Remove the option to split fwd/back functions and update SymbolicToPure

5 files changed, 439 insertions, 776 deletions

diff --git a/compiler/Config.ml b/compiler/Config.ml
index 3b0070c0..6fd866e8 100644
--- a/compiler/Config.ml
+++ b/compiler/Config.ml
@@ -92,69 +92,6 @@ let loop_fixed_point_max_num_iters = 2
 
 (** {1 Translation} *)
 
-(** If true, do not define separate forward/backward functions, but make the
-    forward functions return the backward function.
-
-    Example:
-    {[
-      (* Rust *)
-      pub fn list_nth<'a, T>(l: &'a mut List<T>, i: u32) -> &'a mut T {
-          match l {
-              List::Nil => {
-                  panic!()
-              }
-              List::Cons(x, tl) => {
-                  if i == 0 {
-                      x
-                  } else {
-                      list_nth(tl, i - 1)
-                  }
-              }
-          }
-      }
-
-      (* Translation, if return_back_funs = false *)
-      def list_nth (T : Type) (l : List T) (i : U32) : Result T :=
-        match l with
-        | List.Cons x tl =>
-          if i = 0#u32
-          then Result.ret x
-          else do
-                 let i0 ← i - 1#u32
-                 list_nth T tl i0
-        | List.Nil => Result.fail .panic
-
-      def list_nth_back
-        (T : Type) (l : List T) (i : U32) (ret : T) : Result (List T) :=
-        match l with
-        | List.Cons x tl =>
-          if i = 0#u32
-          then Result.ret (List.Cons ret tl)
-          else
-            do
-              let i0 ← i - 1#u32
-              let tl0 ← list_nth_back T tl i0 ret
-              Result.ret (List.Cons x tl0)
-        | List.Nil => Result.fail .panic
-
-      (* Translation, if return_back_funs = true *)
-      def list_nth (T: Type) (ls : List T) (i : U32) :
-        Result (T × (T → Result (List T))) :=
-        match ls with
-        | List.Cons x tl =>
-          if i = 0#u32
-          then Result.ret (x, (λ ret => return (ret :: ls)))
-          else do
-            let i0 ← i - 1#u32
-            let (x, back) ← list_nth ls i0
-            Return.ret (x,
-              (λ ret => do
-               let ls ← back ret
-               return (x :: ls)))
-    ]}
- *)
-let return_back_funs = ref true
-
 (** Forbids using field projectors for structures.
 
     If we don't use field projectors, whenever we symbolically expand a structure
diff --git a/compiler/ExtractBase.ml b/compiler/ExtractBase.ml
index 5aa8323e..04686705 100644
--- a/compiler/ExtractBase.ml
+++ b/compiler/ExtractBase.ml
@@ -471,8 +471,7 @@ type names_map_init = {
   assumed_adts : (assumed_ty * string) list;
   assumed_structs : (assumed_ty * string) list;
   assumed_variants : (assumed_ty * VariantId.id * string) list;
-  assumed_llbc_functions :
-    (A.assumed_fun_id * RegionGroupId.id option * string) list;
+  assumed_llbc_functions : (A.assumed_fun_id * string) list;
   assumed_pure_functions : (pure_assumed_fun_id * string) list;
 }
 
@@ -1052,63 +1051,28 @@ let assumed_variants () : (assumed_ty * VariantId.id * string) list =
         (* No Fuel::Succ on purpose *)
       ]
 
-let assumed_llbc_functions () :
-    (A.assumed_fun_id * T.RegionGroupId.id option * string) list =
-  let rg0 = Some T.RegionGroupId.zero in
-  let regular : (A.assumed_fun_id * T.RegionGroupId.id option * string) list =
-    match !backend with
-    | FStar | Coq | HOL4 ->
-        [
-          (ArrayIndexShared, None, "array_index_usize");
-          (ArrayToSliceShared, None, "array_to_slice");
-          (ArrayRepeat, None, "array_repeat");
-          (SliceIndexShared, None, "slice_index_usize");
-        ]
-    | Lean ->
-        [
-          (ArrayIndexShared, None, "Array.index_usize");
-          (ArrayToSliceShared, None, "Array.to_slice");
-          (ArrayRepeat, None, "Array.repeat");
-          (SliceIndexShared, None, "Slice.index_usize");
-        ]
-  in
-  let mut_funs : (A.assumed_fun_id * T.RegionGroupId.id option * string) list =
-    if !Config.return_back_funs then
-      match !backend with
-      | FStar | Coq | HOL4 ->
-          [
-            (ArrayIndexMut, None, "array_index_mut_usize");
-            (ArrayToSliceMut, None, "array_to_slice_mut");
-            (SliceIndexMut, None, "slice_index_mut_usize");
-          ]
-      | Lean ->
-          [
-            (ArrayIndexMut, None, "Array.index_mut_usize");
-            (ArrayToSliceMut, None, "Array.to_slice_mut");
-            (SliceIndexMut, None, "Slice.index_mut_usize");
-          ]
-    else
-      match !backend with
-      | FStar | Coq | HOL4 ->
-          [
-            (ArrayIndexMut, None, "array_index_usize");
-            (ArrayIndexMut, rg0, "array_update_usize");
-            (ArrayToSliceMut, None, "array_to_slice");
-            (ArrayToSliceMut, rg0, "array_from_slice");
-            (SliceIndexMut, None, "slice_index_usize");
-            (SliceIndexMut, rg0, "slice_update_usize");
-          ]
-      | Lean ->
-          [
-            (ArrayIndexMut, None, "Array.index_usize");
-            (ArrayIndexMut, rg0, "Array.update_usize");
-            (ArrayToSliceMut, None, "Array.to_slice");
-            (ArrayToSliceMut, rg0, "Array.from_slice");
-            (SliceIndexMut, None, "Slice.index_usize");
-            (SliceIndexMut, rg0, "Slice.update_usize");
-          ]
-  in
-  regular @ mut_funs
+let assumed_llbc_functions () : (A.assumed_fun_id * string) list =
+  match !backend with
+  | FStar | Coq | HOL4 ->
+      [
+        (ArrayIndexShared, "array_index_usize");
+        (ArrayIndexMut, "array_index_mut_usize");
+        (ArrayToSliceShared, "array_to_slice");
+        (ArrayToSliceMut, "array_to_slice_mut");
+        (ArrayRepeat, "array_repeat");
+        (SliceIndexShared, "slice_index_usize");
+        (SliceIndexMut, "slice_index_mut_usize");
+      ]
+  | Lean ->
+      [
+        (ArrayIndexShared, "Array.index_usize");
+        (ArrayIndexMut, "Array.index_mut_usize");
+        (ArrayToSliceShared, "Array.to_slice");
+        (ArrayToSliceMut, "Array.to_slice_mut");
+        (ArrayRepeat, "Array.repeat");
+        (SliceIndexShared, "Slice.index_usize");
+        (SliceIndexMut, "Slice.index_mut_usize");
+      ]
 
 let assumed_pure_functions () : (pure_assumed_fun_id * string) list =
   match !backend with
diff --git a/compiler/ExtractBuiltin.ml b/compiler/ExtractBuiltin.ml
index ee8d4831..3ea5655a 100644
--- a/compiler/ExtractBuiltin.ml
+++ b/compiler/ExtractBuiltin.ml
@@ -213,11 +213,7 @@ let mk_builtin_types_map () =
 
 let builtin_types_map = mk_memoized mk_builtin_types_map
 
-type builtin_fun_info = {
-  rg : Types.RegionGroupId.id option;
-  extract_name : string;
-}
-[@@deriving show]
+type builtin_fun_info = { extract_name : string } [@@deriving show]
 
 (** The assumed functions.
 
