path: root/src/PureMicroPasses.ml

(** The following module defines micro-passes which operate on the pure AST *)

open Pure
open PureUtils
open TranslateCore
module V = Values

(** The local logger *)
let log = L.pure_micro_passes_log

type config = {
  decompose_monadic_let_bindings : bool;
      (** Some provers like F* don't support the decomposition of return values
          in monadic let-bindings:
          ```
          // NOT supported in F*
          let (x, y) <-- f ();
          ...
          ```

          In such situations, we might want to introduce an intermediate
          assignment:
          ```
          let tmp <-- f ();
          let (x, y) = tmp in
          ...
          ```
       *)
  unfold_monadic_let_bindings : bool;
      (** Controls the unfolding of monadic let-bindings to explicit matches:
          
          `y <-- f x; ...`

          becomes:
          
          `match f x with | Failure -> Failure | Return y -> ...`

          
          This is useful when extracting to F*: the support for monadic
          definitions is not super powerful.
          Note that when [undolf_monadic_let_bindings] is true, setting
          [decompose_monadic_let_bindings] to true and only makes the code
          more verbose.
       *)
  filter_useless_monadic_calls : bool;
      (** Controls whether we try to filter the calls to monadic functions
          (which can fail) when their outputs are not used.
          
          See the comments for [expression_contains_child_call_in_all_paths]
          for additional explanations.
          
          TODO: rename to [filter_useless_monadic_calls]
       *)
  filter_useless_functions : bool;
      (** If [filter_useless_monadic_calls] is activated, some functions
          become useless: if this option is true, we don't extract them.

          The calls to functions which always get filtered are:
          - the forward functions with unit return value
          - the backward functions which don't output anything (backward
            functions coming from rust functions with no mutable borrows
            as input values - note that if a function doesn't take mutable
            borrows as inputs, it can't return mutable borrows; we actually
            dynamically check for that).
       *)
}
(** A configuration to control the application of the passes *)

(** Small utility.

    We sometimes have to insert new fresh variables in a function body, in which
    case we need to make their indices greater than the indices of all the variables
    in the body.
    TODO: things would be simpler if we used a better representation of the
    variables indices...
 *)
let get_body_min_var_counter (body : fun_body) : VarId.generator =
  (* Find the max id in the input variables - some of them may have been
   * filtered from the body *)
  let min_input_id =
    List.fold_left
      (fun id (var : var) -> VarId.max id var.id)
      VarId.zero body.inputs
  in
  let obj =
    object
      inherit [_] reduce_expression

      method zero _ = min_input_id

      method plus id0 id1 _ = VarId.max (id0 ()) (id1 ())
      (* Get the maximum *)

      method! visit_var _ v _ = v.id
      (** For the patterns *)

      method! visit_Var _ vid _ = vid
      (** For the rvalues *)
    end
  in
  (* Find the max counter in the body *)
  let id = obj#visit_expression () body.body.e () in
  VarId.generator_from_incr_id id

type pn_ctx = {
  pure_vars : string VarId.Map.t;
      (** Information about the pure variables used in the synthesized program *)
  llbc_vars : string V.VarId.Map.t;
      (** Information about the LLBC variables used in the original program *)
}
(** "pretty-name context": see [compute_pretty_names] *)

(** This function computes pretty names for the variables in the pure AST. It
    relies on the "meta"-place information in the AST to generate naming
    constraints, and then uses those to compute the names.
    
    The way it works is as follows:
    - we only modify the names of the unnamed variables
    - whenever we see an rvalue/pattern which is exactly an unnamed variable,
      and this value is linked to some meta-place information which contains
      a name and an empty path, we consider we should use this name
    - we try to propagate naming constraints on the pure variables use in the
      synthesized programs, and also on the LLBC variables from the original
      program (information about the LLBC variables is stored in the meta-places)
      
      
    Something important is that, for every variable we find, the name of this
    variable can be influenced by the information we find *below* in the AST.

    For instance, the following situations happen:
    
    - let's say we evaluate:
      ```
      match (ls : List<T>) {
        List::Cons(x, hd) => {
          ...
        }
      }
      ```
      
      Actually, in MIR, we get:
      ```
      tmp := discriminant(ls);
      switch tmp {
        0 => {
          x := (ls as Cons).0; // (i)
          hd := (ls as Cons).1; // (ii)
          ...
        }
      }
      ```
      If `ls` maps to a symbolic value `s0` upon evaluating the match in symbolic
      mode, we expand this value upon evaluating `tmp = discriminant(ls)`.
      However, at this point, we don't know which should be the names of
      the symbolic values we introduce for the fields of `Cons`!

      Let's imagine we have (for the `Cons` branch): `s0 ~~> Cons s1 s2`.
      The assigments at (i) and (ii) lead to the following binding in the
      evaluation context:
      ```
      x -> s1
      hd -> s2
      ```
      
      When generating the symbolic AST, we save as meta-information that we
      assign `s1` to the place `x` and `s2` to the place `hd`. This way,
      we learn we can use the names `x` and `hd` for the variables which are
      introduced by the match:
      ```
      match ls with
      | Cons x hd -> ...
      | ...
      ```
   - Assignments:
     `let x [@mplace=lp] = v [@mplace = rp} in ...`
     
     We propagate naming information across the assignments. This is important
     because many reassignments using temporary, anonymous variables are
     introduced during desugaring.
   
