Rollup merge of #112757 - Danvil:patch-1, r=Mark-Simulacrum

Use BorrowFlag instead of explicit isize

The integer type tracking borrow count has a typedef called `BorrowFlag`. This type should be used instead of explicit `isize`.

library/core/src/cell.rs

@@ -1374,7 +1374,7 @@ fn clone(&self) -> Self {
         debug_assert!(is_reading(borrow));
         // Prevent the borrow counter from overflowing into
         // a writing borrow.
-        assert!(borrow != isize::MAX);
+        assert!(borrow != BorrowFlag::MAX);
         self.borrow.set(borrow + 1);
         BorrowRef { borrow: self.borrow }
     }
@@ -1756,7 +1756,7 @@ fn clone(&self) -> BorrowRefMut<'b> {
         let borrow = self.borrow.get();
         debug_assert!(is_writing(borrow));
         // Prevent the borrow counter from underflowing.
-        assert!(borrow != isize::MIN);
+        assert!(borrow != BorrowFlag::MIN);
         self.borrow.set(borrow - 1);
         BorrowRefMut { borrow: self.borrow }
     }