feat(engine/phases): intro simplify_hoisting

This commit introduces a phase that simplifies the pattern:

```
/// Note `let` below is NOT monadic
let hoistN = e in body
```

into `body[hoistN/e]`, when:

 - the local ident `hoistN` is of kind `SideEffectHoistVar` (we know it was produced by the side effect utils module);
 - `body` contains exactly one occurence of `hoistN`.

That simplifies greatly the extracted code.

engine/backends/fstar/fstar_backend.ml

@@ -1357,6 +1357,7 @@ module TransformToInputLanguage =
   |> Phases.Functionalize_loops
   |> Phases.Reject.As_pattern
   |> Phases.Traits_specs
+  |> Phases.Simplify_hoisting
   |> SubtypeToInputLanguage
   |> Identity
   ]

engine/lib/diagnostics.ml

@@ -42,6 +42,7 @@ module Phase = struct
     | FunctionalizeLoops
     | TraitsSpecs
     | SimplifyMatchReturn
+    | SimplifyHoisting
     | DropNeedlessReturns
     | DummyA
     | DummyB

engine/lib/local_ident.ml

@@ -13,7 +13,9 @@ module T = struct
 
   let make_final name = { name; id = mk_id Final 0 }
   let is_final { id; _ } = [%matches? Final] @@ fst id
-  let is_side_effect_hoist_var {id; _} = [%matches? SideEffectHoistVar] @@ fst id
+
+  let is_side_effect_hoist_var { id; _ } =
+    [%matches? SideEffectHoistVar] @@ fst id
 end
 
 include Base.Comparator.Make (T)

engine/lib/local_ident.mli

@@ -1,17 +1,12 @@
 module T : sig
   type kind =
-    Typ
-    (** type namespace *)
-  | Cnst
-    (** Generic constant namespace *)
-  | Expr
-    (** Expression namespace *)
-  | LILifetime
-    (** Lifetime namespace *)
-  | Final
-    (** Frozen identifier: such an identifier will *not* be rewritten by the name policy *)
-  | SideEffectHoistVar
-    (** A variable generated by `Side_effect_utils` *)
+    | Typ  (** type namespace *)
+    | Cnst  (** Generic constant namespace *)
+    | Expr  (** Expression namespace *)
+    | LILifetime  (** Lifetime namespace *)
+    | Final
+        (** Frozen identifier: such an identifier will *not* be rewritten by the name policy *)
+    | SideEffectHoistVar  (** A variable generated by `Side_effect_utils` *)
   [@@deriving show, yojson, hash, compare, sexp, eq]
 
   type id [@@deriving show, yojson, hash, compare, sexp, eq]
@@ -21,10 +16,10 @@ module T : sig
   type t = { name : string; id : id }
   [@@deriving show, yojson, hash, compare, sexp, eq]
 
-  (** Creates a frozen final local identifier: such an indentifier won't be rewritten by a name policy *)
   val make_final : string -> t
-  val is_final : t -> bool
+  (** Creates a frozen final local identifier: such an indentifier won't be rewritten by a name policy *)
 
+  val is_final : t -> bool
   val is_side_effect_hoist_var : t -> bool
 end
 

engine/lib/phases.ml

@@ -15,3 +15,4 @@ module Traits_specs = Phase_traits_specs.Make
 module Drop_needless_returns = Phase_drop_needless_returns.Make
 module Drop_sized_trait = Phase_drop_sized_trait.Make
 module Simplify_match_return = Phase_simplify_match_return.Make
+module Simplify_hoisting = Phase_simplify_hoisting.Make

engine/lib/phases/phase_simplify_hoisting.ml

@@ -0,0 +1,66 @@
+open! Prelude
+
+module Make (F : Features.T) =
+  Phase_utils.MakeMonomorphicPhase
+    (F)
+    (struct
+      let phase_id = Diagnostics.Phase.SimplifyHoisting
+
+      open Ast.Make (F)
+      module U = Ast_utils.Make (F)
+      module Visitors = Ast_visitors.Make (F)
+
+      module Error = Phase_utils.MakeError (struct
+        let ctx = Diagnostics.Context.Phase phase_id
+      end)
+
+      let inline_matches =
+        object
+          inherit [_] Visitors.map as super
+
+          method! visit_expr () e =
+            match e with
+            | {
+             e =
+               Let
+                 {
+                   monadic = None;
+                   lhs =
+                     {
+                       p =
+                         PBinding
+                           {
+                             mut = Immutable;
+                             mode = ByValue;
+                             var;
+                             subpat = None;
+                             _;
+                           };
+                       _;
+                     };
+                   rhs;
+                   body;
+                 };
+             _;
+            }
+              when Local_ident.is_side_effect_hoist_var var ->
+                let body, count =
+                  (object
+                     inherit [_] Visitors.mapreduce as super
+                     method zero = 0
+                     method plus = ( + )
+
+                     method! visit_expr () e =
+                       match e.e with
+                       | LocalVar v when [%eq: Local_ident.t] v var -> (rhs, 1)
+                       | _ -> super#visit_expr () e
+                  end)
+                    #visit_expr
+                    () body
+                in
+                if [%eq: int] count 1 then body else super#visit_expr () e
+            | _ -> super#visit_expr () e
+        end
+
+      let ditems = List.map ~f:(inline_matches#visit_item ())
+    end)

engine/lib/phases/phase_simplify_hoisting.mli

@@ -0,0 +1,4 @@
+(** This phase rewrites `let pat = match ... { ... => ..., ... => return ... }; e`
+    into `match ... { ... => let pat = ...; e}`. *)
+
+module Make : Phase_utils.UNCONSTRAINTED_MONOMORPHIC_PHASE