InstrNode: abstract instruction index

Summary:
Now that the cost analysis doesn't hackily compute the instruction index, we can make this an abstract type to ensure `List.nth_exn` in `OneInstrPerNode` will not fail.

Depends on D7618320

Reviewed By: sblackshear

Differential Revision: D7628908

fbshipit-source-id: 89e8618

infer/src/absint/ProcCfg.ml

@@ -74,9 +74,11 @@ module DefaultNode = struct
 end
 
 module InstrNode = struct
-  type t = Procdesc.Node.t * int
+  type instr_index = int [@@deriving compare]
 
-  type id = Procdesc.Node.id * int [@@deriving compare]
+  type t = Procdesc.Node.t * instr_index
+
+  type id = Procdesc.Node.id * instr_index [@@deriving compare]
 
   let kind (t, _) = Procdesc.Node.get_kind t
 

infer/src/absint/ProcCfg.mli

@@ -82,7 +82,13 @@ end
 
 module DefaultNode : Node with type t = Procdesc.Node.t and type id = Procdesc.Node.id
 
-module InstrNode : Node with type t = Procdesc.Node.t * int and type id = Procdesc.Node.id * int
+module InstrNode : sig
+  type instr_index
+
+  include Node
+          with type t = Procdesc.Node.t * instr_index
+           and type id = Procdesc.Node.id * instr_index
+end
 
 (** Forward CFG with no exceptional control-flow *)
 module Normal :