   - Given back values (introduced by backward functions):
     Let's say we have the following Rust code:
     ```
     let py = id(&mut x);
     *py = 2;
     assert!(x == 2);
     ```
     
     After desugaring, we get the following MIR:
     ```
     ^0 = &mut x; // anonymous variable
     py = id(move ^0);
     *py += 2;
     assert!(x == 2);
     ```
     
     We want this to be translated as:
     ```
     let py = id_fwd x in
     let py1 = py + 2 in
     let x1 = id_back x py1 in // <-- x1 is "given back": doesn't appear in the original MIR
     assert(x1 = 2);
     ```

     We want to notice that the value given back by `id_back` is given back for "x",
     so we should use "x" as the basename (hence the resulting name "x1"). However,
     this is non-trivial, because after desugaring the input argument given to `id`
     is not `&mut x` but `move ^0` (i.e., it comes from a temporary, anonymous
     variable). For this reason, we use the meta-place "&mut x" as the meta-place
     for the given back value (this is done during the synthesis), and propagate
     naming information *also* on the LLBC variables (which are referenced by the
     meta-places).

     This way, because of `^0 = &mut x`, we can propagate the name "x" to the place
     `^0`, then to the given back variable across the function call.
   
 *)
let compute_pretty_names (def : fun_decl) : fun_decl =
  (* Small helpers *)
  (* 
   * When we do branchings, we need to merge (the constraints saved in) the
   * contexts returned by the different branches.
   *
   * Note that by doing so, some mappings from var id to name
   * in one context may be overriden by the ones in the other context.
   *
   * This should be ok because:
   * - generally, the overriden variables should have been introduced *inside*
   *   the branches, in which case we don't care
   * - or they were introduced before, in which case the naming should generally
   *   be consistent? In the worse case, it isn't, but it leads only to less
   *   readable code, not to unsoundness. This case should be pretty rare,
   *   also.
   *)
  let merge_ctxs (ctx0 : pn_ctx) (ctx1 : pn_ctx) : pn_ctx =
    let pure_vars =
      VarId.Map.fold
        (fun id name ctx -> VarId.Map.add id name ctx)
        ctx0.pure_vars ctx1.pure_vars
    in
    let llbc_vars =
      V.VarId.Map.fold
        (fun id name ctx -> V.VarId.Map.add id name ctx)
        ctx0.llbc_vars ctx1.llbc_vars
    in
    { pure_vars; llbc_vars }
  in
  let empty_ctx =
    { pure_vars = VarId.Map.empty; llbc_vars = V.VarId.Map.empty }
  in
  let merge_ctxs_ls (ctxs : pn_ctx list) : pn_ctx =
    List.fold_left (fun ctx0 ctx1 -> merge_ctxs ctx0 ctx1) empty_ctx ctxs
  in

  (*
   * The way we do is as follows:
   * - we explore the expressions
   * - we register the variables introduced by the let-bindings
   * - we use the naming information we find (through the variables and the
   *   meta-places) to update our context (i.e., maps from variable ids to
   *   names)
   * - we use this information to update the names of the variables used in the
   *   expressions
   *)

  (* Register a variable for constraints propagation - used when an variable is
   * introduced (left-hand side of a left binding) *)
  let register_var (ctx : pn_ctx) (v : var) : pn_ctx =
    assert (not (VarId.Map.mem v.id ctx.pure_vars));
    match v.basename with
    | None -> ctx
    | Some name ->
        let pure_vars = VarId.Map.add v.id name ctx.pure_vars in
        { ctx with pure_vars }
  in
  (* Update a variable - used to update an expression after we computed constraints *)
  let update_var (ctx : pn_ctx) (v : var) (mp : mplace option) : var =
    match v.basename with
    | Some _ -> v
    | None -> (
        match VarId.Map.find_opt v.id ctx.pure_vars with
        | Some basename -> { v with basename = Some basename }
        | None ->
            if Option.is_some mp then
              match
                V.VarId.Map.find_opt (Option.get mp).var_id ctx.llbc_vars
              with
              | None -> v
              | Some basename -> { v with basename = Some basename }
            else v)
  in
  (* Update an pattern - used to update an expression after we computed constraints *)
  let update_typed_pattern ctx (lv : typed_pattern) : typed_pattern =
    let obj =
      object
        inherit [_] map_typed_pattern

        method! visit_PatVar _ v mp = PatVar (update_var ctx v mp, mp)
      end
    in
    obj#visit_typed_pattern () lv
  in

  (* Register an mplace the first time we find one *)
  let register_mplace (mp : mplace) (ctx : pn_ctx) : pn_ctx =
    match (V.VarId.Map.find_opt mp.var_id ctx.llbc_vars, mp.name) with
    | None, Some name ->
        let llbc_vars = V.VarId.Map.add mp.var_id name ctx.llbc_vars in
        { ctx with llbc_vars }
    | _ -> ctx
  in

