flambda-backend: Remove var_constraint (#1880)

* Remove [var_constraint], which was redundant

* bootstrap

typing/jkind.ml

@@ -45,7 +45,6 @@ module Sort = struct
 
   let void = Const Void
   let value = Const Value
-
   let float64 = Const Float64
 
   let some_value = Some value
@@ -59,6 +58,7 @@ module Sort = struct
 
   let new_var () = Var (ref None)
 
+  (* Post-condition: If the result is a [Var v], then [!v] is [None]. *)
   let rec get : t -> t = function
     | Const _ as t -> t
     | Var r as t -> begin match !r with
@@ -70,20 +70,6 @@ module Sort = struct
         end
     end
 
-  (* This is constant-time if [var] was just returned from a previous call to
-     [get]. That's because [var] will always be [None] in that case.
-  *)
-  let var_constraint : var -> const option = fun r ->
-    match !r with
-    | None -> None
-    | Some t -> begin
-        match get t with
-        | Const const -> Some const
-        | Var { contents = None } -> None
-        | Var _ ->
-            Misc.fatal_error "[get] should return [Const _] or [Var None]"
-      end
-
   let default_value : t option = Some (Const Value)
   let default_void : t option = Some (Const Void)
   let default_float64 : t option = Some (Const Float64)
@@ -515,6 +501,7 @@ let sub_desc d1 d2 = match d1, d2 with
   | Var v1, Var v2 -> if v1 == v2 then Equal else Not_sub
   | Const _, Var _ | Var _, Const _ -> Not_sub
 
+(* Post-condition: If the result is [Var v], then [!v] is [None]. *)
 let get_internal (lay : internal) : desc = match lay with
   | Any -> Const Any
   | Immediate -> Const Immediate

typing/jkind.mli

@@ -50,11 +50,6 @@ module Sort : sig
   (** A sort variable that can be unified during type-checking. *)
   type var
 
-  (** Return the concrete constraint placed on the variable. This check is
-      constant-time if [var] was just returned from [Jkind.get].
-  *)
-  val var_constraint : var -> const option
-
   (** Create a new sort variable that can be unified. *)
   val new_var : unit -> t
 

typing/subst.ml

@@ -109,13 +109,7 @@ let with_additional_action (config : additional_action_config) s =
           | Const Immediate -> immediate
           | Const Immediate64 -> immediate64
           | Const Float64 -> float64
-          | Var var -> begin
-              match Jkind.Sort.var_constraint var with
-              | Some Void -> void
-              | Some Value -> value
-              | Some Float64 -> float64
-              | None -> raise(Error (loc, Unconstrained_jkind_variable))
-            end
+          | Var _ -> raise(Error (loc, Unconstrained_jkind_variable))
         in
         Prepare_for_saving prepare_jkind
   in
@@ -228,6 +222,8 @@ let newpersty desc =
   create_expr
     desc ~level:generic_level ~scope:Btype.lowest_level ~id:!new_id
 
+(* CR layouts: remove this. While we're still developing, though, it might
+   be nice to get the location of this kind of error. *)
 (* We use a ref instead of passing [loc] as an argument to [typexp]
    because the ref requires no modifications to the body of [typexp],
    reducing the chance of merge conflicts. This location is not critical --