offset_from: always allow if both pointers are the same

Author: RalfJung

Diff for: library/core/src/ptr/const_ptr.rs

@@ -627,11 +627,9 @@ impl<T: ?Sized> *const T {
     /// If any of the following conditions are violated, the result is Undefined
     /// Behavior:
     ///
-    /// * Both `self` and `origin` must be either in bounds or one
-    ///   byte past the end of the same [allocated object].
-    ///
-    /// * Both pointers must be *derived from* a pointer to the same object.
-    ///   (See below for an example.)
+    /// * If `self` and `origin` point to different addresses, then they must both be *derived from*
+    ///   a pointer to the same [allocated object], and they must be either in bounds or one byte
+    ///   past the end of that object. (See below for an example.)
     ///
     /// * The distance between the pointers, in bytes, must be an exact multiple
     ///   of the size of `T`.
@@ -695,13 +693,13 @@ impl<T: ?Sized> *const T {
     /// let ptr2 = Box::into_raw(Box::new(1u8)) as *const u8;
     /// let diff = (ptr2 as isize).wrapping_sub(ptr1 as isize);
     /// // Make ptr2_other an "alias" of ptr2, but derived from ptr1.
-    /// let ptr2_other = (ptr1 as *const u8).wrapping_offset(diff);
+    /// let ptr2_other = (ptr1 as *const u8).wrapping_offset(diff).wrapping_offset(1);
     /// assert_eq!(ptr2 as usize, ptr2_other as usize);
     /// // Since ptr2_other and ptr2 are derived from pointers to different objects,
     /// // computing their offset is undefined behavior, even though
-    /// // they point to the same address!
+    /// // they point to addresses that are in-bounds of the same object!
     /// unsafe {
-    ///     let zero = ptr2_other.offset_from(ptr2); // Undefined Behavior
+    ///     let one = ptr2_other.offset_from(ptr2); // Undefined Behavior
     /// }
     /// ```
     #[stable(feature = "ptr_offset_from", since = "1.47.0")]

Diff for: library/core/src/ptr/mut_ptr.rs

@@ -801,11 +801,9 @@ impl<T: ?Sized> *mut T {
     /// If any of the following conditions are violated, the result is Undefined
     /// Behavior:
     ///
-    /// * Both `self` and `origin` must be either in bounds or one
-    ///   byte past the end of the same [allocated object].
-    ///
-    /// * Both pointers must be *derived from* a pointer to the same object.
-    ///   (See below for an example.)
+    /// * If `self` and `origin` point to different addresses, then they must both be *derived from*
+    ///   a pointer to the same [allocated object], and they must be either in bounds or one byte
+    ///   past the end of that object. (See below for an example.)
     ///
     /// * The distance between the pointers, in bytes, must be an exact multiple
     ///   of the size of `T`.
@@ -869,13 +867,13 @@ impl<T: ?Sized> *mut T {
     /// let ptr2 = Box::into_raw(Box::new(1u8));
     /// let diff = (ptr2 as isize).wrapping_sub(ptr1 as isize);
     /// // Make ptr2_other an "alias" of ptr2, but derived from ptr1.
-    /// let ptr2_other = (ptr1 as *mut u8).wrapping_offset(diff);
+    /// let ptr2_other = (ptr1 as *mut u8).wrapping_offset(diff).wrapping_offset(1);
     /// assert_eq!(ptr2 as usize, ptr2_other as usize);
     /// // Since ptr2_other and ptr2 are derived from pointers to different objects,
     /// // computing their offset is undefined behavior, even though
-    /// // they point to the same address!
+    /// // they point to addresses that are in-bounds of the same object!
     /// unsafe {
-    ///     let zero = ptr2_other.offset_from(ptr2); // Undefined Behavior
+    ///     let one = ptr2_other.offset_from(ptr2); // Undefined Behavior
     /// }
     /// ```
     #[stable(feature = "ptr_offset_from", since = "1.47.0")]