Deprecate offset_to; switch core&alloc to using offset_from instead

Bonus: might make code than uses `.len()` on slice iterators faster

2 files changed, 14 insertions, 9 deletions

diff --git a/src/libcore/ptr.rs b/src/libcore/ptr.rs
index 5a54de06b5e..c1e150e9fb9 100644
--- a/src/libcore/ptr.rs
+++ b/src/libcore/ptr.rs
@@ -677,6 +677,7 @@ impl<T: ?Sized> *const T {
     ///
     /// ```
     /// #![feature(offset_to)]
+    /// #![allow(deprecated)]
     ///
     /// fn main() {
     ///     let a = [0; 5];
@@ -689,14 +690,15 @@ impl<T: ?Sized> *const T {
     /// }
     /// ```
     #[unstable(feature = "offset_to", issue = "41079")]
+    #[rustc_deprecated(since = "1.27.0", reason = "Replaced by `wrapping_offset_from`, with the \
+        opposite argument order.  If you're writing unsafe code, consider `offset_from`.")]
     #[inline]
     pub fn offset_to(self, other: *const T) -> Option<isize> where T: Sized {
         let size = mem::size_of::<T>();
         if size == 0 {
             None
         } else {
-            let diff = (other as isize).wrapping_sub(self as isize);
-            Some(diff / size as isize)
+            Some(other.wrapping_offset_from(self))
         }
     }
 
@@ -1442,6 +1444,7 @@ impl<T: ?Sized> *mut T {
     ///
     /// ```
     /// #![feature(offset_to)]
+    /// #![allow(deprecated)]
     ///
     /// fn main() {
     ///     let mut a = [0; 5];
@@ -1454,14 +1457,15 @@ impl<T: ?Sized> *mut T {
     /// }
     /// ```
     #[unstable(feature = "offset_to", issue = "41079")]
+    #[rustc_deprecated(since = "1.27.0", reason = "Replaced by `wrapping_offset_from`, with the \
+        opposite argument order.  If you're writing unsafe code, consider `offset_from`.")]
     #[inline]
     pub fn offset_to(self, other: *const T) -> Option<isize> where T: Sized {
         let size = mem::size_of::<T>();
         if size == 0 {
             None
         } else {
-            let diff = (other as isize).wrapping_sub(self as isize);
-            Some(diff / size as isize)
+            Some(other.wrapping_offset_from(self))
         }
     }
 
diff --git a/src/libcore/slice/mod.rs b/src/libcore/slice/mod.rs
index 0f1b7cb8fcc..0a22028da81 100644
--- a/src/libcore/slice/mod.rs
+++ b/src/libcore/slice/mod.rs
@@ -1185,7 +1185,7 @@ macro_rules! iterator {
 
             #[inline]
             fn size_hint(&self) -> (usize, Option<usize>) {
-                let exact = ptrdistance(self.ptr, self.end);
+                let exact = unsafe { ptrdistance(self.ptr, self.end) };
                 (exact, Some(exact))
             }
 
@@ -1593,10 +1593,11 @@ unsafe impl<'a, T> TrustedLen for IterMut<'a, T> {}
 // Return the number of elements of `T` from `start` to `end`.
 // Return the arithmetic difference if `T` is zero size.
 #[inline(always)]
-fn ptrdistance<T>(start: *const T, end: *const T) -> usize {
-    match start.offset_to(end) {
-        Some(x) => x as usize,
-        None => (end as usize).wrapping_sub(start as usize),
+unsafe fn ptrdistance<T>(start: *const T, end: *const T) -> usize {
+    if mem::size_of::<T>() == 0 {
+        (end as usize).wrapping_sub(start as usize)
+    } else {
+        end.offset_from(start) as usize
     }
 }