match_for_printing
diff --git a/middle_end/flambda2/terms/flambda.mli b/middle_end/flambda2/terms/flambda.mli
index fbee4974aae..e496dd5243a 100644
--- a/middle_end/flambda2/terms/flambda.mli
+++ b/middle_end/flambda2/terms/flambda.mli
@@ -484,8 +484,8 @@ module Function_params_and_body : sig
     body:expr ->
     free_names_of_body:Name_occurrences.t Or_unknown.t ->
     my_closure:Variable.t ->
-    my_region:Variable.t ->
-    my_ghost_region:Variable.t ->
+    my_region:Variable.t option ->
+    my_ghost_region:Variable.t option ->
     my_depth:Variable.t ->
     t
 
@@ -507,8 +507,8 @@ module Function_params_and_body : sig
       body:expr ->
       my_closure:Variable.t ->
       is_my_closure_used:bool Or_unknown.t ->
-      my_region:Variable.t ->
-      my_ghost_region:Variable.t ->
+      my_region:Variable.t option ->
+      my_ghost_region:Variable.t option ->
       my_depth:Variable.t ->
       free_names_of_body:Name_occurrences.t Or_unknown.t ->
       'a) ->
@@ -533,8 +533,8 @@ module Function_params_and_body : sig
       body1:expr ->
       body2:expr ->
       my_closure:Variable.t ->
-      my_region:Variable.t ->
-      my_ghost_region:Variable.t ->
+      my_region:Variable.t option ->
+      my_ghost_region:Variable.t option ->
       my_depth:Variable.t ->
       'a) ->
     'a
diff --git a/middle_end/flambda2/to_cmm/to_cmm_set_of_closures.ml b/middle_end/flambda2/to_cmm/to_cmm_set_of_closures.ml
index bc32d396182..7066da7ccbe 100644
--- a/middle_end/flambda2/to_cmm/to_cmm_set_of_closures.ml
+++ b/middle_end/flambda2/to_cmm/to_cmm_set_of_closures.ml
@@ -458,23 +458,34 @@ let params_and_body0 env res code_id ~result_arity ~fun_dbg
     Env.enter_function_body env ~return_continuation ~return_continuation_arity
       ~exn_continuation
   in
-  (* [my_region] can be referenced in [Begin_try_region] primitives so must be
-     in the environment; however it should never end up in actual generated
-     code, so we don't need any binder for it (this is why we can ignore
+  (* [my_region] can be referenced in [Begin_region] primitives so must be in
+     the environment; however it should never end up in actual generated code,
+     so we don't need any binder for it (this is why we can ignore
      [_bound_var]). If it does end up in generated code, Selection will complain
      and refuse to compile the code. *)
-  let env, my_region_var = Env.create_bound_parameter env my_region in
+  let env, my_region_var =
+    match my_region with
+    | None -> env, None
+    | Some my_region ->
+      let env, region = Env.create_bound_parameter env my_region in
+      env, Some region
+  in
   (* Similarly for [my_ghost_region]. *)
   let env, my_ghost_region_var =
-    Env.create_bound_parameter env my_ghost_region
+    match my_ghost_region with
+    | None -> env, None
+    | Some my_ghost_region ->
+      let env, region = Env.create_bound_parameter env my_ghost_region in
+      env, Some region
   in
   (* Translate the arg list and body *)
   let env, fun_params = C.function_bound_parameters env params in
   let fun_body, fun_body_free_vars, res = translate_expr env res body in
   let fun_free_vars =
     C.remove_vars_with_machtype
-      (C.remove_var_with_provenance
-         (C.remove_var_with_provenance fun_body_free_vars my_ghost_region_var)
+      (C.remove_var_opt_with_provenance
+         (C.remove_var_opt_with_provenance fun_body_free_vars
+            my_ghost_region_var)
          my_region_var)
       fun_params
   in
diff --git a/middle_end/flambda2/to_cmm/to_cmm_shared.ml b/middle_end/flambda2/to_cmm/to_cmm_shared.ml
index 7da2054eee5..bb408a0129f 100644
--- a/middle_end/flambda2/to_cmm/to_cmm_shared.ml
+++ b/middle_end/flambda2/to_cmm/to_cmm_shared.ml
@@ -37,6 +37,11 @@ let remove_var_with_provenance free_vars var =
   let v = Backend_var.With_provenance.var var in
   Backend_var.Set.remove v free_vars
 
+let remove_var_opt_with_provenance free_vars var =
+  match var with
+  | None -> free_vars
+  | Some var -> remove_var_with_provenance free_vars var
+
 let remove_vars_with_machtype free_vars vars =
   List.fold_left
     (fun free_vars (cmm_var, _machtype) ->
diff --git a/middle_end/flambda2/to_cmm/to_cmm_shared.mli b/middle_end/flambda2/to_cmm/to_cmm_shared.mli
index f10d1d61744..2adddc260ff 100644
--- a/middle_end/flambda2/to_cmm/to_cmm_shared.mli
+++ b/middle_end/flambda2/to_cmm/to_cmm_shared.mli
@@ -27,6 +27,11 @@ val remove_skipped_args : 'a list -> _ To_cmm_env.param_type list -> 'a list
 val remove_var_with_provenance :
   To_cmm_env.free_vars -> Backend_var.With_provenance.t -> To_cmm_env.free_vars
 
+val remove_var_opt_with_provenance :
+  To_cmm_env.free_vars ->
+  Backend_var.With_provenance.t option ->
+  To_cmm_env.free_vars
+
 val remove_vars_with_machtype :
   To_cmm_env.free_vars ->
   (Backend_var.With_provenance.t * _) list ->