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