Auto merge of #22838 - petrochenkov:bytelit, r=alexcrichton

This patch changes the type of byte string literals from `&[u8]` to `&[u8; N]`.
It also implements some necessary traits (`IntoBytes`, `Seek`, `Read`, `BufRead`) for fixed-size arrays (also related to #21725) and adds test for #17233, which seems to be resolved.

Fixes #18465
[breaking-change]

src/libcore/array.rs

@@ -23,10 +23,34 @@ use marker::{Copy, Sized};
 use option::Option;
 use slice::{Iter, IterMut, SliceExt};
 
+/// Utility trait implemented only on arrays of fixed size
+///
+/// This trait can be used to implement other traits on fixed-size arrays
+/// without causing much metadata bloat.
+#[unstable(feature = "core")]
+pub trait FixedSizeArray<T> {
+    /// Converts the array to immutable slice
+    fn as_slice(&self) -> &[T];
+    /// Converts the array to mutable slice
+    fn as_mut_slice(&mut self) -> &mut [T];
+}
+
 // macro for implementing n-ary tuple functions and operations
 macro_rules! array_impls {
     ($($N:expr)+) => {
         $(
+            #[unstable(feature = "core")]
+            impl<T> FixedSizeArray<T> for [T; $N] {
+                #[inline]
+                fn as_slice(&self) -> &[T] {
+                    &self[..]
+                }
+                #[inline]
+                fn as_mut_slice(&mut self) -> &mut [T] {
+                    &mut self[..]
+                }
+            }
+
             #[stable(feature = "rust1", since = "1.0.0")]
             impl<T:Copy> Clone for [T; $N] {
                 fn clone(&self) -> [T; $N] {

src/libcore/lib.rs

@@ -129,6 +129,7 @@ pub mod default;
 /* Core types and methods on primitives */
 
 pub mod any;
+pub mod array;
 pub mod atomic;
 pub mod cell;
 pub mod char;
@@ -151,7 +152,6 @@ mod bool {
 
 // note: does not need to be public
 mod tuple;
-mod array;
 
 #[doc(hidden)]
 mod core {

src/libcore/result.rs

@@ -448,7 +448,8 @@ impl<T, E> Result<T, E> {
     /// ```
     /// use std::old_io::IoResult;
     ///
-    /// let mut buffer = &mut b"1\n2\n3\n4\n";
+    /// let mut buffer: &[u8] = b"1\n2\n3\n4\n";
+    /// let mut buffer = &mut buffer;
     ///
     /// let mut sum = 0;
     ///