  (* Register the fact that [name] can be used for the pure variable identified
   * by [var_id] (will add this name in the map if the variable is anonymous) *)
  let add_pure_var_constraint (var_id : VarId.id) (name : string) (ctx : pn_ctx)
      : pn_ctx =
    let pure_vars =
      if VarId.Map.mem var_id ctx.pure_vars then ctx.pure_vars
      else VarId.Map.add var_id name ctx.pure_vars
    in
    { ctx with pure_vars }
  in
  (* Similar to [add_pure_var_constraint], but for LLBC variables *)
  let add_llbc_var_constraint (var_id : V.VarId.id) (name : string)
      (ctx : pn_ctx) : pn_ctx =
    let llbc_vars =
      if V.VarId.Map.mem var_id ctx.llbc_vars then ctx.llbc_vars
      else V.VarId.Map.add var_id name ctx.llbc_vars
    in
    { ctx with llbc_vars }
  in
  (* Add a constraint: given a variable id and an associated meta-place, try to
   * extract naming information from the meta-place and save it *)
  let add_constraint (mp : mplace) (var_id : VarId.id) (ctx : pn_ctx) : pn_ctx =
    (* Register the place *)
    let ctx = register_mplace mp ctx in
    (* Update the variable name *)
    match (mp.name, mp.projection) with
    | Some name, [] ->
        (* Check if the variable already has a name - if not: insert the new name *)
        let ctx = add_pure_var_constraint var_id name ctx in
        let ctx = add_llbc_var_constraint mp.var_id name ctx in
        ctx
    | _ -> ctx
  in
  (* Specific case of constraint on rvalues *)
  let add_right_constraint (mp : mplace) (rv : texpression) (ctx : pn_ctx) :
      pn_ctx =
    (* Register the place *)
    let ctx = register_mplace mp ctx in
    (* Add the constraint *)
    match (unmeta rv).e with Var vid -> add_constraint mp vid ctx | _ -> ctx
  in
  (* Specific case of constraint on left values *)
  let add_left_constraint (lv : typed_pattern) (ctx : pn_ctx) : pn_ctx =
    let obj =
      object (self)
        inherit [_] reduce_typed_pattern

        method zero _ = empty_ctx

        method plus ctx0 ctx1 _ = merge_ctxs (ctx0 ()) (ctx1 ())

        method! visit_PatVar _ v mp () =
          (* Register the variable *)
          let ctx = register_var (self#zero ()) v in
          (* Register the mplace information if there is such information *)
          match mp with Some mp -> add_constraint mp v.id ctx | None -> ctx
      end
    in
    let ctx1 = obj#visit_typed_pattern () lv () in
    merge_ctxs ctx ctx1
  in

  (* This is used to propagate constraint information about places in case of
   * variable reassignments: we try to propagate the information from the
   * rvalue to the left *)
  let add_left_right_constraint (lv : typed_pattern) (re : texpression)
      (ctx : pn_ctx) : pn_ctx =
    (* We propagate constraints across variable reassignments: `^0 = x`,
     * if the destination doesn't have naming information *)
    match lv.value with
    | PatVar (({ id = _; basename = None; ty = _ } as lvar), lmp) ->
        if
          (* Check that there is not already a name for the variable *)
          VarId.Map.mem lvar.id ctx.pure_vars
        then ctx
        else
          (* We ignore the left meta-place information: it should have been taken
           * care of by [add_left_constraint]. We try to use the right meta-place
           * information *)
          let add (name : string) (ctx : pn_ctx) : pn_ctx =
            (* Add the constraint for the pure variable *)
            let ctx = add_pure_var_constraint lvar.id name ctx in
            (* Add the constraint for the LLBC variable *)
            match lmp with
            | None -> ctx
            | Some lmp -> add_llbc_var_constraint lmp.var_id name ctx
          in
          (* We try to use the right-place information *)
          let rmp, re = opt_unmeta_mplace re in
          let ctx =
            match rmp with
            | Some { var_id; name; projection = [] } -> (
                if Option.is_some name then add (Option.get name) ctx
                else
                  match V.VarId.Map.find_opt var_id ctx.llbc_vars with
                  | None -> ctx
                  | Some name -> add name ctx)
            | _ -> ctx
          in
          (* We try to use the rvalue information, if it is a variable *)
          let ctx =
            match (unmeta re).e with
            | Var rvar_id -> (
                match VarId.Map.find_opt rvar_id ctx.pure_vars with
                | None -> ctx
                | Some name -> add name ctx)
            | _ -> ctx
          in
          ctx
    | _ -> ctx
  in

  (* *)
  let rec update_texpression (e : texpression) (ctx : pn_ctx) :
      pn_ctx * texpression =
    let ty = e.ty in
    let ctx, e =
      match e.e with
      | Var _ -> (* Nothing to do *) (ctx, e.e)
      | Const _ -> (* Nothing to do *) (ctx, e.e)
      | App (app, arg) ->
          let ctx, app = update_texpression app ctx in
          let ctx, arg = update_texpression arg ctx in
          let e = App (app, arg) in
          (ctx, e)
      | Abs (x, e) -> update_abs x e ctx
      | Qualif _ -> (* nothing to do *) (ctx, e.e)
      | Let (monadic, lb, re, e) -> update_let monadic lb re e ctx
      | Switch (scrut, body) -> update_switch_body scrut body ctx
      | Meta (meta, e) -> update_meta meta e ctx
    in
    (ctx, { e; ty })
  (* *)
  and update_abs (x : typed_pattern) (e : texpression) (ctx : pn_ctx) :
      pn_ctx * expression =
    (* We first add the left-constraint *)
    let ctx = add_left_constraint x ctx in
    (* Update the expression, and add additional constraints *)
    let ctx, e = update_texpression e ctx in
    (* Update the abstracted value *)
    let x = update_typed_pattern ctx x in
    (* Put together *)
    (ctx, Abs (x, e))
  (* *)
  and update_let (monadic : bool) (lv : typed_pattern) (re : texpression)
      (e : texpression) (ctx : pn_ctx) : pn_ctx * expression =
    (* We first add the left-constraint *)
    let ctx = add_left_constraint lv ctx in
    (* Then we try to propagate the right-constraints to the left, in case
     * the left constraints didn't give naming information *)
    let ctx = add_left_right_constraint lv re ctx in
    let ctx, re = update_texpression re ctx in
    let ctx, e = update_texpression e ctx in
    let lv = update_typed_pattern ctx lv in
    (ctx, Let (monadic, lv, re, e))
  (* *)
  and update_switch_body (scrut : texpression) (body : switch_body)
      (ctx : pn_ctx) : pn_ctx * expression =
    let ctx, scrut = update_texpression scrut ctx in