@@ -227,19 +223,10 @@ type builtin_fun_info = {
  *)
 let builtin_funs () : (pattern * bool list option * builtin_fun_info list) list
     =
-  let rg0 = Some Types.RegionGroupId.zero in
   (* Small utility *)
   let mk_fun (rust_name : string) (extract_name : string option)
-      (filter : bool list option) (with_back : bool) (back_no_suffix : bool) :
+      (filter : bool list option) :
       pattern * bool list option * builtin_fun_info list =
-    (* [back_no_suffix] is used to control whether the backward function should
-       have the suffix "_back" or not (if not, then the forward function has the
-       prefix "_fwd", and is filtered anyway). This is pertinent only if we split
-       the fwd/back functions. *)
-    let back_no_suffix = back_no_suffix && not !Config.return_back_funs in
-    (* Same for the [with_back] option: this is pertinent only if we split
-       the fwd/back functions *)
-    let with_back = with_back && not !Config.return_back_funs in
     let rust_name =
       try parse_pattern rust_name
       with Failure _ ->
@@ -251,68 +238,51 @@ let builtin_funs () : (pattern * bool list option * builtin_fun_info list) list
       | Some name -> split_on_separator name
     in
     let basename = flatten_name extract_name in
-    let fwd_suffix = if with_back && back_no_suffix then "_fwd" else "" in
-    let fwd = [ { rg = None; extract_name = basename ^ fwd_suffix } ] in
-    let back_suffix = if with_back && back_no_suffix then "" else "_back" in
-    let back =
-      if with_back then [ { rg = rg0; extract_name = basename ^ back_suffix } ]
-      else []
-    in
-    (rust_name, filter, fwd @ back)
+    let f = [ { extract_name = basename } ] in
+    (rust_name, filter, f)
   in
   [
-    mk_fun "core::mem::replace" None None true false;
+    mk_fun "core::mem::replace" None None;
     mk_fun "core::slice::{[@T]}::len"
       (Some (backend_choice "slice::len" "Slice::len"))
-      None true false;
+      None;
     mk_fun "alloc::vec::{alloc::vec::Vec<@T, alloc::alloc::Global>}::new"
-      (Some "alloc::vec::Vec::new") None false false;
+      (Some "alloc::vec::Vec::new") None;
     mk_fun "alloc::vec::{alloc::vec::Vec<@T, @A>}::push" None
-      (Some [ true; false ])
-      true true;
+      (Some [ true; false ]);
     mk_fun "alloc::vec::{alloc::vec::Vec<@T, @A>}::insert" None
-      (Some [ true; false ])
-      true true;
+      (Some [ true; false ]);
     mk_fun "alloc::vec::{alloc::vec::Vec<@T, @A>}::len" None
-      (Some [ true; false ])
-      true false;
+      (Some [ true; false ]);
     mk_fun "alloc::vec::{alloc::vec::Vec<@T, @A>}::index" None
-      (Some [ true; true; false ])
-      true false;
+      (Some [ true; true; false ]);
     mk_fun "alloc::vec::{alloc::vec::Vec<@T, @A>}::index_mut" None
-      (Some [ true; true; false ])
-      true false;
-    mk_fun "alloc::boxed::{Box<@T>}::deref" None
-      (Some [ true; false ])
-      true false;
-    mk_fun "alloc::boxed::{Box<@T>}::deref_mut" None
-      (Some [ true; false ])
-      true false;
-    mk_fun "core::slice::index::{[@T]}::index" None None true false;
-    mk_fun "core::slice::index::{[@T]}::index_mut" None None true false;
-    mk_fun "core::array::{[@T; @C]}::index" None None true false;
-    mk_fun "core::array::{[@T; @C]}::index_mut" None None true false;
+      (Some [ true; true; false ]);
+    mk_fun "alloc::boxed::{Box<@T>}::deref" None (Some [ true; false ]);
+    mk_fun "alloc::boxed::{Box<@T>}::deref_mut" None (Some [ true; false ]);
+    mk_fun "core::slice::index::{[@T]}::index" None None;
+    mk_fun "core::slice::index::{[@T]}::index_mut" None None;
+    mk_fun "core::array::{[@T; @C]}::index" None None;
+    mk_fun "core::array::{[@T; @C]}::index_mut" None None;
     mk_fun "core::slice::index::{core::ops::range::Range<usize>}::get"
-      (Some "core::slice::index::RangeUsize::get") None true false;
+      (Some "core::slice::index::RangeUsize::get") None;
     mk_fun "core::slice::index::{core::ops::range::Range<usize>}::get_mut"
-      (Some "core::slice::index::RangeUsize::get_mut") None true false;
+      (Some "core::slice::index::RangeUsize::get_mut") None;
     mk_fun "core::slice::index::{core::ops::range::Range<usize>}::index"
-      (Some "core::slice::index::RangeUsize::index") None true false;
+      (Some "core::slice::index::RangeUsize::index") None;
     mk_fun "core::slice::index::{core::ops::range::Range<usize>}::index_mut"
-      (Some "core::slice::index::RangeUsize::index_mut") None true false;
+      (Some "core::slice::index::RangeUsize::index_mut") None;
     mk_fun "core::slice::index::{core::ops::range::Range<usize>}::get_unchecked"
-      (Some "core::slice::index::RangeUsize::get_unchecked") None false false;
+      (Some "core::slice::index::RangeUsize::get_unchecked") None;
     mk_fun
       "core::slice::index::{core::ops::range::Range<usize>}::get_unchecked_mut"
-      (Some "core::slice::index::RangeUsize::get_unchecked_mut") None false
-      false;
-    mk_fun "core::slice::index::{usize}::get" None None true false;
-    mk_fun "core::slice::index::{usize}::get_mut" None None true false;
-    mk_fun "core::slice::index::{usize}::get_unchecked" None None false false;
-    mk_fun "core::slice::index::{usize}::get_unchecked_mut" None None false
-      false;
-    mk_fun "core::slice::index::{usize}::index" None None true false;
-    mk_fun "core::slice::index::{usize}::index_mut" None None true false;
+      (Some "core::slice::index::RangeUsize::get_unchecked_mut") None;
+    mk_fun "core::slice::index::{usize}::get" None None;
+    mk_fun "core::slice::index::{usize}::get_mut" None None;
+    mk_fun "core::slice::index::{usize}::get_unchecked" None None;
+    mk_fun "core::slice::index::{usize}::get_unchecked_mut" None None;
+    mk_fun "core::slice::index::{usize}::index" None None;
+    mk_fun "core::slice::index::{usize}::index_mut" None None;
   ]
 
 let mk_builtin_funs_map () =
@@ -412,10 +382,9 @@ type builtin_trait_decl_info = {
 [@@deriving show]
 
