combine and keep attributes of same length

d.R

l1 <- structure(list(x = 1), attr = 1)
l2 <- structure(list(y = 2), attr = 2)

d <- function(...) {
  
  dots <- match.call(expand.dots = FALSE)$...
  attr_ls <- lapply(dots, function(x) attributes(get(as.character(x))))
  attr_ls <- lapply(purrr::transpose(attr_ls), unlist)
  
  out <- c(...)
  ln_out <- length(out)
  
  if (!all(unlist(lapply(attr_ls, function(x) length(x) == ln_out)))) {
    stop("All attributes must have the same length as the output object.")
  }
  
  attributes(out) <- attr_ls
  out
  
}

# works
d(l1, l2)

# test: throws error
l3 <- structure(list(y = 2))
d(l1, l3)

# test: throws error
l4 <- structure(list(y = 2), attr = c("a", "b"))
d(l1, l4)