message: Assert alloc_str_concat is not called with empty strings
  Summary: Documentation assertion.
diff: @@ -311,6 +311,11 @@ impl Heap {
  }
  pub(crate) fn alloc_str_concat<'v>(&'v self, x: &str, y: &str) -> Value<'v> {
  + // If either strings is empty, we should not be calling this function
  + // but reuse non-empty string object instead.
  + debug_assert!(!x.is_empty());
  + debug_assert!(!y.is_empty());
  +
  self.alloc_str_init(x.len() + y.len(), |dest| unsafe {
  copy_nonoverlapping(x.as_ptr(), dest, x.len());
  copy_nonoverlapping(y.as_ptr(), dest.add(x.len()), y.len())
  