 let builtin_trait_decls_info () =
-  let rg0 = Some Types.RegionGroupId.zero in
   let mk_trait (rust_name : string) ?(extract_name : string option = None)
       ?(parent_clauses : string list = []) ?(types : string list = [])
-      ?(methods : (string * bool) list = []) () : builtin_trait_decl_info =
+      ?(methods : string list = []) () : builtin_trait_decl_info =
     let rust_name = parse_pattern rust_name in
     let extract_name =
       match extract_name with
@@ -443,22 +412,14 @@ let builtin_trait_decls_info () =
       List.map mk_type types
     in
     let methods =
-      let mk_method (item_name, with_back) =
+      let mk_method item_name =
         (* TODO: factor out with builtin_funs_info *)
         let basename =
           if !record_fields_short_names then item_name
           else extract_name ^ "_" ^ item_name
         in
-        let back_no_suffix = false in
-        let fwd_suffix = if with_back && back_no_suffix then "_fwd" else "" in
-        let fwd = [ { rg = None; extract_name = basename ^ fwd_suffix } ] in
-        let back_suffix = if with_back && back_no_suffix then "" else "_back" in
-        let back =
-          if with_back then
-            [ { rg = rg0; extract_name = basename ^ back_suffix } ]
-          else []
-        in
-        (item_name, fwd @ back)
+        let fwd = [ { extract_name = basename } ] in
+        (item_name, fwd)
       in
       List.map mk_method methods
     in
@@ -474,21 +435,17 @@ let builtin_trait_decls_info () =
   in
   [
     (* Deref *)
-    mk_trait "core::ops::deref::Deref" ~types:[ "Target" ]
-      ~methods:[ ("deref", true) ]
+    mk_trait "core::ops::deref::Deref" ~types:[ "Target" ] ~methods:[ "deref" ]
       ();
     (* DerefMut *)
     mk_trait "core::ops::deref::DerefMut" ~parent_clauses:[ "derefInst" ]
-      ~methods:[ ("deref_mut", true) ]
-      ();
+      ~methods:[ "deref_mut" ] ();
     (* Index *)
-    mk_trait "core::ops::index::Index" ~types:[ "Output" ]
-      ~methods:[ ("index", true) ]
+    mk_trait "core::ops::index::Index" ~types:[ "Output" ] ~methods:[ "index" ]
       ();
     (* IndexMut *)
     mk_trait "core::ops::index::IndexMut" ~parent_clauses:[ "indexInst" ]
-      ~methods:[ ("index_mut", true) ]
-      ();
+      ~methods:[ "index_mut" ] ();
     (* Sealed *)
     mk_trait "core::slice::index::private_slice_index::Sealed" ();
     (* SliceIndex *)
@@ -496,12 +453,12 @@ let builtin_trait_decls_info () =
       ~types:[ "Output" ]
       ~methods:
         [
-          ("get", true);
-          ("get_mut", true);
-          ("get_unchecked", false);
-          ("get_unchecked_mut", false);
-          ("index", true);
-          ("index_mut", true);
+          "get";
+          "get_mut";
+          "get_unchecked";
+          "get_unchecked_mut";
+          "index";
+          "index_mut";
         ]
       ();
   ]
diff --git a/compiler/Main.ml b/compiler/Main.ml
index 4a2d01dc..664ec067 100644
--- a/compiler/Main.ml
+++ b/compiler/Main.ml
@@ -120,9 +120,6 @@ let () =
         " Generate a default lakefile.lean (Lean only)" );
       ("-print-llbc", Arg.Set print_llbc, " Print the imported LLBC");
       ("-k", Arg.Clear fail_hard, " Do not fail hard in case of error");
-      ( "-split-fwd-back",
-        Arg.Clear return_back_funs,
-        " Split the forward and backward functions." );
       ( "-tuple-nested-proj",
         Arg.Set use_nested_tuple_projectors,
         " Use nested projectors for tuples (e.g., (0, 1).snd.fst instead of \
diff --git a/compiler/SymbolicToPure.ml b/compiler/SymbolicToPure.ml
index 3a50e495..859d6f17 100644
--- a/compiler/SymbolicToPure.ml
+++ b/compiler/SymbolicToPure.ml
@@ -805,11 +805,9 @@ let bs_ctx_register_forward_call (call_id : V.FunCallId.id) (forward : S.call)
     that we need to call. This function may be [None] if it has to be ignored
     (because it does nothing).
  *)
-let bs_ctx_register_backward_call (abs : V.abs) (effect_info : fun_effect_info)
-    (call_id : V.FunCallId.id) (back_id : T.RegionGroupId.id)
-    (inherited_args : texpression list) (back_args : texpression list)
-    (generics : generic_args) (output_ty : ty) (ctx : bs_ctx) :
-    bs_ctx * texpression option =
+let bs_ctx_register_backward_call (abs : V.abs) (call_id : V.FunCallId.id)
+    (back_id : T.RegionGroupId.id) (back_args : texpression list) (ctx : bs_ctx)
+    : bs_ctx * texpression option =
   (* Insert the abstraction in the call informations *)
   let info = V.FunCallId.Map.find call_id ctx.calls in
   let calls = V.FunCallId.Map.add call_id info ctx.calls in
@@ -819,29 +817,9 @@ let bs_ctx_register_backward_call (abs : V.abs) (effect_info : fun_effect_info)
   let abstractions =
     V.AbstractionId.Map.add abs.abs_id (abs, back_args) abstractions
   in
-  (* Compute the expression corresponding to the function *)
-  let func =
-    if !Config.return_back_funs then
-      (* Lookup the variable introduced for the backward function *)
-      RegionGroupId.Map.find back_id (Option.get info.back_funs)
-    else
-      (* Retrieve the fun_id *)
-      let fun_id =
-        match info.forward.call_id with
-        | S.Fun (fid, _) ->
-            let fid = translate_fun_id_or_trait_method_ref ctx fid in
-            Fun (FromLlbc (fid, None, Some back_id))
-        | S.Unop _ | S.Binop _ -> raise (Failure "Unreachable")
-      in
-      let args = List.append inherited_args back_args in
-      let input_tys = (List.map (fun (x : texpression) -> x.ty)) args in
-      let ret_ty =
-        if effect_info.can_fail then mk_result_ty output_ty else output_ty
-      in
-      let func_ty = mk_arrows input_tys ret_ty in
-      let func = { id = FunOrOp fun_id; generics } in
-      Some { e = Qualif func; ty = func_ty }
-  in
+  (* Compute the expression corresponding to the function.
+     We simply lookup the variable introduced for the backward function. *)
+  let func = RegionGroupId.Map.find back_id (Option.get info.back_funs) in
   (* Update the context and return *)
   ({ ctx with calls; abstractions }, func)
 
@@ -1124,20 +1102,34 @@ let translate_fun_sig_with_regions_hierarchy_to_decomposed
     let inputs_no_state =
       List.map (fun ty -> (Some "ret", ty)) inputs_no_state
     in
-    (* In case we merge the forward/backward functions:
-       we consider the backward function as stateful and potentially failing
+    (* We consider a backward function as stateful and potentially failing
        **only if it has inputs** (for the "potentially failing": if it has
        not inputs, we directly evaluate it in the body of the forward function).
+
+       For instance, we do the following:
+       {[
+         // Rust
+         fn push<T, 'a>(v : &mut Vec<T>, x : T) { ... }
+
+         (* Generated code: before doing unit elimination.
+            We return (), as well as the backward function; as the backward
+            function doesn't consume any inputs, it is a value that we compute
+            directly in the body of [push].
+          *)
+         let push T (v : Vec T) (x : T) : Result (() * Vec T) = ...
+
+         (* Generated code: after doing unit elimination, if we simplify the merged
+            fwd/back functions (see below). *)
+         let push T (v : Vec T) (x : T) : Result (Vec T) = ...
+       ]}
     *)
     let back_effect_info =
-      if !Config.return_back_funs then
-        let b = inputs_no_state <> [] in
-        {
-          back_effect_info with
-          stateful = back_effect_info.stateful && b;
-          can_fail = back_effect_info.can_fail && b;
-        }
-      else back_effect_info
+      let b = inputs_no_state <> [] in
+      {
+        back_effect_info with
+        stateful = back_effect_info.stateful && b;
+        can_fail = back_effect_info.can_fail && b;
+      }
     in
     let state =
       if back_effect_info.stateful then [ (None, mk_state_ty) ] else []
@@ -1145,8 +1137,7 @@ let translate_fun_sig_with_regions_hierarchy_to_decomposed
     let inputs = inputs_no_state @ state in
     let output_names, outputs = compute_back_outputs_for_gid gid in
     let filter =
-      !Config.simplify_merged_fwd_backs
-      && !Config.return_back_funs && inputs = [] && outputs = []
+      !Config.simplify_merged_fwd_backs && inputs = [] && outputs = []
     in
     let info =
       {
@@ -1186,7 +1177,7 @@ let translate_fun_sig_with_regions_hierarchy_to_decomposed
       }
     in
     let ignore_output =
-      if !Config.return_back_funs && !Config.simplify_merged_fwd_backs then
+      if !Config.simplify_merged_fwd_backs then
         ty_is_unit fwd_output
         && List.exists
              (fun (info : back_sg_info) -> not info.filter)
@@ -1296,10 +1287,10 @@ let compute_back_tys (dsg : Pure.decomposed_fun_sig)
     (subst : (generic_args * trait_instance_id) option) : ty option list =
   List.map (Option.map snd) (compute_back_tys_with_info dsg subst)
 
-(** In case we merge the fwd/back functions: compute the output type of
-    a function, from a decomposed signature. *)
+(** Compute the output type of a function, from a decomposed signature
+    (the output type contains the type of the value returned by the forward
+    function as well as the types of the returned backward functions). *)
 let compute_output_ty_from_decomposed (dsg : Pure.decomposed_fun_sig) : ty =
-  assert !Config.return_back_funs;
   (* Compute the arrow types for all the backward functions *)
   let back_tys = List.filter_map (fun x -> x) (compute_back_tys dsg None) in
   (* Group the forward output and the types of the backward functions *)
@@ -1315,8 +1306,8 @@ let compute_output_ty_from_decomposed (dsg : Pure.decomposed_fun_sig) : ty =
   in
   mk_output_ty_from_effect_info effect_info output
 
