specialize zip: Introduce TrustedRandomAccess trait

src/libcore/iter_private.rs

@@ -0,0 +1,27 @@
+// Copyright 2016 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.
+
+
+use iter::ExactSizeIterator;
+
+/// An iterator whose items are random accessible efficiently
+///
+/// # Safety
+///
+/// The iterator's .len() and size_hint() must be exact.
+///
+/// .get_unchecked() must return distinct mutable references for distinct
+/// indices (if applicable), and must return a valid reference if index is in
+/// 0..self.len().
+#[doc(hidden)]
+pub unsafe trait TrustedRandomAccess : ExactSizeIterator {
+    unsafe fn get_unchecked(&mut self, i: usize) -> Self::Item;
+}
+

src/libcore/lib.rs

@@ -156,4 +156,5 @@ pub mod hash;
 pub mod fmt;
 
 // note: does not need to be public
+mod iter_private;
 mod tuple;