Update the code generated for tuple projectors

3 files changed, 47 insertions, 18 deletions

diff --git a/compiler/Config.ml b/compiler/Config.ml
index 2bb1ca34..3b0070c0 100644
--- a/compiler/Config.ml
+++ b/compiler/Config.ml
@@ -469,3 +469,7 @@ let use_tuple_structs = ref true
 
 let backend_has_tuple_projectors () =
   match !backend with Lean -> true | Coq | FStar | HOL4 -> false
+
+(** We we use nested projectors for tuple (like: [(0, 1).snd.fst]) or do
+    we use better projector syntax? *)
+let use_nested_tuple_projectors = ref false
diff --git a/compiler/Extract.ml b/compiler/Extract.ml
index d7ef5f34..dbca4f8f 100644
--- a/compiler/Extract.ml
+++ b/compiler/Extract.ml
@@ -601,25 +601,46 @@ and extract_field_projector (ctx : extraction_ctx) (fmt : F.formatter)
             | Lean ->
                 (* Tuples in Lean are syntax sugar for nested products/pairs,
                    so we need to map the field id accordingly.
-                   A field id i maps to:
-                   (.2)^i if i is the last element of the tuple
-                   (.2)^i.1 otherwise
-                   where (.2)^i denotes .2 repeated i times.
-                   For example, 3 maps to .2.2.2 if the tuple has 4 fields and
-                   to .2.2.2.1 if it has more than 4 fields.
-                   Note that the first "." is added below *)
+
+                   We give two possibilities:
+                   - either we use the custom syntax [.#i], like in: [(0, 1).#1]
+                   - or we introduce nested projections which use the field
+                     projectors [.1] and [.2], like in: [(0, 1).2.1]
+
+                     This necessary in some situations, for instance if we have
+                     in Rust:
+                     {[
+                       struct Tuple(u32, (u32, u32));
+                     ]}
+
+                     The issue comes from the fact that in Lean [A * B * C] and [A * (B *
+                     C)] are the same type.  As a result, in Rust, field 1 of [Tuple] is
+                     the pair (an element of type [(u32, u32)]), however in Lean it would
+                     be the first element of the pair (an element of type [u32]). If such
+                     situations happen, we allow to force using the nested projectors by
+                     providing the proper command line argument.  TODO: we can actually
+                     check the type to determine exactly when we need to use nested
+                     projectors and when we don't.
+
+                     When using nested projectors, a field id i maps to:
+                     - (.2)^i if i is the last element of the tuple
+                     - (.2)^i.1 otherwise
+                     where (.2)^i denotes .2 repeated i times.
+                     For example, 3 maps to .2.2.2 if the tuple has 4 fields and
+                     to .2.2.2.1 if it has more than 4 fields.
+                     Note that the first "." is added below.
+                *)
                 let field_id = FieldId.to_int proj.field_id in
-                (* Helper: repeat "2.2.2..."  *)
-                let rec repeat_snd n =
-                  match n with
-                  | 0 -> ""
-                  | 1 -> "2"
-                  | _ -> "2." ^ repeat_snd (n - 1)
-                in
-                let twos_prefix = repeat_snd field_id in
-                if field_id + 1 = Option.get num_fields then twos_prefix
-                else if field_id = 0 then "1"
-                else twos_prefix ^ ".1"
+                if !Config.use_nested_tuple_projectors then
+                  (* Nested projection: "2.2.2..."  *)
+                  if field_id = 0 then "1"
+                  else
+                    let twos_prefix =
+                      String.concat "." (Collections.List.repeat field_id "2")
+                    in
+                    if field_id + 1 = Option.get num_fields then twos_prefix
+                    else twos_prefix ^ ".1"
+                else "#" ^ string_of_int field_id
           else ctx_get_field proj.adt_id proj.field_id ctx
         in
         (* Open a box *)
diff --git a/compiler/Main.ml b/compiler/Main.ml
index 0b8ec439..4a2d01dc 100644
--- a/compiler/Main.ml
+++ b/compiler/Main.ml
@@ -123,6 +123,10 @@ let () =
       ( "-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 \
+         (0, 1).1)." );
     ]
   in