[report] avoid embarrassing "object `null` could be null" message

Summary:
A long-standing easter egg from infer error messages is the "object
`null` could be null and is dereferenced at line ...". I tried to fix
this but the part that generates the first "null" in the message and the
part that generates the second one are very far apart and it's hard to
see how to make the second part aware of the first in a clean way.

Instead, hack around it by detecting if the string representing the
value is literally `null` and in that case chop `could be null ` from
the error messages...

Reviewed By: jeremydubreil

Differential Revision: D14972324

fbshipit-source-id: ccc48ce6b

infer/src/IR/Localise.ml

@@ -180,9 +180,11 @@ let deref_str_null_ proc_name_opt problem_str_ =
   {tags= Tags.create (); value_pre= Some (pointer_or_object ()); value_post= None; problem_str}
 
 
+let could_be_null_and_prefix = "could be null and "
+
 (** dereference strings for null dereference *)
 let deref_str_null proc_name_opt =
-  let problem_str = "could be null and is dereferenced" in
+  let problem_str = could_be_null_and_prefix ^ "is dereferenced" in
   deref_str_null_ proc_name_opt problem_str
 
 
@@ -412,7 +414,11 @@ let dereference_string proc_name deref_str value_str access_opt loc =
             ^ MF.monospaced_to_string weak_var_str
             ^ ", a weak pointer captured in the block, and is dereferenced without a null check"
       | None, None ->
-          deref_str.problem_str
+          (* hack to avoid dumb message "null could be null" *)
+          if String.equal value_str "null" then
+            String.chop_prefix deref_str.problem_str ~prefix:could_be_null_and_prefix
+            |> Option.value ~default:deref_str.problem_str
+          else deref_str.problem_str
     in
     [problem_str ^ " " ^ at_line tags loc]
   in