Auto merge of #95837 - scottmcm:ptr-offset-from-unsigned, r=oli-obk

Add `sub_ptr` on pointers (the `usize` version of `offset_from`)

We have `add`/`sub` which are the `usize` versions of `offset`, this adds the `usize` equivalent of `offset_from`.  Like how `.add(d)` replaced a whole bunch of `.offset(d as isize)`, you can see from the changes here that it's fairly common that code actually knows the order between the pointers and *wants* a `usize`, not an `isize`.

As a bonus, this can do `sub nuw`+`udiv exact`, rather than `sub`+`sdiv exact`, which can be optimized slightly better because it doesn't have to worry about negatives.  That's why the slice iterators weren't using `offset_from`, though I haven't updated that code in this PR because slices are so perf-critical that I'll do it as its own change.

This is an intrinsic, like `offset_from`, so that it can eventually be allowed in CTFE.  It also allows checking the extra safety condition -- see the test confirming that CTFE catches it if you pass the pointers in the wrong order.

6 files changed, 6 insertions, 5 deletions

diff --git a/library/alloc/src/lib.rs b/library/alloc/src/lib.rs
index ecebc7ed9ac..fd21b367118 100644
--- a/library/alloc/src/lib.rs
+++ b/library/alloc/src/lib.rs
@@ -127,6 +127,7 @@
 #![feature(pattern)]
 #![feature(ptr_internals)]
 #![feature(ptr_metadata)]
+#![feature(ptr_sub_ptr)]
 #![feature(receiver_trait)]
 #![feature(set_ptr_value)]
 #![feature(slice_group_by)]
diff --git a/library/alloc/src/slice.rs b/library/alloc/src/slice.rs
index 5bdf36a63a2..199b3c9d029 100644
--- a/library/alloc/src/slice.rs
+++ b/library/alloc/src/slice.rs
@@ -1056,7 +1056,7 @@ where
         fn drop(&mut self) {
             // `T` is not a zero-sized type, and these are pointers into a slice's elements.
             unsafe {
-                let len = self.end.offset_from(self.start) as usize;
+                let len = self.end.sub_ptr(self.start);
                 ptr::copy_nonoverlapping(self.start, self.dest, len);
             }
         }
diff --git a/library/alloc/src/vec/drain.rs b/library/alloc/src/vec/drain.rs
index 1bff19d05c1..5cdee0bd4da 100644
--- a/library/alloc/src/vec/drain.rs
+++ b/library/alloc/src/vec/drain.rs
@@ -163,7 +163,7 @@ impl<T, A: Allocator> Drop for Drain<'_, T, A> {
             // it from the original vec but also avoid creating a &mut to the front since that could
             // invalidate raw pointers to it which some unsafe code might rely on.
             let vec_ptr = vec.as_mut().as_mut_ptr();
-            let drop_offset = drop_ptr.offset_from(vec_ptr) as usize;
+            let drop_offset = drop_ptr.sub_ptr(vec_ptr);
             let to_drop = ptr::slice_from_raw_parts_mut(vec_ptr.add(drop_offset), drop_len);
             ptr::drop_in_place(to_drop);
         }
diff --git a/library/alloc/src/vec/in_place_collect.rs b/library/alloc/src/vec/in_place_collect.rs
index 282af8cc33f..55dcb84ad16 100644
--- a/library/alloc/src/vec/in_place_collect.rs
+++ b/library/alloc/src/vec/in_place_collect.rs
@@ -250,7 +250,7 @@ where
         let sink =
             self.try_fold::<_, _, Result<_, !>>(sink, write_in_place_with_drop(end)).unwrap();
         // iteration succeeded, don't drop head
-        unsafe { ManuallyDrop::new(sink).dst.offset_from(dst_buf) as usize }
+        unsafe { ManuallyDrop::new(sink).dst.sub_ptr(dst_buf) }
     }
 }
 
diff --git a/library/alloc/src/vec/in_place_drop.rs b/library/alloc/src/vec/in_place_drop.rs
index 354d25c2389..1b1ef9130fa 100644
--- a/library/alloc/src/vec/in_place_drop.rs
+++ b/library/alloc/src/vec/in_place_drop.rs
@@ -10,7 +10,7 @@ pub(super) struct InPlaceDrop<T> {
 
 impl<T> InPlaceDrop<T> {
     fn len(&self) -> usize {
-        unsafe { self.dst.offset_from(self.inner) as usize }
+        unsafe { self.dst.sub_ptr(self.inner) }
     }
 }
 
diff --git a/library/alloc/src/vec/into_iter.rs b/library/alloc/src/vec/into_iter.rs
index a7df6f59b59..9b84a1d9b4b 100644
--- a/library/alloc/src/vec/into_iter.rs
+++ b/library/alloc/src/vec/into_iter.rs
@@ -169,7 +169,7 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
         let exact = if mem::size_of::<T>() == 0 {
             self.end.addr().wrapping_sub(self.ptr.addr())
         } else {
-            unsafe { self.end.offset_from(self.ptr) as usize }
+            unsafe { self.end.sub_ptr(self.ptr) }
         };
         (exact, Some(exact))
     }