From a4ee9c6e96025fa2b5eb254e4ccdd4c6910f5f60 Mon Sep 17 00:00:00 2001
From: Ulrik Sverdrup <bluss@users.noreply.github.com>
Date: Tue, 13 Sep 2016 20:51:39 +0200
Subject: [PATCH] core: Use primitive indexing in slice's Index/IndexMut

[T]'s Index implementation is normally not used for indexing, instead
the compiler supplied indexing is used.

Use the compiler supplied version in Index/IndexMut.

This removes an inconsistency:

Compiler supplied bound check failures look like this:

thread 'main' panicked at 'index out of bounds: the len is 3 but the index is 4'

If you convince Rust to use the Index impl for slices, bounds check
failure looks like this instead:

thread 'main' panicked at 'assertion failed: index < self.len()'

The latter is used if you for example use Index generically::

   use std::ops::Index;
   fn foo<T: ?Sized>(x: &T) where T: Index<usize> { &x[4]; }

   foo(&[1, 2, 3][..])
---
 src/libcore/slice.rs                          | 8 ++++----
 src/test/run-fail/bounds-check-no-overflow.rs | 2 +-
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/src/libcore/slice.rs b/src/libcore/slice.rs
index b22bdb43414fd..9755623365961 100644
--- a/src/libcore/slice.rs
+++ b/src/libcore/slice.rs
@@ -520,8 +520,8 @@ impl<T> ops::Index<usize> for [T] {
     type Output = T;
 
     fn index(&self, index: usize) -> &T {
-        assert!(index < self.len());
-        unsafe { self.get_unchecked(index) }
+        // NB built-in indexing
+        &(*self)[index]
     }
 }
 
@@ -530,8 +530,8 @@ impl<T> ops::Index<usize> for [T] {
 impl<T> ops::IndexMut<usize> for [T] {
     #[inline]
     fn index_mut(&mut self, index: usize) -> &mut T {
-        assert!(index < self.len());
-        unsafe { self.get_unchecked_mut(index) }
+        // NB built-in indexing
+        &mut (*self)[index]
     }
 }
 
diff --git a/src/test/run-fail/bounds-check-no-overflow.rs b/src/test/run-fail/bounds-check-no-overflow.rs
index 4d502cb2106b1..3d1cbb446e848 100644
--- a/src/test/run-fail/bounds-check-no-overflow.rs
+++ b/src/test/run-fail/bounds-check-no-overflow.rs
@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// error-pattern:assertion failed: index < self.len()
+// error-pattern:index out of bounds
 
 use std::usize;
 use std::mem::size_of;