    let ctx, body =
      match body with
      | If (e_true, e_false) ->
          let ctx1, e_true = update_texpression e_true ctx in
          let ctx2, e_false = update_texpression e_false ctx in
          let ctx = merge_ctxs ctx1 ctx2 in
          (ctx, If (e_true, e_false))
      | Match branches ->
          let ctx_branches_ls =
            List.map
              (fun br ->
                let ctx = add_left_constraint br.pat ctx in
                let ctx, branch = update_texpression br.branch ctx in
                let pat = update_typed_pattern ctx br.pat in
                (ctx, { pat; branch }))
              branches
          in
          let ctxs, branches = List.split ctx_branches_ls in
          let ctx = merge_ctxs_ls ctxs in
          (ctx, Match branches)
    in
    (ctx, Switch (scrut, body))
  (* *)
  and update_meta (meta : meta) (e : texpression) (ctx : pn_ctx) :
      pn_ctx * expression =
    let ctx =
      match meta with
      | Assignment (mp, rvalue, rmp) ->
          let ctx = add_right_constraint mp rvalue ctx in
          let ctx =
            match (mp.projection, rmp) with
            | [], Some { var_id; name; projection = [] } -> (
                let name =
                  match name with
                  | Some name -> Some name
                  | None -> V.VarId.Map.find_opt var_id ctx.llbc_vars
                in
                match name with
                | None -> ctx
                | Some name -> add_llbc_var_constraint mp.var_id name ctx)
            | _ -> ctx
          in
          ctx
      | MPlace mp -> add_right_constraint mp e ctx
    in
    let ctx, e = update_texpression e ctx in
    let e = mk_meta meta e in
    (ctx, e.e)
  in

  let body =
    match def.body with
    | None -> None
    | Some body ->
        let input_names =
          List.filter_map
            (fun (v : var) ->
              match v.basename with
              | None -> None
              | Some name -> Some (v.id, name))
            body.inputs
        in
        let ctx =
          {
            pure_vars = VarId.Map.of_list input_names;
            llbc_vars = V.VarId.Map.empty;
          }
        in
        let _, body_exp = update_texpression body.body ctx in
        Some { body with body = body_exp }
  in
  { def with body }

(** Remove the meta-information *)
let remove_meta (def : fun_decl) : fun_decl =
  match def.body with
  | None -> def
  | Some body ->
      let body = { body with body = PureUtils.remove_meta body.body } in
      { def with body = Some body }

(** Inline the useless variable (re-)assignments:

    A lot of intermediate variable assignments are introduced through the
    compilation to MIR and by the translation itself (and the variable used
    on the left is often unnamed).

    Note that many of them are just variable "reassignments": `let x = y in ...`.
    Some others come from ??
    
    TODO: how do we call that when we introduce intermediate variable assignments
    for the arguments of a function call?

    [inline_named]: if `true`, inline all the assignments of the form
    `let VAR = VAR in ...`, otherwise inline only the ones where the variable
    on the left is anonymous.
    
    [inline_pure]: if `true`, inline all the pure assignments where the variable
    on the left is anonymous, but the assignments where the r-expression is
    a non-primitive function call (i.e.: inline the binops, ADT constructions,
    etc.).

    TODO: we have a smallish issue which is that rvalues should be merged with
    expressions... For now, this forces us to substitute whenever we can, but
    leave the let-bindings where they are, and eliminated them in a subsequent
    pass (if they are useless).
 *)
let inline_useless_var_reassignments (inline_named : bool) (inline_pure : bool)
    (def : fun_decl) : fun_decl =
  let obj =
    object (self)
      inherit [_] map_expression as super

      method! visit_Let (env : texpression VarId.Map.t) monadic lv re e =
        (* In order to filter, we need to check first that:
         * - the let-binding is not monadic
         * - the left-value is a variable
         *)
        match (monadic, lv.value) with
        | false, PatVar (lv_var, _) ->
            (* We can filter if: *)
            let filter = false in
            (* 1. Either:
             *   - the left variable is unnamed or [inline_named] is true
             *   - the right-expression is a variable
             *)
            let filter =
              match (inline_named, lv_var.basename) with
              | true, _ | _, None -> is_var re
              | _ -> filter
            in
            (* 2. Or:
             *   - the left variable is an unnamed variable
             *   - the right-expression is a value or a primitive function call
             *)
            let filter =
              if inline_pure then
                let app, _ = destruct_apps re in
                match app.e with
                | Const _ | Var _ -> true (* constant or variable *)
                | Qualif qualif -> (
                    match qualif.id with
                    | AdtCons _ | Proj _ -> true (* ADT constructor *)
                    | Func (Unop _ | Binop _) ->
                        true (* primitive function call *)
                    | Func (Regular _) ->
                        false (* non-primitive function call *))
                | _ -> filter
              else false
            in

            (* Update the environment and continue the exploration *)
            let re = self#visit_texpression env re in
            (* TODO: once rvalues and expressions are merged, filter the
             * let-binding (note that for now we leave it, expect it to
             * become useless, and wait for a subsequent pass to filter it) *)
            (* let env = add_subst lv_var.id re env in *)
            let env = if filter then VarId.Map.add lv_var.id re env else env in
            let e = self#visit_texpression env e in
            Let (monadic, lv, re, e)
        | _ -> super#visit_Let env monadic lv re e
      (** Visit the let-bindings to filter the useless ones (and update
          the substitution map while doing so *)

      method! visit_Var (env : texpression VarId.Map.t) (vid : VarId.id) =
        match VarId.Map.find_opt vid env with
        | None -> (* No substitution *) super#visit_Var env vid
        | Some ne ->
            (* Substitute - note that we need to reexplore, because
             * there may be stacked substitutions, if we have:
             * var0 --> var1
             * var1 --> var2.
             *)
            self#visit_expression env ne.e
      (** Substitute the variables *)
    end
  in
  match def.body with
  | None -> def
  | Some body ->
      let body =
        { body with body = obj#visit_texpression VarId.Map.empty body.body }
      in
      { def with body = Some body }

