Be more explicit about how and why we need fallback in targets of casts

src/test/run-pass/cast-does-fallback.rs

@@ -0,0 +1,16 @@
+// 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.
+
+pub fn main() {
+    let cap = 512 * 512;
+    cap as u8;
+    // Assert `cap` did not get inferred to `u8` and overflowed.
+    assert_ne!(cap, 0);
+}