expand comment

src/librustc_mir/interpret/memory.rs

@@ -759,6 +759,8 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> Memory<'a, 'mir, 'tcx, M> {
         // SAFE: The above indexing would have panicked if there weren't at least `size` bytes
         // behind `src` and `dest`. Also, we use the overlapping-safe `ptr::copy` if `src` and
         // `dest` could possibly overlap.
+        // The pointers above remain valid even if the `HashMap` table moved around because they
+        // point into the `Vec` storing the bytes.
         unsafe {
             assert_eq!(size.bytes() as usize as u64, size.bytes());
             if src.alloc_id == dest.alloc_id {