doc: use ptr::addr in offset_from docs

library/core/src/ptr/const_ptr.rs

@@ -567,9 +567,8 @@ impl<T: PointeeSized> *const T {
     /// needed for `const`-compatibility: the distance between pointers into *different* allocated
     /// objects is not known at compile-time. However, the requirement also exists at
     /// runtime and may be exploited by optimizations. If you wish to compute the difference between
-    /// pointers that are not guaranteed to be from the same allocation, use `(self as isize -
-    /// origin as isize) / size_of::<T>()`.
-    // FIXME: recommend `addr()` instead of `as usize` once that is stable.
+    /// pointers that are not guaranteed to be from the same allocation, use
+    /// `(self.addr() as isize - origin.addr() as isize) / size_of::<T>()`.
     ///
     /// [`add`]: #method.add
     /// [allocation]: crate::ptr#allocation

library/core/src/ptr/mut_ptr.rs

@@ -748,9 +748,8 @@ impl<T: PointeeSized> *mut T {
     /// needed for `const`-compatibility: the distance between pointers into *different* allocated
     /// objects is not known at compile-time. However, the requirement also exists at
     /// runtime and may be exploited by optimizations. If you wish to compute the difference between
-    /// pointers that are not guaranteed to be from the same allocation, use `(self as isize -
-    /// origin as isize) / size_of::<T>()`.
-    // FIXME: recommend `addr()` instead of `as usize` once that is stable.
+    /// pointers that are not guaranteed to be from the same allocation, use
+    /// `(self.addr() as isize - origin.addr() as isize) / size_of::<T>()`.
     ///
     /// [`add`]: #method.add
     /// [allocation]: crate::ptr#allocation

library/core/src/ptr/non_null.rs

@@ -736,9 +736,8 @@ impl<T: PointeeSized> NonNull<T> {
     /// needed for `const`-compatibility: the distance between pointers into *different* allocated
     /// objects is not known at compile-time. However, the requirement also exists at
     /// runtime and may be exploited by optimizations. If you wish to compute the difference between
-    /// pointers that are not guaranteed to be from the same allocation, use `(self as isize -
-    /// origin as isize) / size_of::<T>()`.
-    // FIXME: recommend `addr()` instead of `as usize` once that is stable.
+    /// pointers that are not guaranteed to be from the same allocation, use
+    /// `(self.addr() as isize - origin.addr() as isize) / size_of::<T>()`.
     ///
     /// [`add`]: #method.add
     /// [allocation]: crate::ptr#allocation