-let translate_fun_sig_from_decomposed (dsg : Pure.decomposed_fun_sig)
-    (gid : RegionGroupId.id option) : fun_sig =
+let translate_fun_sig_from_decomposed (dsg : Pure.decomposed_fun_sig) : fun_sig
+    =
   let generics = dsg.generics in
   let llbc_generics = dsg.llbc_generics in
   let preds = dsg.preds in
@@ -1329,27 +1320,10 @@ let translate_fun_sig_from_decomposed (dsg : Pure.decomposed_fun_sig)
            (gid, info.effect_info))
          (RegionGroupId.Map.bindings dsg.back_sg))
   in
-  let mk_output_ty = mk_output_ty_from_effect_info in
   let inputs, output =
-    (* Two cases depending on whether we split the forward/backward functions or not *)
-    if !Config.return_back_funs then (
-      assert (gid = None);
-      let output = compute_output_ty_from_decomposed dsg in
-      let inputs = dsg.fwd_inputs in
-      (inputs, output))
-    else
-      match gid with
-      | None ->
-          let effect_info = dsg.fwd_info.effect_info in
-          let output = mk_output_ty effect_info dsg.fwd_output in
-          (dsg.fwd_inputs, output)
-      | Some gid ->
-          let back_sg = RegionGroupId.Map.find gid dsg.back_sg in
-          let effect_info = back_sg.effect_info in
-          let inputs = dsg.fwd_inputs @ List.map snd back_sg.inputs in
-          let output = mk_simpl_tuple_ty back_sg.outputs in
-          let output = mk_output_ty effect_info output in
-          (inputs, output)
+    let output = compute_output_ty_from_decomposed dsg in
+    let inputs = dsg.fwd_inputs in
+    (inputs, output)
   in
   { generics; llbc_generics; preds; inputs; output; fwd_info; back_effect_info }
 
