Update compiler/rustc_const_eval/src/const_eval/machine.rs

Co-authored-by: Ralf Jung <[email protected]>

Author: zachs18

compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -284,6 +284,8 @@ impl<'tcx> CompileTimeInterpCx<'tcx> {
             // `ptr-int` with null, so we can reduce this case to a `scalar_may_be_null` test.
             (Scalar::Int(int), Scalar::Ptr(ptr, _)) | (Scalar::Ptr(ptr, _), Scalar::Int(int)) => {
                 let int = int.to_target_usize(*self.tcx);
+                // The `wrapping_neg` here may produce a value that is not
+                // a valid target usize any more... but `wrapping_offset` handles that correctly.
                 let offset_ptr = ptr.wrapping_offset(Size::from_bytes(int.wrapping_neg()), self);
                 if !self.scalar_may_be_null(Scalar::from_pointer(offset_ptr, self))? {
                     // `ptr.wrapping_sub(int)` is definitely not equal to `0`, so `ptr != int`