Revert "Compute the solutions to the var system once."

This reverts commit dcc64e8.
This take too much memory. The good solution could be the use of the
unit to go through only one solution for the var system.

lib/unification/AC.ml

@@ -260,18 +260,33 @@ end = struct
     in
     aux env Bitv.empty (Array.length solutions - 1)
 
-  let get_var_envs env solutions bitvars_cover =
-    let rec aux acc env coverage = function
-      | -1 -> (env, coverage)::acc
+  let iterate_var_subsets env shape_coverage system solutions bitvars_cover k =
+    let mask = Bitv.all_until (system.System.nb_atom - 1) in
+    let rec aux env coverage = function
+      | -1 ->
+          (* Check if the subset is large enough *)
+          if Bitv.(equal mask coverage) then (
+            let final_env, stack = Env.commit env in
+            match
+              let* _ =
+                if List.is_empty stack then Done
+                else Syntactic.process_stack final_env (Stack.of_list stack)
+              in
+              Syntactic.occur_check final_env
+            with
+            | Syntactic.FailUnif _ | FailedOccurCheck _ -> ()
+            | Done ->
+                  k final_env
+          )
       | n ->
-          let envs = aux acc env coverage (n-1) in
+          aux env coverage (n-1);
           match merge_env env solutions.(n) with
           | env ->
               let coverage = Bitv.(coverage || bitvars_cover.(n)) in
-              aux envs env coverage (n-1)
-          | exception Bail -> failwith "Impossible"
+              aux env coverage (n-1)
+          | exception Bail -> ()
     in
-    aux [] env Bitv.empty (Array.length solutions - 1)
+    aux env shape_coverage (Array.length solutions - 1)
 
   let get_first_assoc_type sol {System. assoc_type; first_var; _} =
     let rec aux i =
@@ -351,8 +366,6 @@ end = struct
     let shapes_sols =
       List.map (fun (system, solutions) -> system, extract_solutions env system solutions) shapes_sols
     in
-    let mask_var = Bitv.all_until (system.System.nb_atom - 1) in
-    let var_envs = get_var_envs env env_sols sol_coverages in
     let rec combine_shapes env shapes_sols acc_coverage k =
       match shapes_sols with
       | (system, (env_sols, bitvars)) :: t ->
@@ -361,24 +374,7 @@ end = struct
             combine_shapes env_sol t Bitv.(acc_coverage || coverage) k
           )
       | [] -> (* Here we could have a custom occur check to avoid the iteration on var *)
-          List.iter (fun (var_env, coverage) ->
-            if Bitv.(equal mask_var (coverage || acc_coverage)) then (
-              (* TODO: This merge_env could be specialised, because we only have new element in
-                 partial and thuse only this to merge. *)
-              let env = merge_env env var_env in
-              let final_env, stack = Env.commit env in
-              match
-                let* _ =
-                  if List.is_empty stack then Done
-                  else Syntactic.process_stack final_env (Stack.of_list stack)
-                in
-                Syntactic.occur_check final_env
-              with
-              | Syntactic.FailUnif _ | FailedOccurCheck _ -> ()
-              | Done ->
-                    k final_env
-            )
-            else ()) var_envs
+          iterate_var_subsets env acc_coverage system env_sols sol_coverages k
     in
     let n_solutions = ref 0 in
     (* TODO: We should sort shapes_sol by the number of first_var, the bigger first as it is