Bring back slice::ref_slice as slice::from_ref.

These functions were deprecated and removed in 1.5, but such simple
functionality shouldn't require using unsafe code, and it isn't
cluttering libstd too much.

src/liballoc/slice.rs

@@ -119,6 +119,8 @@ pub use core::slice::{SplitN, RSplitN, SplitNMut, RSplitNMut};
 pub use core::slice::{RSplit, RSplitMut};
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use core::slice::{from_raw_parts, from_raw_parts_mut};
+#[stable(feature = "from_ref", since = "1.22.0")]
+pub use core::slice::{from_ref, from_ref_mut};
 #[unstable(feature = "slice_get_slice", issue = "35729")]
 pub use core::slice::SliceIndex;
 

src/libcore/slice/mod.rs

@@ -2450,6 +2450,22 @@ pub unsafe fn from_raw_parts_mut<'a, T>(p: *mut T, len: usize) -> &'a mut [T] {
     mem::transmute(Repr { data: p, len: len })
 }
 
+/// Converts a reference to T into a slice of length 1 (without copying).
+#[stable(feature = "from_ref", since = "1.22.0")]
+pub fn from_ref<T>(s: &T) -> &[T] {
+    unsafe {
+        from_raw_parts(s, 1)
+    }
+}
+
+/// Converts a reference to T into a slice of length 1 (without copying).
+#[stable(feature = "from_ref", since = "1.22.0")]
+pub fn from_ref_mut<T>(s: &mut T) -> &mut [T] {
+    unsafe {
+        from_raw_parts_mut(s, 1)
+    }
+}
+
 // This function is public only because there is no other way to unit test heapsort.
 #[unstable(feature = "sort_internals", reason = "internal to sort module", issue = "0")]
 #[doc(hidden)]