(** Given a forward or backward function call, is there, for every execution
    path, a child backward function called later with exactly the same input
    list prefix? We use this to filter useless function calls: if there are
    such child calls, we can remove this one (in case its outputs are not
    used).
    We do this check because we can't simply remove function calls whose
    outputs are not used, as they might fail. However, if a function fails,
    its children backward functions then fail on the same inputs (ignoring
    the additional inputs those receive).
    
    For instance, if we have:
    ```
    fn f<'a>(x : &'a mut T);
    ```
    
    We often have  things like this in the synthesized code:
    ```
    _ <-- f x;
    ...
    nx <-- f@back'a x y;
    ...
    ```

    In this situation, we can remove the call `f x`.
 *)
let expression_contains_child_call_in_all_paths (ctx : trans_ctx)
    (fun_id0 : fun_id) (tys0 : ty list) (args0 : texpression list)
    (e : texpression) : bool =
  let check_call (fun_id1 : fun_id) (tys1 : ty list) (args1 : texpression list)
      : bool =
    (* Check the fun_ids, to see if call1's function is a child of call0's function *)
    match (fun_id0, fun_id1) with
    | Regular (id0, rg_id0), Regular (id1, rg_id1) ->
        (* Both are "regular" calls: check if they come from the same rust function *)
        if id0 = id1 then
          (* Same rust functions: check the regions hierarchy *)
          let call1_is_child =
            match (rg_id0, rg_id1) with
            | None, _ ->
                (* The function used in call0 is the forward function: the one
                 * used in call1 is necessarily a child *)
                true
            | Some _, None ->
                (* Opposite of previous case *)
                false
            | Some rg_id0, Some rg_id1 ->
                if rg_id0 = rg_id1 then true
                else
                  (* We need to use the regions hierarchy *)
                  (* First, lookup the signature of the LLBC function *)
                  let sg =
                    LlbcAstUtils.lookup_fun_sig id0 ctx.fun_context.fun_decls
                  in
                  (* Compute the set of ancestors of the function in call1 *)
                  let call1_ancestors =
                    LlbcAstUtils.list_parent_region_groups sg rg_id1
                  in
                  (* Check if the function used in call0 is inside *)
                  T.RegionGroupId.Set.mem rg_id0 call1_ancestors
          in
          (* If call1 is a child, then we need to check if the input arguments
           * used in call0 are a prefix of the input arguments used in call1
           * (note call1 being a child, it will likely consume strictly more
           * given back values).
           * *)
          if call1_is_child then
            let call1_args =
              Collections.List.prefix (List.length args0) args1
            in
            let args = List.combine args0 call1_args in
            (* Note that the input values are expressions, *which may contain
             * meta-values* (which we need to ignore). *)
            let input_eq (v0, v1) =
              PureUtils.remove_meta v0 = PureUtils.remove_meta v1
            in
            (* Compare the input types and the prefix of the input arguments *)
            tys0 = tys1 && List.for_all input_eq args
          else (* Not a child *)
            false
        else (* Not the same function *)
          false
    | _ -> false
  in

  let visitor =
    object (self)
      inherit [_] reduce_expression

      method zero _ = false

      method plus b0 b1 _ = b0 () && b1 ()

      method! visit_texpression env e =
        match e.e with
        | Var _ | Const _ -> fun _ -> false
        | Let (_, _, re, e) -> (
            match opt_destruct_function_call re with
            | None -> fun () -> self#visit_texpression env e ()
            | Some (func1, tys1, args1) ->
                let call_is_child = check_call func1 tys1 args1 in
                if call_is_child then fun () -> true
                else fun () -> self#visit_texpression env e ())
        | App _ -> (
            fun () ->
              match opt_destruct_function_call e with
              | Some (func1, tys1, args1) -> check_call func1 tys1 args1
              | None -> false)
        | Abs (_, e) -> self#visit_texpression env e
        | Qualif _ ->
            (* Note that this case includes functions without arguments *)
            fun () -> false
        | Meta (_, e) -> self#visit_texpression env e
        | Switch (_, body) -> self#visit_switch_body env body

      method! visit_switch_body env body =
        match body with
        | If (e1, e2) ->
            fun () ->
              self#visit_texpression env e1 ()
              && self#visit_texpression env e2 ()
        | Match branches ->
            fun () ->
              List.for_all
                (fun br -> self#visit_texpression env br.branch ())
                branches
    end
  in
  visitor#visit_texpression () e ()

(** Filter the useless assignments (removes the useless variables, filters
    the function calls) *)
let filter_useless (filter_monadic_calls : bool) (ctx : trans_ctx)
    (def : fun_decl) : fun_decl =
  (* We first need a transformation on *left-values*, which filters the useless
   * variables and tells us whether the value contains any variable which has
   * not been replaced by `_` (in which case we need to keep the assignment,
   * etc.).
   * 
   * This is implemented as a map-reduce.
   *
   * Returns: ( filtered_left_value, *all_dummies* )
   *
   * `all_dummies`:
   * If the returned boolean is true, it means that all the variables appearing
   * in the filtered left-value are *dummies* (meaning that if this left-value
   * appears at the left of a let-binding, this binding might potentially be
   * removed).
   *)
  let lv_visitor =
    object
      inherit [_] mapreduce_typed_pattern

      method zero _ = true

      method plus b0 b1 _ = b0 () && b1 ()

      method! visit_PatVar env v mp =
        if VarId.Set.mem v.id env then (PatVar (v, mp), fun _ -> false)
        else (PatDummy, fun _ -> true)
    end
  in
  let filter_typed_pattern (used_vars : VarId.Set.t) (lv : typed_pattern) :
      typed_pattern * bool =
    let lv, all_dummies = lv_visitor#visit_typed_pattern used_vars lv in
    (lv, all_dummies ())
  in