@@ -1933,16 +1907,14 @@ and translate_panic (ctx : bs_ctx) : texpression =
     *)
     match ctx.bid with
     | None ->
-        if !Config.return_back_funs then
-          let back_tys = compute_back_tys ctx.sg None in
-          let back_tys = List.filter_map (fun x -> x) back_tys in
-          let tys =
-            if ctx.sg.fwd_info.ignore_output then back_tys
-            else ctx.sg.fwd_output :: back_tys
-          in
-          let output = mk_simpl_tuple_ty tys in
-          mk_output output
-        else mk_output ctx.sg.fwd_output
+        let back_tys = compute_back_tys ctx.sg None in
+        let back_tys = List.filter_map (fun x -> x) back_tys in
+        let tys =
+          if ctx.sg.fwd_info.ignore_output then back_tys
+          else ctx.sg.fwd_output :: back_tys
+        in
+        let output = mk_simpl_tuple_ty tys in
+        mk_output output
     | Some bid ->
         let output =
           mk_simpl_tuple_ty (RegionGroupId.Map.find bid ctx.sg.back_sg).outputs
@@ -2080,107 +2052,103 @@ and translate_function_call (call : S.call) (e : S.expression) (ctx : bs_ctx) :
             (List.concat [ fuel; args; [ state_var ] ], ctx, Some nstate_var)
           else (List.concat [ fuel; args ], ctx, None)
         in
-        (* If we do not split the forward/backward functions: generate the
-           variables for the backward functions returned by the forward
+        (* Generate the variables for the backward functions returned by the forward
            function. *)
         let ctx, ignore_fwd_output, back_funs_map, back_funs =
-          if !Config.return_back_funs then (
-            (* We need to compute the signatures of the backward functions. *)
-            let sg = Option.get call.sg in
-            let decls_ctx = ctx.decls_ctx in
-            let dsg =
-              translate_fun_sig_with_regions_hierarchy_to_decomposed decls_ctx
-                fid call.regions_hierarchy sg
-                (List.map (fun _ -> None) sg.inputs)
-            in
-            log#ldebug
-              (lazy ("dsg.generics:\n" ^ show_generic_params dsg.generics));
-            let tr_self, all_generics =
-              match call.trait_method_generics with
-              | None -> (UnknownTrait __FUNCTION__, generics)
-              | Some (all_generics, tr_self) ->
-                  let all_generics =
-                    ctx_translate_fwd_generic_args ctx all_generics
-                  in
-                  let tr_self =
-                    translate_fwd_trait_instance_id ctx.type_ctx.type_infos
-                      tr_self
+          (* We need to compute the signatures of the backward functions. *)
+          let sg = Option.get call.sg in
+          let decls_ctx = ctx.decls_ctx in
+          let dsg =
+            translate_fun_sig_with_regions_hierarchy_to_decomposed decls_ctx fid
+              call.regions_hierarchy sg
+              (List.map (fun _ -> None) sg.inputs)
+          in
+          log#ldebug
+            (lazy ("dsg.generics:\n" ^ show_generic_params dsg.generics));
+          let tr_self, all_generics =
+            match call.trait_method_generics with
+            | None -> (UnknownTrait __FUNCTION__, generics)
+            | Some (all_generics, tr_self) ->
+                let all_generics =
+                  ctx_translate_fwd_generic_args ctx all_generics
+                in
+                let tr_self =
+                  translate_fwd_trait_instance_id ctx.type_ctx.type_infos
+                    tr_self
+                in
+                (tr_self, all_generics)
+          in
+          let back_tys =
+            compute_back_tys_with_info dsg (Some (all_generics, tr_self))
+          in
+          (* Introduce variables for the backward functions *)
+          (* Compute a proper basename for the variables *)
+          let back_fun_name =
+            let name =
+              match fid with
+              | FunId (FAssumed fid) -> (
+                  match fid with
+                  | BoxNew -> "box_new"
+                  | BoxFree -> "box_free"
+                  | ArrayRepeat -> "array_repeat"
+                  | ArrayIndexShared -> "index_shared"
+                  | ArrayIndexMut -> "index_mut"
+                  | ArrayToSliceShared -> "to_slice_shared"
+                  | ArrayToSliceMut -> "to_slice_mut"
+                  | SliceIndexShared -> "index_shared"
+                  | SliceIndexMut -> "index_mut")
+              | FunId (FRegular fid) | TraitMethod (_, _, fid) -> (
+                  let decl =
+                    FunDeclId.Map.find fid ctx.fun_ctx.llbc_fun_decls
                   in
-                  (tr_self, all_generics)
+                  match Collections.List.last decl.name with
+                  | PeIdent (s, _) -> s
+                  | PeImpl _ ->
+                      (* We shouldn't get there *)
+                      raise (Failure "Unexpected"))
             in
-            let back_tys =
-              compute_back_tys_with_info dsg (Some (all_generics, tr_self))
-            in
-            (* Introduce variables for the backward functions *)
-            (* Compute a proper basename for the variables *)
-            let back_fun_name =
-              let name =
-                match fid with
-                | FunId (FAssumed fid) -> (
-                    match fid with
-                    | BoxNew -> "box_new"
-                    | BoxFree -> "box_free"
-                    | ArrayRepeat -> "array_repeat"
-                    | ArrayIndexShared -> "index_shared"
-                    | ArrayIndexMut -> "index_mut"
-                    | ArrayToSliceShared -> "to_slice_shared"
-                    | ArrayToSliceMut -> "to_slice_mut"
-                    | SliceIndexShared -> "index_shared"
-                    | SliceIndexMut -> "index_mut")
-                | FunId (FRegular fid) | TraitMethod (_, _, fid) -> (
-                    let decl =
-                      FunDeclId.Map.find fid ctx.fun_ctx.llbc_fun_decls
-                    in
-                    match Collections.List.last decl.name with
-                    | PeIdent (s, _) -> s
-                    | PeImpl _ ->
-                        (* We shouldn't get there *)
-                        raise (Failure "Unexpected"))
-              in
-              name ^ "_back"
-            in
-            let ctx, back_vars =
-              fresh_opt_vars
-                (List.map
-                   (fun ty ->
-                     match ty with
-                     | None -> None
-                     | Some (back_sg, ty) ->
-                         (* We insert a name for the variable only if the function
-                            can fail: if it can fail, it means the call returns a backward
-                            function. Otherwise, we it directly returns the value given
-                            back by the backward function, which means we shouldn't
-                            give it a name like "back..." (it doesn't make sense) *)
-                         let name =
-                           if back_sg.effect_info.can_fail then
-                             Some back_fun_name
-                           else None
-                         in
-                         Some (name, ty))
-                   back_tys)
-                ctx
-            in
-            let back_funs =
-              List.filter_map
-                (fun v ->
-                  match v with
-                  | None -> None
-                  | Some v -> Some (mk_typed_pattern_from_var v None))
-                back_vars
-            in
-            let gids =
-              List.map
-                (fun (g : T.region_var_group) -> g.id)
-                call.regions_hierarchy
-            in
-            let back_vars =
-              List.map (Option.map mk_texpression_from_var) back_vars
-            in
-            let back_funs_map =
-              RegionGroupId.Map.of_list (List.combine gids back_vars)
-            in
-            (ctx, dsg.fwd_info.ignore_output, Some back_funs_map, back_funs))
-          else (ctx, false, None, [])
+            name ^ "_back"
+          in
+          let ctx, back_vars =
+            fresh_opt_vars
+              (List.map
+                 (fun ty ->
+                   match ty with
+                   | None -> None
+                   | Some (back_sg, ty) ->
+                       (* We insert a name for the variable only if the function
+                          can fail: if it can fail, it means the call returns a backward
+                          function. Otherwise, we it directly returns the value given
+                          back by the backward function, which means we shouldn't
+                          give it a name like "back..." (it doesn't make sense) *)
+                       let name =
+                         if back_sg.effect_info.can_fail then Some back_fun_name
+                         else None
+                       in
+                       Some (name, ty))
+                 back_tys)
+              ctx
+          in
+          let back_funs =
+            List.filter_map
+              (fun v ->
+                match v with
+                | None -> None
+                | Some v -> Some (mk_typed_pattern_from_var v None))
+              back_vars
+          in
+          let gids =
+            List.map
+              (fun (g : T.region_var_group) -> g.id)
+              call.regions_hierarchy
+          in
+          let back_vars =
+            List.map (Option.map mk_texpression_from_var) back_vars
+          in
+          let back_funs_map =
+            RegionGroupId.Map.of_list (List.combine gids back_vars)
+          in
+          (ctx, dsg.fwd_info.ignore_output, Some back_funs_map, back_funs)
         in
         (* Compute the pattern for the destination *)
         let ctx, dest = fresh_var_for_symbolic_value call.dest ctx in
@@ -2407,19 +2375,6 @@ and translate_end_abstraction_fun_call (ectx : C.eval_ctx) (abs : V.abs)
         raise (Failure "Unreachable")
   in
   let effect_info = get_fun_effect_info ctx fun_id None (Some rg_id) in
-  let generics = ctx_translate_fwd_generic_args ctx call.generics in
-  (* Retrieve the original call and the parent abstractions *)
-  let _forward, backwards = get_abs_ancestors ctx abs call_id in
-  (* Retrieve the values consumed when we called the forward function and
-   * ended the parent backward functions: those give us part of the input
-   * values (rem: for now, as we disallow nested lifetimes, there can't be
-   * parent backward functions).
-   * Note that the forward inputs **include the fuel and the input state**
-   * (if we use those). *)
-  let fwd_inputs = call_info.forward_inputs in
-  let back_ancestors_inputs =
-    List.concat (List.map (fun (_abs, args) -> args) backwards)
-  in
   (* Retrieve the values consumed upon ending the loans inside this
    * abstraction: those give us the remaining input values *)
   let back_inputs = abs_to_consumed ctx ectx abs in
@@ -2434,11 +2389,6 @@ and translate_end_abstraction_fun_call (ectx : C.eval_ctx) (abs : V.abs)
       ([ back_state ], ctx, Some nstate)
     else ([], ctx, None)
   in
-  (* Concatenate all the inpus *)
-  let inherited_inputs =
-    if !Config.return_back_funs then []
-    else List.concat [ fwd_inputs; back_ancestors_inputs ]
-  in
   let back_inputs = List.append back_inputs back_state in
   (* Retrieve the values given back by this function: those are the output
    * values. We rely on the fact that there are no nested borrows to use the
@@ -2459,58 +2409,33 @@ and translate_end_abstraction_fun_call (ectx : C.eval_ctx) (abs : V.abs)
   (* Retrieve the function id, and register the function call in the context
      if necessary.Arith_status *)
   let ctx, func =
-    bs_ctx_register_backward_call abs effect_info call_id rg_id inherited_inputs
-      back_inputs generics output.ty ctx
+    bs_ctx_register_backward_call abs call_id rg_id back_inputs ctx
   in
   (* Translate the next expression *)
   let next_e = translate_expression e ctx in
   (* Put everything together *)
-  let inputs = List.append inherited_inputs back_inputs in
+  let inputs = back_inputs in
   let args_mplaces = List.map (fun _ -> None) inputs in
   let args =
     List.map
       (fun (arg, mp) -> mk_opt_mplace_texpression mp arg)
       (List.combine inputs args_mplaces)
   in
-  (* **Optimization**:
-     =================
-     We do a small optimization here if we split the forward/backward functions.
-     If the backward function doesn't have any output, we don't introduce any function
-     call.
-     See the comment in {!Config.filter_useless_monadic_calls}.
-
-     TODO: use an option to disallow backward functions from updating the state.
-     TODO: a backward function which only gives back shared borrows shouldn't
-     update the state (state updates should only be used for mutable borrows,
-     with objects like Rc for instance).
-  *)
-  if
-    (not !Config.return_back_funs)
-    && !Config.filter_useless_monadic_calls
-    && outputs = [] && nstate = None
-  then (
-    (* No outputs - we do a small sanity check: the backward function
-       should have exactly the same number of inputs as the forward:
-       this number can be different only if the forward function returned
-       a value containing mutable borrows, which can't be the case... *)
-    assert (List.length inputs = List.length fwd_inputs);
-    next_e)
-  else
-    (* The backward function might also have been filtered if we do not
-       split the forward/backward functions *)
-    match func with
-    | None -> next_e
-    | Some func ->
-        log#ldebug
-          (lazy
-            (let args = List.map (texpression_to_string ctx) args in
-             "func: "
-             ^ texpression_to_string ctx func
-             ^ "\nfunc type: "
-             ^ pure_ty_to_string ctx func.ty
-             ^ "\n\nargs:\n" ^ String.concat "\n" args));
-        let call = mk_apps func args in
-        mk_let effect_info.can_fail output call next_e
+  (* The backward function might have been filtered it does nothing
+     (consumes unit and returns unit). *)
+  match func with
+  | None -> next_e
+  | Some func ->
+      log#ldebug
+        (lazy
+          (let args = List.map (texpression_to_string ctx) args in
+           "func: "
+           ^ texpression_to_string ctx func
+           ^ "\nfunc type: "
+           ^ pure_ty_to_string ctx func.ty
+           ^ "\n\nargs:\n" ^ String.concat "\n" args));
+      let call = mk_apps func args in
+      mk_let effect_info.can_fail output call next_e
 
