special forms of UseT

Summary:
"This type is incompatible with" errors have the potential to be a lot more descriptive, if we can keep track of what caused us to flow one def type into another def type.

This diff is a lot of boilerplate nonsense to add an "op" property to `UseT`, and a couple of examples of those ops being set when we deconstruct some other type into the final `Type.t -> UseT Type.t`.

Reviewed By: avikchaudhuri

Differential Revision: D3208938

fb-gh-sync-id: 4fc3fafb4a01d4bdf8d490a71ab99e2975ceb76f
fbshipit-source-id: 4fc3fafb4a01d4bdf8d490a71ab99e2975ceb76f

src/typing/debug_js.ml

@@ -274,7 +274,8 @@ and _json_of_use_t_impl json_cx t = Hh_json.(
  "kind", JSON_String (string_of_use_ctor t)
  ] @
  match t with
- | UseT t -> [
+ | UseT (op, t) -> [
+ "use", JSON_String (string_of_use_op op);
  "type", _json_of_t json_cx t
  ]
 
@@ -941,7 +942,7 @@ and dump_use_t cx t =
  dump_use_t_ ISet.empty cx t
 
 and dump_use_t_ stack cx t = match t with
- | UseT t -> dump_t_ stack cx t
+ | UseT (_, t) -> dump_t_ stack cx t
 
  | SetPropT (_, (_, n), t) ->
  spf "SetPropT(%s: %s)" n (dump_t_ stack cx t)

src/typing/flow_error.ml

@@ -213,7 +213,7 @@ end = struct
 
  (* for when a t has been extracted from a use_t *)
  let flow_err_use_t cx trace msg ?extra lower upper =
- flow_err cx trace msg ?extra lower (UseT upper)
+ flow_err cx trace msg ?extra lower (UseT (UnknownUse, upper))
 
 end