add a ui test for the inference variable warning

Author: mikeyhew

src/test/ui/inference-variable-behind-raw-pointer.rs

@@ -0,0 +1,15 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn main() {
+    let data = std::ptr::null();
+    let _ = &data as *const *const ();
+    if data.is_null() {}
+}

src/test/ui/inference-variable-behind-raw-pointer.stderr

@@ -0,0 +1,8 @@
+warning[E0619]: the type of this value must be known in this context
+  --> $DIR/inference-variable-behind-raw-pointer.rs:14:13
+   |
+14 |     if data.is_null() {}
+   |             ^^^^^^^
+   |
+   = note: this will be made into a hard error in a future version of the compiler
+