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`.

library/core/src/ptr/const_ptr.rs

 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 @@ pub const fn guaranteed_ne(self, other: *const T) -> Option<bool>
     #[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

library/core/src/ptr/mut_ptr.rs

 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 @@ pub const fn guaranteed_ne(self, other: *mut T) -> Option<bool>
     #[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

tests/codegen/ptr-arithmetic.rs

+// compile-flags: -O -Z merge-functions=disabled
+// ignore-debug (the extra assertions get in the way)
+
+#![crate_type = "lib"]
+
+// CHECK-LABEL: ptr @i32_add(
+// CHECK-SAME: [[WORD:i[0-9]+]] noundef %n)
+#[no_mangle]
+pub unsafe fn i32_add(p: *const i32, n: usize) -> *const i32 {
+    // CHECK: %[[TEMP:.+]] = getelementptr inbounds i32, ptr %p, [[WORD]] %n
+    // CHECK: ret ptr %[[TEMP]]
+    p.add(n)
+}
+
+// Ensure we tell LLVM that the negation in `sub` can't overflow.
+
+// CHECK-LABEL: ptr @i32_sub(
+// CHECK-SAME: [[WORD:i[0-9]+]] noundef %n)
+#[no_mangle]
+pub unsafe fn i32_sub(p: *const i32, n: usize) -> *const i32 {
+    // CHECK: %[[DELTA:.+]] = sub nsw [[WORD]] 0, %n
+    // CHECK: %[[TEMP:.+]] = getelementptr inbounds i32, ptr %p, [[WORD]] %[[DELTA]]
+    // CHECK: ret ptr %[[TEMP]]
+    p.sub(n)
+}
+
+// CHECK-LABEL: ptr @i32_offset(
+// CHECK-SAME: [[WORD:i[0-9]+]] noundef %d)
+#[no_mangle]
+pub unsafe fn i32_offset(p: *const i32, d: isize) -> *const i32 {
+    // CHECK: %[[TEMP:.+]] = getelementptr inbounds i32, ptr %p, [[WORD]] %d
+    // CHECK: ret ptr %[[TEMP]]
+    p.offset(d)
+}