mark FIXMES for all the places found that are probably offset_from

Author: Gankra

core/src/slice/sort.rs

@@ -269,6 +269,8 @@ where
     // Returns the number of elements between pointers `l` (inclusive) and `r` (exclusive).
     fn width<T>(l: *mut T, r: *mut T) -> usize {
         assert!(mem::size_of::<T>() > 0);
+        // FIXME: this should *likely* use `offset_from`, but more
+        // investigation is needed (including running tests in miri).
         (r.addr() - l.addr()) / mem::size_of::<T>()
     }
 

std/src/sys/unix/memchr.rs

@@ -26,6 +26,8 @@ pub fn memrchr(needle: u8, haystack: &[u8]) -> Option<usize> {
                 haystack.len(),
             )
         };
+        // FIXME: this should *likely* use `offset_from`, but more
+        // investigation is needed (including running tests in miri).
         if p.is_null() { None } else { Some(p.addr() - haystack.as_ptr().addr()) }
     }