[nullsafe][TypeOrigin refactor] introduce TypeOrigin.ArrayLengthResult

Summary:
This diff is a part of work teaching Nullsafe to explain decisions it's
making.

Using `New` was definitely not a good idea here, we need a specific
data type value here.

Reviewed By: artempyanykh

Differential Revision: D18450174

fbshipit-source-id: 7ce7cc7e8

infer/src/nullsafe/typeCheck.ml

@@ -1094,7 +1094,7 @@ let typecheck_instr tenv calls_this checks (node : Procdesc.Node.t) idenv curr_p
         EradicateChecks.check_object_dereference ~is_strict_mode tenv find_canonical_duplicate
           curr_pdesc node instr_ref array_exp DereferenceRule.ArrayLengthAccess ta loc ;
       TypeState.add_id id
-        (Typ.mk (Tint Typ.IInt), InferredNullability.create_nonnull TypeOrigin.New)
+        (Typ.mk (Tint Typ.IInt), InferredNullability.create_nonnull TypeOrigin.ArrayLengthResult)
         typestate
   (* All other builtins that are not considered above *)
   | Sil.Call (_, Exp.Const (Const.Cfun pn), _, _, _) when BuiltinDecl.is_declared pn ->

infer/src/nullsafe/typeOrigin.ml

@@ -23,6 +23,7 @@ type t =
   | Formal of Mangled.t  (** A formal parameter *)
   | Proc of proc_origin  (** A procedure call *)
   | New  (** A new object creation *)
+  | ArrayLengthResult  (** integer value - result of accessing array.length *)
   | ONone  (** No origin is known *)
   | Undef  (** Undefined value before initialization *)
 [@@deriving compare]
@@ -42,6 +43,8 @@ let rec to_string = function
       Printf.sprintf "Fun %s" (Typ.Procname.to_simplified_string po.pname)
   | New ->
       "New"
+  | ArrayLengthResult ->
+      "ArrayLength"
   | ONone ->
       "ONone"
   | Undef ->
@@ -76,7 +79,7 @@ let get_description origin =
      But for these issues we currently don't print origins in the error string.
      It is a good idea to change this and start printing origins for these origins as well.
   *)
-  | New | NonnullConst _ ->
+  | New | NonnullConst _ | ArrayLengthResult ->
       None
   (* Two special cases - should not really occur in normal code *)
   | ONone | Undef ->

infer/src/nullsafe/typeOrigin.mli

@@ -22,6 +22,7 @@ type t =
   | Formal of Mangled.t  (** A formal parameter *)
   | Proc of proc_origin  (** A procedure call *)
   | New  (** A new object creation *)
+  | ArrayLengthResult  (** integer value - result of accessing array.length *)
   | ONone  (** No origin is known *)
   | Undef  (** Undefined value before initialization *)
 [@@deriving compare]