  (* We then implement the transformation on *expressions* through a mapreduce.
   * Note that the transformation is bottom-up.
   * The map filters the useless assignments, the reduce computes the set of
   * used variables.
   *)
  let expr_visitor =
    object (self)
      inherit [_] mapreduce_expression as super

      method zero _ = VarId.Set.empty

      method plus s0 s1 _ = VarId.Set.union (s0 ()) (s1 ())

      method! visit_Var _ vid = (Var vid, fun _ -> VarId.Set.singleton vid)
      (** Whenever we visit a variable, we need to register the used variable *)

      method! visit_expression env e =
        match e with
        | Var _ | Const _ | App _ | Qualif _
        | Switch (_, _)
        | Meta (_, _)
        | Abs _ ->
            super#visit_expression env e
        | Let (monadic, lv, re, e) ->
            (* Compute the set of values used in the next expression *)
            let e, used = self#visit_texpression env e in
            let used = used () in
            (* Filter the left values *)
            let lv, all_dummies = filter_typed_pattern used lv in
            (* Small utility - called if we can't filter the let-binding *)
            let dont_filter () =
              let re, used_re = self#visit_texpression env re in
              let used = VarId.Set.union used (used_re ()) in
              (Let (monadic, lv, re, e), fun _ -> used)
            in
            (* Potentially filter the let-binding *)
            if all_dummies then
              if not monadic then
                (* Not a monadic let-binding: simple case *)
                (e.e, fun _ -> used)
              else
                (* Monadic let-binding: trickier.
                 * We can filter if the right-expression is a function call,
                 * under some conditions. *)
                match (filter_monadic_calls, opt_destruct_function_call re) with
                | true, Some (func, tys, args) ->
                    (* We need to check if there is a child call - see
                     * the comments for:
                     * [expression_contains_child_call_in_all_paths] *)
                    let has_child_call =
                      expression_contains_child_call_in_all_paths ctx func tys
                        args e
                    in
                    if has_child_call then (* Filter *)
                      (e.e, fun _ -> used)
                    else (* No child call: don't filter *)
                      dont_filter ()
                | _ ->
                    (* Not a call or not allowed to filter: we can't filter *)
                    dont_filter ()
            else (* There are used variables: don't filter *)
              dont_filter ()
    end
  in
  (* We filter only inside of transparent (i.e., non-opaque) definitions *)
  match def.body with
  | None -> def
  | Some body ->
      (* Visit the body *)
      let body_exp, used_vars = expr_visitor#visit_texpression () body.body in
      (* Visit the parameters - TODO: update: we can filter only if the definition
       * is not recursive (otherwise it might mess up with the decrease clauses:
       * the decrease clauses uses all the inputs given to the function, if some
       * inputs are replaced by '_' we can't give it to the function used in the
       * decreases clause).
       * For now we deactivate the filtering. *)
      let used_vars = used_vars () in
      let inputs_lvs =
        if false then
          List.map
            (fun lv -> fst (filter_typed_pattern used_vars lv))
            body.inputs_lvs
        else body.inputs_lvs
      in
      (* Return *)
      let body = { body with body = body_exp; inputs_lvs } in
      { def with body = Some body }

(** Return `None` if the function is a backward function with no outputs (so
    that we eliminate the definition which is useless).

    Note that the calls to such functions are filtered when translating from
    symbolic to pure. Here, we remove the definitions altogether, because they
    are now useless
  *)
let filter_if_backward_with_no_outputs (config : config) (def : fun_decl) :
    fun_decl option =
  if
    config.filter_useless_functions && Option.is_some def.back_id
    && def.signature.output = mk_result_ty mk_unit_ty
  then None
  else Some def

(** Return `false` if the forward function is useless and should be filtered.

    - a forward function with no output (comes from a Rust function with
      unit return type)
    - the function has mutable borrows as inputs (which is materialized
      by the fact we generated backward functions which were not filtered).

    In such situation, every call to the Rust function will be translated to:
    - a call to the forward function which returns nothing
    - calls to the backward functions
    As a failing backward function implies the forward function also fails,
    we can filter the calls to the forward function, which thus becomes
    useless.
    In such situation, we can remove the forward function definition
    altogether.
  *)
let keep_forward (config : config) (trans : pure_fun_translation) : bool =
  let fwd, backs = trans in
  (* Note that at this point, the output types are no longer seen as tuples:
   * they should be lists of length 1. *)
  if
    config.filter_useless_functions
    && fwd.signature.output = mk_result_ty mk_unit_ty
    && backs <> []
  then false
  else true

(** Convert the unit variables to `()` if they are used as right-values or
    `_` if they are used as left values in patterns. *)
let unit_vars_to_unit (def : fun_decl) : fun_decl =
  (* The map visitor *)
  let obj =
    object
      inherit [_] map_expression as super

      method! visit_PatVar _ v mp =
        if v.ty = mk_unit_ty then PatDummy else PatVar (v, mp)
      (** Replace in patterns *)

      method! visit_texpression env e =
        if e.ty = mk_unit_ty then mk_unit_rvalue
        else super#visit_texpression env e
      (** Replace in "regular" expressions - note that we could limit ourselves
          to variables, but this is more powerful
       *)
    end
  in
  (* Update the body *)
  match def.body with
  | None -> def
  | Some body ->
      let body_exp = obj#visit_texpression () body.body in
      (* Update the input parameters *)
      let inputs_lvs = List.map (obj#visit_typed_pattern ()) body.inputs_lvs in
      (* Return *)
      let body = Some { body with body = body_exp; inputs_lvs } in
      { def with body }

(** Eliminate the box functions like `Box::new`, `Box::deref`, etc. Most of them
    are translated to identity, and `Box::free` is translated to `()`.

