Implement TryFrom for array reference types

There are many cases where a buffer with a static compile-time size is
preferred over a slice with a dynamic size. This allows for performing
a checked conversion from &[T] to &[T; N].

This may also lead to compile-time optimizations involving [T; N] such
as loop unrolling.

src/libcore/array.rs

@@ -21,6 +21,7 @@
 
 use borrow::{Borrow, BorrowMut};
 use cmp::Ordering;
+use convert::TryFrom;
 use fmt;
 use hash::{Hash, self};
 use marker::Unsize;
@@ -57,6 +58,11 @@ unsafe impl<T, A: Unsize<[T]>> FixedSizeArray<T> for A {
     }
 }
 
+/// The error type returned when a conversion from a slice to an array fails.
+#[unstable(feature = "try_from", issue = "33417")]
+#[derive(Debug, Copy, Clone)]
+pub struct TryFromSliceError(());
+
 macro_rules! __impl_slice_eq1 {
     ($Lhs: ty, $Rhs: ty) => {
         __impl_slice_eq1! { $Lhs, $Rhs, Sized }
@@ -123,6 +129,34 @@ macro_rules! array_impls {
                 }
             }
 
+            #[unstable(feature = "try_from", issue = "33417")]
+            impl<'a, T> TryFrom<&'a [T]> for &'a [T; $N] {
+                type Error = TryFromSliceError;
+
+                fn try_from(slice: &[T]) -> Result<&[T; $N], TryFromSliceError> {
+                    if slice.len() == $N {
+                        let ptr = slice.as_ptr() as *const [T; $N];
+                        unsafe { Ok(&*ptr) }
+                    } else {
+                        Err(TryFromSliceError(()))
+                    }
+                }
+            }
+
+            #[unstable(feature = "try_from", issue = "33417")]
+            impl<'a, T> TryFrom<&'a mut [T]> for &'a mut [T; $N] {
+                type Error = TryFromSliceError;
+
+                fn try_from(slice: &mut [T]) -> Result<&mut [T; $N], TryFromSliceError> {
+                    if slice.len() == $N {
+                        let ptr = slice.as_mut_ptr() as *mut [T; $N];
+                        unsafe { Ok(&mut *ptr) }
+                    } else {
+                        Err(TryFromSliceError(()))
+                    }
+                }
+            }
+
             #[stable(feature = "rust1", since = "1.0.0")]
             impl<T: Hash> Hash for [T; $N] {
                 fn hash<H: hash::Hasher>(&self, state: &mut H) {