Added tests for non-obvious builtin indexing

Tests match issues opened on Github.
Tests would not previously compile and pass.

src/test/run-pass/issue-33903.rs

@@ -0,0 +1,19 @@
+// 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.
+
+// Issue 33903:
+// Built-in indexing should be used even when the index is not
+// trivially an integer
+// Only built-in indexing can be used in constant expresssions
+
+const FOO: i32 = [12, 34][0 + 1];
+
+fn main() {}
+

src/test/run-pass/issue-46095.rs

@@ -0,0 +1,39 @@
+// 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.
+
+struct A;
+
+impl A {
+    fn take_mutably(&mut self) {}
+}
+
+fn identity<T>(t: T) -> T {
+    t
+}
+
+// Issue 46095
+// Built-in indexing should be used even when the index is not
+// trivially an integer
+// Overloaded indexing would cause wrapped to be borrowed mutably
+
+fn main() {
+    let mut a1 = A;
+    let mut a2 = A;
+
+    let wrapped = [&mut a1, &mut a2];
+
+    {
+        wrapped[0 + 1 - 1].take_mutably();
+    }
+
+    {
+        wrapped[identity(0)].take_mutably();
+    }
+}