    Note that the box types have already been eliminated during the translation
    from symbolic to pure.
    The reason why we don't eliminate the box functions at the same time is
    that we would need to eliminate them in two different places: when translating
    function calls, and when translating end abstractions. Here, we can do
    something simpler, in one micro-pass.
 *)
let eliminate_box_functions (_ctx : trans_ctx) (def : fun_decl) : fun_decl =
  (* The map visitor *)
  let obj =
    object
      inherit [_] map_expression as super

      method! visit_texpression env e =
        match opt_destruct_function_call e with
        | Some (fun_id, _tys, args) -> (
            match fun_id with
            | Regular (A.Assumed aid, rg_id) -> (
                (* Below, when dealing with the arguments: we consider the very
                 * general case, where functions could be boxed (meaning we
                 * could have: `box_new f x`)
                 * *)
                match (aid, rg_id) with
                | A.BoxNew, _ ->
                    assert (rg_id = None);
                    let arg, args = Collections.List.pop args in
                    mk_apps arg args
                | A.BoxDeref, None ->
                    (* `Box::deref` forward is the identity *)
                    let arg, args = Collections.List.pop args in
                    mk_apps arg args
                | A.BoxDeref, Some _ ->
                    (* `Box::deref` backward is `()` (doesn't give back anything) *)
                    assert (args = []);
                    mk_unit_rvalue
                | A.BoxDerefMut, None ->
                    (* `Box::deref_mut` forward is the identity *)
                    let arg, args = Collections.List.pop args in
                    mk_apps arg args
                | A.BoxDerefMut, Some _ ->
                    (* `Box::deref_mut` back is almost the identity:
                     * let box_deref_mut (x_init : t) (x_back : t) : t = x_back
                     * *)
                    let arg, args =
                      match args with
                      | _ :: given_back :: args -> (given_back, args)
                      | _ -> failwith "Unreachable"
                    in
                    mk_apps arg args
                | A.BoxFree, _ ->
                    assert (args = []);
                    mk_unit_rvalue
                | ( ( A.Replace | A.VecNew | A.VecPush | A.VecInsert | A.VecLen
                    | A.VecIndex | A.VecIndexMut ),
                    _ ) ->
                    super#visit_texpression env e)
            | _ -> super#visit_texpression env e)
        | _ -> super#visit_texpression env e
    end
  in
  (* Update the body *)
  match def.body with
  | None -> def
  | Some body ->
      let body = Some { body with body = obj#visit_texpression () body.body } in
      { def with body }

(** Decompose the monadic let-bindings.

    See the explanations in [config].
 *)
let decompose_monadic_let_bindings (_ctx : trans_ctx) (def : fun_decl) :
    fun_decl =
  match def.body with
  | None -> def
  | Some body ->
      (* Set up the var id generator *)
      let cnt = get_body_min_var_counter body in
      let _, fresh_id = VarId.mk_stateful_generator cnt in
      (* It is a very simple map *)
      let obj =
        object (self)
          inherit [_] map_expression as super

          method! visit_Let env monadic lv re next_e =
            if not monadic then super#visit_Let env monadic lv re next_e
            else
              (* If monadic, we need to check if the left-value is a variable:
               * - if yes, don't decompose
               * - if not, make the decomposition in two steps
               *)
              match lv.value with
              | PatVar _ ->
                  (* Variable: nothing to do *)
                  super#visit_Let env monadic lv re next_e
              | _ ->
                  (* Not a variable: decompose *)
                  (* Introduce a temporary variable to receive the value of the
                   * monadic binding *)
                  let vid = fresh_id () in
                  let tmp : var = { id = vid; basename = None; ty = lv.ty } in
                  let ltmp = mk_typed_pattern_from_var tmp None in
                  let rtmp = mk_texpression_from_var tmp in
                  (* Visit the next expression *)
                  let next_e = self#visit_texpression env next_e in
                  (* Create the let-bindings *)
                  (mk_let true ltmp re (mk_let false lv rtmp next_e)).e
        end
      in
      (* Update the body *)
      let body = Some { body with body = obj#visit_texpression () body.body } in
      (* Return *)
      { def with body }

