Save another BB by using SubUnchecked instead of a call to arith_offset

Probably reasonable anyway since it more obviously drops provenance.

Author: scottmcm

Diff for: core/src/slice/iter.rs

@@ -11,7 +11,7 @@ use crate::iter::{
 use crate::marker::PhantomData;
 use crate::mem::{self, SizedTypeProperties};
 use crate::num::NonZero;
-use crate::ptr::{NonNull, null, without_provenance, without_provenance_mut};
+use crate::ptr::{NonNull, without_provenance, without_provenance_mut};
 use crate::{cmp, fmt};
 
 #[stable(feature = "boxed_slice_into_iter", since = "1.80.0")]

Diff for: core/src/slice/iter/macros.rs

@@ -162,13 +162,13 @@ macro_rules! iterator {
                 // SAFETY: Type invariants.
                 unsafe {
                     if T::IS_ZST {
-                        let byte_end = end_or_len as *const u8;
-                        if byte_end == null() {
+                        let len = end_or_len.addr();
+                        if len == 0 {
                             return None;
                         }
-                        self.end_or_len = byte_end.wrapping_sub(1) as _;
+                        self.end_or_len = without_provenance_mut(len.unchecked_sub(1));
                     } else {
-                        if ptr == crate::intrinsics::transmute::<*const T, NonNull<T>>(end_or_len) {
+                        if ptr == crate::intrinsics::transmute::<$ptr, NonNull<T>>(end_or_len) {
                             return None;
                         }
                         self.ptr = ptr.add(1);