 and translate_end_abstraction_identity (ectx : C.eval_ctx) (abs : V.abs)
     (e : S.expression) (ctx : bs_ctx) : texpression =
@@ -2637,10 +2562,7 @@ and translate_end_abstraction_loop (ectx : C.eval_ctx) (abs : V.abs)
         else ([], ctx, None)
       in
       (* Concatenate all the inputs *)
-      let inputs =
-        if !Config.return_back_funs then List.concat [ back_inputs; back_state ]
-        else List.concat [ fwd_inputs; back_inputs; back_state ]
-      in
+      let inputs = List.concat [ back_inputs; back_state ] in
       (* Retrieve the values given back by this function *)
       let ctx, outputs = abs_to_given_back None abs ctx in
       (* Group the output values together: first the updated inputs *)
@@ -2670,77 +2592,46 @@ and translate_end_abstraction_loop (ectx : C.eval_ctx) (abs : V.abs)
          - or a call to the variable we introduced for the backward function,
            if we merge the forward/backward functions *)
       let func =
-        if !Config.return_back_funs then
-          RegionGroupId.Map.find rg_id (Option.get loop_info.back_funs)
-        else
-          let func_ty = mk_arrows input_tys ret_ty in
-          let func = Fun (FromLlbc (FunId fun_id, Some loop_id, Some rg_id)) in
-          let func = { id = FunOrOp func; generics } in
-          Some { e = Qualif func; ty = func_ty }
+        RegionGroupId.Map.find rg_id (Option.get loop_info.back_funs)
       in
-      (* **Optimization**:
-         =================
-         We do a small optimization here in case we split the forward/backward
-         functions.
-         If the backward function doesn't have any output, we don't introduce
-         any function call.
-         See the comment in {!Config.filter_useless_monadic_calls}.
-
-         TODO: use an option to disallow backward functions from updating the state.
-         TODO: a backward function which only gives back shared borrows shouldn't
-         update the state (state updates should only be used for mutable borrows,
-         with objects like Rc for instance).
-      *)
-      if
-        (not !Config.return_back_funs)
-        && !Config.filter_useless_monadic_calls
-        && outputs = [] && nstate = None
-      then (
-        (* No outputs - we do a small sanity check: the backward function
-           should have exactly the same number of inputs as the forward:
-           this number can be different only if the forward function returned
-           a value containing mutable borrows, which can't be the case... *)
-        assert (List.length inputs = List.length fwd_inputs);
-        next_e)
-      else
-        (* In case we merge the fwd/back functions we filter the backward
-           functions elsewhere *)
-        match func with
-        | None -> next_e
-        | Some func ->
-            let call = mk_apps func args in
-            (* Add meta-information - this is slightly hacky: we look at the
-               values consumed by the abstraction (note that those come from
-               *before* we applied the fixed-point context) and use them to
-               guide the naming of the output vars.
-
-               Also, we need to convert the backward outputs from patterns to
-               variables.
-
-               Finally, in practice, this works well only for loop bodies:
-               we do this only in this case.
-               TODO: improve the heuristics, to give weight to the hints for
-               instance.
-            *)
-            let next_e =
-              if ctx.inside_loop then
-                let consumed_values = abs_to_consumed ctx ectx abs in
-                let var_values = List.combine outputs consumed_values in
-                let var_values =
-                  List.filter_map
-                    (fun (var, v) ->
-                      match var.Pure.value with
-                      | PatVar (var, _) -> Some (var, v)
-                      | _ -> None)
-                    var_values
-                in
-                let vars, values = List.split var_values in
-                mk_emeta_symbolic_assignments vars values next_e
-              else next_e
-            in
+      (* We may have filtered the backward function elsewhere if it doesn't
+         do anything (doesn't consume anything and doesn't return anything) *)
+      match func with
+      | None -> next_e
+      | Some func ->
+          let call = mk_apps func args in
+          (* Add meta-information - this is slightly hacky: we look at the
+             values consumed by the abstraction (note that those come from
+             *before* we applied the fixed-point context) and use them to
+             guide the naming of the output vars.
+
+             Also, we need to convert the backward outputs from patterns to
+             variables.
+
+             Finally, in practice, this works well only for loop bodies:
+             we do this only in this case.
+             TODO: improve the heuristics, to give weight to the hints for
+             instance.
+          *)
+          let next_e =
+            if ctx.inside_loop then
+              let consumed_values = abs_to_consumed ctx ectx abs in
+              let var_values = List.combine outputs consumed_values in
+              let var_values =
+                List.filter_map
+                  (fun (var, v) ->
+                    match var.Pure.value with
+                    | PatVar (var, _) -> Some (var, v)
+                    | _ -> None)
+                  var_values
+              in
+              let vars, values = List.split var_values in
+              mk_emeta_symbolic_assignments vars values next_e
+            else next_e
+          in
 
-            (* Create the let-binding *)
-            mk_let effect_info.can_fail output call next_e)
+          (* Create the let-binding *)
+          mk_let effect_info.can_fail output call next_e)
 
 and translate_global_eval (gid : A.GlobalDeclId.id) (sval : V.symbolic_value)
     (e : S.expression) (ctx : bs_ctx) : texpression =
@@ -3068,48 +2959,40 @@ and translate_forward_end (ectx : C.eval_ctx)
           *)
           let ctx =
             (* Introduce variables for the inputs and the state variable
-               and update the context. *)
-            if !Config.return_back_funs then
-              (* If the forward/backward functions are not split, we need
-                 to introduce fresh variables for the additional inputs,
-                 because they are locally introduced in a lambda *)
-              let back_sg = RegionGroupId.Map.find bid ctx.sg.back_sg in
-              let ctx, backward_inputs_no_state =
-                fresh_vars back_sg.inputs_no_state ctx
-              in
-              let ctx, backward_inputs_with_state =
-                if back_sg.effect_info.stateful then
-                  let ctx, var, _ = bs_ctx_fresh_state_var ctx in
-                  (ctx, backward_inputs_no_state @ [ var ])
-                else (ctx, backward_inputs_no_state)
-              in
-              {
-                ctx with
-                backward_inputs_no_state =
-                  RegionGroupId.Map.add bid backward_inputs_no_state
-                    ctx.backward_inputs_no_state;
-                backward_inputs_with_state =
-                  RegionGroupId.Map.add bid backward_inputs_with_state
-                    ctx.backward_inputs_with_state;
-              }
-            else
-              (* Update the state variable *)
-              let back_state_var =
-                RegionGroupId.Map.find bid ctx.back_state_vars
-              in
-              { ctx with state_var = back_state_var }
+               and update the context.
+
+               We need to introduce fresh variables for the additional inputs,
+               because they are locally introduced in a lambda.
+            *)
+            let back_sg = RegionGroupId.Map.find bid ctx.sg.back_sg in
+            let ctx, backward_inputs_no_state =
+              fresh_vars back_sg.inputs_no_state ctx
+            in
+            let ctx, backward_inputs_with_state =
+              if back_sg.effect_info.stateful then
+                let ctx, var, _ = bs_ctx_fresh_state_var ctx in
+                (ctx, backward_inputs_no_state @ [ var ])
+              else (ctx, backward_inputs_no_state)
+            in
+            {
+              ctx with
+              backward_inputs_no_state =
+                RegionGroupId.Map.add bid backward_inputs_no_state
+                  ctx.backward_inputs_no_state;
+              backward_inputs_with_state =
+                RegionGroupId.Map.add bid backward_inputs_with_state
+                  ctx.backward_inputs_with_state;
+            }
           in
 
           let e = T.RegionGroupId.Map.find bid back_e in
           let finish e =
             (* Wrap in lambdas if necessary *)
-            if !Config.return_back_funs then
-              let inputs =
-                RegionGroupId.Map.find bid ctx.backward_inputs_with_state
-              in
-              let places = List.map (fun _ -> None) inputs in
-              mk_lambdas_from_vars inputs places e
-            else e
+            let inputs =
+              RegionGroupId.Map.find bid ctx.backward_inputs_with_state
+            in
+            let places = List.map (fun _ -> None) inputs in
+            mk_lambdas_from_vars inputs places e
           in
           (ctx, e, finish)
     in
@@ -3131,85 +3014,83 @@ and translate_forward_end (ectx : C.eval_ctx)
      function, if needs be, and lookup the proper expression.
   *)
   let translate_end ctx =
-    if !Config.return_back_funs then
-      (* Compute the output of the forward function *)
-      let fwd_effect_info = ctx.sg.fwd_info.effect_info in
-      let ctx, pure_fwd_var = fresh_var None ctx.sg.fwd_output ctx in
-      let fwd_e = translate_one_end ctx None in
-
-      (* Introduce the backward functions. *)
-      let back_el =
-        List.map
-          (fun ((gid, _) : RegionGroupId.id * back_sg_info) ->
-            translate_one_end ctx (Some gid))
-          (RegionGroupId.Map.bindings ctx.sg.back_sg)
-      in
+    (* Compute the output of the forward function *)
+    let fwd_effect_info = ctx.sg.fwd_info.effect_info in
+    let ctx, pure_fwd_var = fresh_var None ctx.sg.fwd_output ctx in
+    let fwd_e = translate_one_end ctx None in
 
-      (* Compute whether the backward expressions should be evaluated straight
-         away or not (i.e., if we should bind them with monadic let-bindings
-         or not). We evaluate them straight away if they can fail and have no
-         inputs. *)
-      let evaluate_backs =
-        List.map
-          (fun (sg : back_sg_info) ->
-            if !Config.simplify_merged_fwd_backs then
-              sg.inputs = [] && sg.effect_info.can_fail
-            else false)
-          (RegionGroupId.Map.values ctx.sg.back_sg)
-      in
+    (* Introduce the backward functions. *)
+    let back_el =
+      List.map
+        (fun ((gid, _) : RegionGroupId.id * back_sg_info) ->
+          translate_one_end ctx (Some gid))
+        (RegionGroupId.Map.bindings ctx.sg.back_sg)
+    in
 
