Auto merge of #114720 - scottmcm:better-sub, r=workingjubilee

Tell LLVM that the negation in `<*const T>::sub` cannot overflow

Today it's just `sub` <https://rust.godbolt.org/z/8EzEPnMr5>; with this PR it's `sub nsw`.

Author: bors

core/src/ptr/const_ptr.rs

@@ -1,7 +1,7 @@
 use super::*;
 use crate::cmp::Ordering::{self, Equal, Greater, Less};
 use crate::intrinsics::{self, const_eval_select};
-use crate::mem;
+use crate::mem::{self, SizedTypeProperties};
 use crate::slice::{self, SliceIndex};
 
 impl<T: ?Sized> *const T {
@@ -995,14 +995,23 @@ impl<T: ?Sized> *const T {
     #[stable(feature = "pointer_methods", since = "1.26.0")]
     #[must_use = "returns a new pointer rather than modifying its argument"]
     #[rustc_const_stable(feature = "const_ptr_offset", since = "1.61.0")]
+    // We could always go back to wrapping if unchecked becomes unacceptable
+    #[rustc_allow_const_fn_unstable(const_int_unchecked_arith)]
     #[inline(always)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
     pub const unsafe fn sub(self, count: usize) -> Self
     where
         T: Sized,
     {
-        // SAFETY: the caller must uphold the safety contract for `offset`.
-        unsafe { self.offset((count as isize).wrapping_neg()) }
+        if T::IS_ZST {
+            // Pointer arithmetic does nothing when the pointee is a ZST.
+            self
+        } else {
+            // SAFETY: the caller must uphold the safety contract for `offset`.
+            // Because the pointee is *not* a ZST, that means that `count` is
+            // at most `isize::MAX`, and thus the negation cannot overflow.
+            unsafe { self.offset(intrinsics::unchecked_sub(0, count as isize)) }
+        }
     }
 
     /// Calculates the offset from a pointer in bytes (convenience for

core/src/ptr/mut_ptr.rs

@@ -1,6 +1,7 @@
 use super::*;
 use crate::cmp::Ordering::{self, Equal, Greater, Less};
 use crate::intrinsics::{self, const_eval_select};
+use crate::mem::SizedTypeProperties;
 use crate::slice::{self, SliceIndex};
 
 impl<T: ?Sized> *mut T {
@@ -1095,14 +1096,23 @@ impl<T: ?Sized> *mut T {
     #[stable(feature = "pointer_methods", since = "1.26.0")]
     #[must_use = "returns a new pointer rather than modifying its argument"]
     #[rustc_const_stable(feature = "const_ptr_offset", since = "1.61.0")]
+    // We could always go back to wrapping if unchecked becomes unacceptable
+    #[rustc_allow_const_fn_unstable(const_int_unchecked_arith)]
     #[inline(always)]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
     pub const unsafe fn sub(self, count: usize) -> Self
     where
         T: Sized,
     {
-        // SAFETY: the caller must uphold the safety contract for `offset`.
-        unsafe { self.offset((count as isize).wrapping_neg()) }
+        if T::IS_ZST {
+            // Pointer arithmetic does nothing when the pointee is a ZST.
+            self
+        } else {
+            // SAFETY: the caller must uphold the safety contract for `offset`.
+            // Because the pointee is *not* a ZST, that means that `count` is
+            // at most `isize::MAX`, and thus the negation cannot overflow.
+            unsafe { self.offset(intrinsics::unchecked_sub(0, count as isize)) }
+        }
     }
 
     /// Calculates the offset from a pointer in bytes (convenience for