(** Unfold the monadic let-bindings to explicit matches. *)
let unfold_monadic_let_bindings (_ctx : trans_ctx) (def : fun_decl) : fun_decl =
  match def.body with
  | None -> def
  | Some body ->
      (* It is a very simple map *)
      let obj =
        object (_self)
          inherit [_] map_expression as super

          method! visit_Let env monadic lv re e =
            (* We simply do the following transformation:
             * ```
             * pat <-- re; e
             *
             *     ~~>
             *
             * match re with
             * | Fail err -> Fail err
             * | Return pat -> e
             * ```
             *)
            (* TODO: we should use a monad "kind" instead of a boolean *)
            if not monadic then super#visit_Let env monadic lv re e
            else
              (* We don't do the same thing if we use a state-error monad or simply
               * an error monad.
               * Note that some functions always live in the error monad (arithmetic
               * operations, for instance).
               *)
              (* TODO: this information should be computed in SymbolicToPure and
               * store in an enum ("monadic" should be an enum, not a bool). *)
              let re_ty = Option.get (opt_destruct_result re.ty) in
              assert (lv.ty = re_ty);
              let fail_pat = mk_result_fail_pattern lv.ty in
              let fail_value = mk_result_fail_texpression e.ty in
              let fail_branch = { pat = fail_pat; branch = fail_value } in
              let success_pat = mk_result_return_pattern lv in
              let success_branch = { pat = success_pat; branch = e } in
              let switch_body = Match [ fail_branch; success_branch ] in
              let e = Switch (re, switch_body) in
              (* Continue *)
              super#visit_expression env e
        end
      in
      (* Update the body *)
      let body_e = obj#visit_texpression () body.body in
      let body = { body with body = body_e } in
      (* Return *)
      { def with body = Some body }

(** Apply all the micro-passes to a function.

    Will return `None` if the function is a backward function with no outputs.

    [ctx]: used only for printing.
 *)
let apply_passes_to_def (config : config) (ctx : trans_ctx) (def : fun_decl) :
    fun_decl option =
  (* Debug *)
  log#ldebug
    (lazy
      ("PureMicroPasses.apply_passes_to_def: "
      ^ Print.fun_name_to_string def.basename
      ^ " ("
      ^ Print.option_to_string T.RegionGroupId.to_string def.back_id
      ^ ")"));

  (* First, find names for the variables which are unnamed *)
  let def = compute_pretty_names def in
  log#ldebug
    (lazy ("compute_pretty_name:\n\n" ^ fun_decl_to_string ctx def ^ "\n"));

  (* TODO: we might want to leverage more the assignment meta-data, for
   * aggregates for instance. *)

  (* TODO: reorder the branches of the matches/switches *)

  (* The meta-information is now useless: remove it.
   * Rk.: some passes below use the fact that we removed the meta-data
   * (otherwise we would have to "unmeta" expressions before matching) *)
  let def = remove_meta def in
  log#ldebug (lazy ("remove_meta:\n\n" ^ fun_decl_to_string ctx def ^ "\n"));

  (* Remove the backward functions with no outputs.
   * Note that the calls to those functions should already have been removed,
   * when translating from symbolic to pure. Here, we remove the definitions
   * altogether, because they are now useless *)
  let def = filter_if_backward_with_no_outputs config def in

  match def with
  | None -> None
  | Some def ->
      (* Convert the unit variables to `()` if they are used as right-values or
       * `_` if they are used as left values. *)
      let def = unit_vars_to_unit def in
      log#ldebug
        (lazy ("unit_vars_to_unit:\n\n" ^ fun_decl_to_string ctx def ^ "\n"));

      (* Inline the useless variable reassignments *)
      let inline_named_vars = true in
      let inline_pure = true in
      let def =
        inline_useless_var_reassignments inline_named_vars inline_pure def
      in
      log#ldebug
        (lazy
          ("inline_useless_var_assignments:\n\n" ^ fun_decl_to_string ctx def
         ^ "\n"));

      (* Eliminate the box functions - note that the "box" types were eliminated
       * during the symbolic to pure phase: see the comments for [eliminate_box_functions] *)
      let def = eliminate_box_functions ctx def in
      log#ldebug
        (lazy
          ("eliminate_box_functions:\n\n" ^ fun_decl_to_string ctx def ^ "\n"));

      (* Filter the useless variables, assignments, function calls, etc. *)
      let def = filter_useless config.filter_useless_monadic_calls ctx def in
      log#ldebug
        (lazy ("filter_useless:\n\n" ^ fun_decl_to_string ctx def ^ "\n"));

      (* Decompose the monadic let-bindings - F* specific
       * TODO: remove? *)
      let def =
        if config.decompose_monadic_let_bindings then (
          let def = decompose_monadic_let_bindings ctx def in
          log#ldebug
            (lazy
              ("decompose_monadic_let_bindings:\n\n"
             ^ fun_decl_to_string ctx def ^ "\n"));
          def)
        else (
          log#ldebug
            (lazy
              "ignoring decompose_monadic_let_bindings due to the configuration\n");
          def)
      in

      (* Unfold the monadic let-bindings *)
      let def =
        if config.unfold_monadic_let_bindings then (
          let def = unfold_monadic_let_bindings ctx def in
          log#ldebug
            (lazy
              ("unfold_monadic_let_bindings:\n\n" ^ fun_decl_to_string ctx def
             ^ "\n"));
          def)
        else (
          log#ldebug
            (lazy
              "ignoring unfold_monadic_let_bindings due to the configuration\n");
          def)
      in

      (* We are done *)
      Some def

(** Return the forward/backward translations on which we applied the micro-passes.

    Also returns a boolean indicating whether the forward function should be kept
    or not (because useful/useless - `true` means we need to keep the forward
    function).
    Note that we don't "filter" the forward function and return a boolean instead,
    because this function contains useful information to extract the backward
    functions: keeping it is not necessary but more convenient.
 *)
let apply_passes_to_pure_fun_translation (config : config) (ctx : trans_ctx)
    (trans : pure_fun_translation) : bool * pure_fun_translation =
  (* Apply the passes to the individual functions *)
  let forward, backwards = trans in
  let forward = Option.get (apply_passes_to_def config ctx forward) in
  let backwards = List.filter_map (apply_passes_to_def config ctx) backwards in
  let trans = (forward, backwards) in
  (* Compute whether we need to filter the forward function or not *)
  (keep_forward config trans, trans)