-      (* Introduce variables for the backward functions.
-         We lookup the LLBC definition in an attempt to derive pretty names
-         for those functions. *)
-      let _, back_vars = fresh_back_vars_for_current_fun ctx in
+    (* Compute whether the backward expressions should be evaluated straight
+       away or not (i.e., if we should bind them with monadic let-bindings
+       or not). We evaluate them straight away if they can fail and have no
+       inputs. *)
+    let evaluate_backs =
+      List.map
+        (fun (sg : back_sg_info) ->
+          if !Config.simplify_merged_fwd_backs then
+            sg.inputs = [] && sg.effect_info.can_fail
+          else false)
+        (RegionGroupId.Map.values ctx.sg.back_sg)
+    in
 
-      (* Create the return expressions *)
-      let vars =
-        let back_vars = List.filter_map (fun x -> x) back_vars in
-        if ctx.sg.fwd_info.ignore_output then back_vars
-        else pure_fwd_var :: back_vars
-      in
-      let vars = List.map mk_texpression_from_var vars in
-      let ret = mk_simpl_tuple_texpression vars in
-
-      (* Introduce a fresh input state variable for the forward expression *)
-      let _ctx, state_var, state_pat =
-        if fwd_effect_info.stateful then
-          let ctx, var, pat = bs_ctx_fresh_state_var ctx in
-          (ctx, [ var ], [ pat ])
-        else (ctx, [], [])
-      in
+    (* Introduce variables for the backward functions.
+       We lookup the LLBC definition in an attempt to derive pretty names
+       for those functions. *)
+    let _, back_vars = fresh_back_vars_for_current_fun ctx in
+
+    (* Create the return expressions *)
+    let vars =
+      let back_vars = List.filter_map (fun x -> x) back_vars in
+      if ctx.sg.fwd_info.ignore_output then back_vars
+      else pure_fwd_var :: back_vars
+    in
+    let vars = List.map mk_texpression_from_var vars in
+    let ret = mk_simpl_tuple_texpression vars in
+
+    (* Introduce a fresh input state variable for the forward expression *)
+    let _ctx, state_var, state_pat =
+      if fwd_effect_info.stateful then
+        let ctx, var, pat = bs_ctx_fresh_state_var ctx in
+        (ctx, [ var ], [ pat ])
+      else (ctx, [], [])
+    in
 
-      let state_var = List.map mk_texpression_from_var state_var in
-      let ret = mk_simpl_tuple_texpression (state_var @ [ ret ]) in
-      let ret = mk_result_return_texpression ret in
+    let state_var = List.map mk_texpression_from_var state_var in
+    let ret = mk_simpl_tuple_texpression (state_var @ [ ret ]) in
+    let ret = mk_result_return_texpression ret in
 
-      (* Introduce all the let-bindings *)
+    (* Introduce all the let-bindings *)
 
-      (* Combine:
-         - the backward variables
-         - whether we should evaluate the expression for the backward function
-           (i.e., should we use a monadic let-binding or not - we do if the
-           backward functions don't have inputs and can fail)
-         - the expressions for the backward functions
-      *)
-      let back_vars_els =
-        List.filter_map
-          (fun (v, (eval, el)) ->
-            match v with None -> None | Some v -> Some (v, eval, el))
-          (List.combine back_vars (List.combine evaluate_backs back_el))
-      in
-      let e =
-        List.fold_right
-          (fun (var, evaluate, back_e) e ->
-            mk_let evaluate (mk_typed_pattern_from_var var None) back_e e)
-          back_vars_els ret
-      in
-      (* Bind the expression for the forward output *)
-      let fwd_var = mk_typed_pattern_from_var pure_fwd_var None in
-      let pat = mk_simpl_tuple_pattern (state_pat @ [ fwd_var ]) in
-      mk_let fwd_effect_info.can_fail pat fwd_e e
-    else translate_one_end ctx ctx.bid
+    (* Combine:
+       - the backward variables
+       - whether we should evaluate the expression for the backward function
+         (i.e., should we use a monadic let-binding or not - we do if the
+         backward functions don't have inputs and can fail)
+       - the expressions for the backward functions
+    *)
+    let back_vars_els =
+      List.filter_map
+        (fun (v, (eval, el)) ->
+          match v with None -> None | Some v -> Some (v, eval, el))
+        (List.combine back_vars (List.combine evaluate_backs back_el))
+    in
+    let e =
+      List.fold_right
+        (fun (var, evaluate, back_e) e ->
+          mk_let evaluate (mk_typed_pattern_from_var var None) back_e e)
+        back_vars_els ret
+    in
+    (* Bind the expression for the forward output *)
+    let fwd_var = mk_typed_pattern_from_var pure_fwd_var None in
+    let pat = mk_simpl_tuple_pattern (state_pat @ [ fwd_var ]) in
+    mk_let fwd_effect_info.can_fail pat fwd_e e
   in
 
   (* If we are (re-)entering a loop, we need to introduce a call to the
@@ -3279,24 +3160,22 @@ and translate_forward_end (ectx : C.eval_ctx)
          backward functions of the outer function.
       *)
       let ctx, back_funs_map, back_funs =
-        if !Config.return_back_funs then
-          let ctx, back_vars = fresh_back_vars_for_current_fun ctx in
-          let back_funs =
-            List.filter_map
-              (fun v ->
-                match v with
-                | None -> None
-                | Some v -> Some (mk_typed_pattern_from_var v None))
-              back_vars
-          in
-          let gids = RegionGroupId.Map.keys ctx.sg.back_sg in
-          let back_funs_map =
-            RegionGroupId.Map.of_list
-              (List.combine gids
-                 (List.map (Option.map mk_texpression_from_var) back_vars))
-          in
-          (ctx, Some back_funs_map, back_funs)
-        else (ctx, None, [])
+        let ctx, back_vars = fresh_back_vars_for_current_fun ctx in
+        let back_funs =
+          List.filter_map
+            (fun v ->
+              match v with
+              | None -> None
+              | Some v -> Some (mk_typed_pattern_from_var v None))
+            back_vars
+        in
+        let gids = RegionGroupId.Map.keys ctx.sg.back_sg in
+        let back_funs_map =
+          RegionGroupId.Map.of_list
+            (List.combine gids
+               (List.map (Option.map mk_texpression_from_var) back_vars))
+        in
+        (ctx, Some back_funs_map, back_funs)
       in
 
