Rollup merge of rust-lang#40943 - Amanieu:offset_to, r=alexcrichton

Add ptr::offset_to

This PR adds a method to calculate the signed distance (in number of elements) between two pointers. The resulting value can then be passed to `offset` to get one pointer from the other. This is similar to pointer subtraction in C/C++.

There are 2 special cases:

- If the distance is not a multiple of the element size then the result is rounded towards zero. (in C/C++ this is UB)
-  ZST return `None`, while normal types return `Some(isize)`. This forces the user to handle the ZST case in unsafe code. (C/C++ doesn't have ZSTs)

src/doc/unstable-book/src/SUMMARY.md

@@ -124,6 +124,7 @@
 - [no_debug](no-debug.md)
 - [non_ascii_idents](non-ascii-idents.md)
 - [nonzero](nonzero.md)
+- [offset_to](offset-to.md)
 - [omit_gdb_pretty_printer_section](omit-gdb-pretty-printer-section.md)
 - [on_unimplemented](on-unimplemented.md)
 - [once_poison](once-poison.md)

src/doc/unstable-book/src/offset-to.md

@@ -0,0 +1,7 @@
+# `offset_to`
+
+The tracking issue for this feature is: [#41079]
+
+[#41079]: https://github.com/rust-lang/rust/issues/41079
+
+------------------------

src/libcollections/lib.rs

@@ -62,6 +62,7 @@
 #![feature(untagged_unions)]
 #![cfg_attr(not(test), feature(str_checked_slicing))]
 #![cfg_attr(test, feature(rand, test))]
+#![feature(offset_to)]
 
 #![no_std]
 

src/libcollections/vec.rs

@@ -2074,14 +2074,10 @@ impl<T> Iterator for IntoIter<T> {
 
     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) {
-        let diff = (self.end as usize) - (self.ptr as usize);
-        let size = mem::size_of::<T>();
-        let exact = diff /
-                    (if size == 0 {
-                         1
-                     } else {
-                         size
-                     });
+        let exact = match self.ptr.offset_to(self.end) {
+            Some(x) => x as usize,
+            None => (self.end as usize).wrapping_sub(self.ptr as usize),
+        };
         (exact, Some(exact))
     }
 

src/libcore/ptr.rs

@@ -500,6 +500,44 @@ impl<T: ?Sized> *const T {
             intrinsics::arith_offset(self, count)
         }
     }
+
+    /// Calculates the distance between two pointers. The returned value is in
+    /// units of T: the distance in bytes is divided by `mem::size_of::<T>()`.
+    ///
+    /// If the address different between the two pointers ia not a multiple of
+    /// `mem::size_of::<T>()` then the result of the division is rounded towards
+    /// zero.
+    ///
+    /// This function returns `None` if `T` is a zero-sized typed.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// #![feature(offset_to)]
+    ///
+    /// fn main() {
+    ///     let a = [0; 5];
+    ///     let ptr1: *const i32 = &a[1];
+    ///     let ptr2: *const i32 = &a[3];
+    ///     assert_eq!(ptr1.offset_to(ptr2), Some(2));
+    ///     assert_eq!(ptr2.offset_to(ptr1), Some(-2));
+    ///     assert_eq!(unsafe { ptr1.offset(2) }, ptr2);
+    ///     assert_eq!(unsafe { ptr2.offset(-2) }, ptr1);
+    /// }
+    /// ```
+    #[unstable(feature = "offset_to", issue = "41079")]
+    #[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)
+        }
+    }
 }
 
 #[lang = "mut_ptr"]
@@ -653,6 +691,44 @@ impl<T: ?Sized> *mut T {
             Some(&mut *self)
         }
     }
+
+    /// Calculates the distance between two pointers. The returned value is in
+    /// units of T: the distance in bytes is divided by `mem::size_of::<T>()`.
+    ///
+    /// If the address different between the two pointers ia not a multiple of
+    /// `mem::size_of::<T>()` then the result of the division is rounded towards
+    /// zero.
+    ///
+    /// This function returns `None` if `T` is a zero-sized typed.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// #![feature(offset_to)]
+    ///
+    /// fn main() {
+    ///     let mut a = [0; 5];
+    ///     let ptr1: *mut i32 = &mut a[1];
+    ///     let ptr2: *mut i32 = &mut a[3];
+    ///     assert_eq!(ptr1.offset_to(ptr2), Some(2));
+    ///     assert_eq!(ptr2.offset_to(ptr1), Some(-2));
+    ///     assert_eq!(unsafe { ptr1.offset(2) }, ptr2);
+    ///     assert_eq!(unsafe { ptr2.offset(-2) }, ptr1);
+    /// }
+    /// ```
+    #[unstable(feature = "offset_to", issue = "41079")]
+    #[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)
+        }
+    }
 }
 
 // Equality for pointers

src/libcore/slice/mod.rs

@@ -1498,9 +1498,10 @@ unsafe impl<'a, T> TrustedLen for IterMut<'a, T> {}
 // Return the arithmetic difference if `T` is zero size.
 #[inline(always)]
 fn ptrdistance<T>(start: *const T, end: *const T) -> usize {
-    let diff = (end as usize).wrapping_sub(start as usize);
-    let size = mem::size_of::<T>();
-    diff / (if size == 0 { 1 } else { size })
+    match start.offset_to(end) {
+        Some(x) => x as usize,
+        None => (end as usize).wrapping_sub(start as usize),
+    }
 }
 
 // Extension methods for raw pointers, used by the iterators