rustc: Require that vector indices are uints

This commit tightens up the restriction on types used to index slices to require
exactly `uint` indices. Previously any integral type was accepted, but this
leads to a few subtle problems:

  * 64-bit indices don't make much sense on 32-bit systems
  * Signed indices for slices used as negative indexing isn't implemented

This was discussed at the recent work week, and also has some discussion on
issue #10453.

Closes #10453

src/test/compile-fail/indexing-requires-a-uint.rs

@@ -0,0 +1,25 @@
+// Copyright 2014 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.
+
+// Make sure that indexing an array is only valid with a `uint`, not any other
+// integral type.
+
+fn main() {
+    fn bar<T>(_: T) {}
+    [0][0u8]; //~ ERROR: mismatched types
+
+    [0][0]; // should infer to be a uint
+
+    let i = 0;      // i is an IntVar
+    [0][i];         // i should be locked to uint
+    bar::<int>(i);  // i should not be re-coerced back to an int
+    //~^ ERROR: mismatched types
+}
+