       (* Introduce patterns *)
@@ -3438,91 +3317,58 @@ and translate_loop (loop : S.loop) (ctx : bs_ctx) : texpression =
   (* The output type of the loop function *)
   let fwd_effect_info = { ctx.sg.fwd_info.effect_info with is_rec = true } in
   let back_effect_infos, output_ty =
-    if !Config.return_back_funs then
-      (* The loop backward functions consume the same additional inputs as the parent
-         function, but have custom outputs *)
-      let back_sgs = RegionGroupId.Map.bindings ctx.sg.back_sg in
-      let given_back_tys = RegionGroupId.Map.values rg_to_given_back_tys in
-      let back_info_tys =
-        List.map
-          (fun (((id, back_sg), given_back) : (_ * back_sg_info) * ty list) ->
-            (* Remark: the effect info of the backward function for the loop
-               is almost the same as for the backward function of the parent function.
-               Quite importantly, the fact that the function is stateful and/or can fail
-               mostly depends on whether it has inputs or not, and the backward functions
-               for the loops have the same inputs as the backward functions for the parent
-               function.
-            *)
-            let effect_info = back_sg.effect_info in
-            let effect_info = { effect_info with is_rec = true } in
-            (* Compute the input/output types *)
-            let inputs = List.map snd back_sg.inputs in
-            let outputs = given_back in
-            (* Filter if necessary *)
-            let ty =
-              if
-                !Config.simplify_merged_fwd_backs && inputs = [] && outputs = []
-              then None
-              else
-                let output = mk_simpl_tuple_ty outputs in
-                let output =
-                  mk_back_output_ty_from_effect_info effect_info inputs output
-                in
-                let ty = mk_arrows inputs output in
-                Some ty
-            in
-            ((id, effect_info), ty))
-          (List.combine back_sgs given_back_tys)
-      in
-      let back_info = List.map fst back_info_tys in
-      let back_info = RegionGroupId.Map.of_list back_info in
-      let back_tys = List.filter_map snd back_info_tys in
-      let output =
-        if ctx.sg.fwd_info.ignore_output then back_tys
-        else ctx.sg.fwd_output :: back_tys
-      in
-      let output = mk_simpl_tuple_ty output in
-      let effect_info = ctx.sg.fwd_info.effect_info in
-      let output =
-        if effect_info.stateful then mk_simpl_tuple_ty [ mk_state_ty; output ]
-        else output
-      in
-      let output =
-        if effect_info.can_fail && inputs <> [] then mk_result_ty output
-        else output
-      in
-      (back_info, output)
-    else
-      let back_info =
-        RegionGroupId.Map.of_list
-          (List.map
-             (fun ((id, back_sg) : _ * back_sg_info) ->
-               (id, { back_sg.effect_info with is_rec = true }))
-             (RegionGroupId.Map.bindings ctx.sg.back_sg))
-      in
-      let output =
-        match ctx.bid with
-        | None ->
-            (* Forward function: same type as the parent function *)
-            (translate_fun_sig_from_decomposed ctx.sg None).output
-        | Some rg_id ->
-            (* Backward function: custom return type *)
-            let doutputs =
-              T.RegionGroupId.Map.find rg_id rg_to_given_back_tys
-            in
-            let output = mk_simpl_tuple_ty doutputs in
-            let fwd_effect_info = ctx.sg.fwd_info.effect_info in
-            let output =
-              if fwd_effect_info.stateful then
-                mk_simpl_tuple_ty [ mk_state_ty; output ]
-              else output
-            in
-            let output =
-              if fwd_effect_info.can_fail then mk_result_ty output else output
-            in
-            output
-      in
-      (back_info, output)
+    (* The loop backward functions consume the same additional inputs as the parent
+       function, but have custom outputs *)
+    let back_sgs = RegionGroupId.Map.bindings ctx.sg.back_sg in
+    let given_back_tys = RegionGroupId.Map.values rg_to_given_back_tys in
+    let back_info_tys =
+      List.map
+        (fun (((id, back_sg), given_back) : (_ * back_sg_info) * ty list) ->
+          (* Remark: the effect info of the backward function for the loop
+             is almost the same as for the backward function of the parent function.
+             Quite importantly, the fact that the function is stateful and/or can fail
+             mostly depends on whether it has inputs or not, and the backward functions
+             for the loops have the same inputs as the backward functions for the parent
+             function.
+          *)
+          let effect_info = back_sg.effect_info in
+          let effect_info = { effect_info with is_rec = true } in
+          (* Compute the input/output types *)
+          let inputs = List.map snd back_sg.inputs in
+          let outputs = given_back in
+          (* Filter if necessary *)
+          let ty =
+            if !Config.simplify_merged_fwd_backs && inputs = [] && outputs = []
+            then None
+            else
+              let output = mk_simpl_tuple_ty outputs in
+              let output =
+                mk_back_output_ty_from_effect_info effect_info inputs output
+              in
+              let ty = mk_arrows inputs output in
+              Some ty
+          in
+          ((id, effect_info), ty))
+        (List.combine back_sgs given_back_tys)
+    in
+    let back_info = List.map fst back_info_tys in
+    let back_info = RegionGroupId.Map.of_list back_info in
+    let back_tys = List.filter_map snd back_info_tys in
+    let output =
+      if ctx.sg.fwd_info.ignore_output then back_tys
+      else ctx.sg.fwd_output :: back_tys
+    in
+    let output = mk_simpl_tuple_ty output in
+    let effect_info = ctx.sg.fwd_info.effect_info in
+    let output =
+      if effect_info.stateful then mk_simpl_tuple_ty [ mk_state_ty; output ]
+      else output
+    in
+    let output =
+      if effect_info.can_fail && inputs <> [] then mk_result_ty output
+      else output
+    in
+    (back_info, output)
   in
 
   (* Add the loop information in the context *)
@@ -3708,21 +3554,19 @@ let wrap_in_match_fuel (fuel0 : VarId.id) (fuel : VarId.id) (body : texpression)
 let translate_fun_decl (ctx : bs_ctx) (body : S.expression option) : fun_decl =
   (* Translate *)
   let def = ctx.fun_decl in
-  let bid = ctx.bid in
+  assert (ctx.bid = None);
   log#ldebug
     (lazy
       ("SymbolicToPure.translate_fun_decl: "
       ^ name_to_string ctx def.name
-      ^ " ("
-      ^ Print.option_to_string T.RegionGroupId.to_string bid
-      ^ ")\n"));
+      ^ "\n"));
 
   (* Translate the declaration *)
   let def_id = def.def_id in
   let llbc_name = def.name in
   let name = name_to_string ctx llbc_name in
   (* Translate the signature *)
-  let signature = translate_fun_sig_from_decomposed ctx.sg ctx.bid in
+  let signature = translate_fun_sig_from_decomposed ctx.sg in
   let regions_hierarchy =
     FunIdMap.find (FRegular def_id) ctx.fun_ctx.regions_hierarchies
   in
@@ -3732,7 +3576,7 @@ let translate_fun_decl (ctx : bs_ctx) (body : S.expression option) : fun_decl =
     | None -> None
     | Some body ->
         let effect_info =
-          get_fun_effect_info ctx (FunId (FRegular def_id)) None bid
+          get_fun_effect_info ctx (FunId (FRegular def_id)) None None
         in
         let body = translate_expression body ctx in
         (* Add a match over the fuel, if necessary *)
@@ -3760,37 +3604,8 @@ let translate_fun_decl (ctx : bs_ctx) (body : S.expression option) : fun_decl =
           if effect_info.stateful_group then [ mk_state_var ctx.state_var ]
           else []
         in
-        (* Compute the list of (properly ordered) backward input variables *)
-        let backward_inputs : var list =
-          match bid with
-          | None -> []
-          | Some back_id ->
-              assert (not !Config.return_back_funs);
-              let parents_ids =
-                list_ordered_ancestor_region_groups regions_hierarchy back_id
-              in
-              let backward_ids = List.append parents_ids [ back_id ] in
-              List.concat
-                (List.map
-                   (fun id ->
-                     T.RegionGroupId.Map.find id ctx.backward_inputs_no_state)
-                   backward_ids)
-        in
-        (* Introduce the backward input state (the state at call site of the
-         * *backward* function), if necessary *)
-        let back_state =
-          if effect_info.stateful && Option.is_some bid then
-            let state_var =
-              RegionGroupId.Map.find (Option.get bid) ctx.back_state_vars
-            in
-            [ mk_state_var state_var ]
-          else []
-        in
         (* Group the inputs together *)
-        let inputs =
-          List.concat
-            [ fuel; ctx.forward_inputs; fwd_state; backward_inputs; back_state ]
-        in
+        let inputs = List.concat [ fuel; ctx.forward_inputs; fwd_state ] in
         let inputs_lvs =
           List.map (fun v -> mk_typed_pattern_from_var v None) inputs
         in
@@ -3799,16 +3614,10 @@ let translate_fun_decl (ctx : bs_ctx) (body : S.expression option) : fun_decl =
           (lazy
             ("SymbolicToPure.translate_fun_decl: "
             ^ name_to_string ctx def.name
-            ^ " ("
-            ^ Print.option_to_string T.RegionGroupId.to_string bid
-            ^ ")" ^ "\n- forward_inputs: "
+            ^ "\n- inputs: "
             ^ String.concat ", " (List.map show_var ctx.forward_inputs)
-            ^ "\n- fwd_state: "
+            ^ "\n- state: "
             ^ String.concat ", " (List.map show_var fwd_state)
-            ^ "\n- backward_inputs: "
-            ^ String.concat ", " (List.map show_var backward_inputs)
-            ^ "\n- back_state: "
-            ^ String.concat ", " (List.map show_var back_state)
             ^ "\n- signature.inputs: "
             ^ String.concat ", "
                 (List.map (pure_ty_to_string ctx) signature.inputs)));
@@ -3837,7 +3646,6 @@ let translate_fun_decl (ctx : bs_ctx) (body : S.expression option) : fun_decl =
       kind = def.kind;
       num_loops;
       loop_id;
-      back_id = bid;
       llbc_name;
       name;
       signature;