Add even more notes on aliasing&mutation in archive slide

Author: brookslogan

R/grouped_epi_archive.R

@@ -336,6 +336,13 @@ grouped_epi_archive =
                 # copies.
                 if (address(as_of_archive$DT) == address(private$ungrouped$DT)) {
                   # `as_of` aliased its the full `$DT`; copy before mutating:
+                  #
+                  # Note: this step is probably unneeded; we're fine with
+                  # aliasing of the DT or its columns: vanilla operations aren't
+                  # going to mutate them in-place if they are aliases, and we're
+                  # not performing mutation (unlike the situation with
+                  # `fill_through_version` where we do mutate a `DT` and don't
+                  # want aliasing).
                   as_of_archive$DT <- copy(as_of_archive$DT)
                 }
                 dt_key = data.table::key(as_of_archive$DT)

R/methods-epi_archive.R

@@ -805,12 +805,13 @@ group_by.epi_archive = function(.data, ..., .add=FALSE, .drop=dplyr::group_by_dr
 #'   x$slide(new_var = comp(old_var), before = 119)
 #'   ```
 #'
-#' Mutation and aliasing: `epix_slide` and `$slide` will not mutate the input
-#' archives, but may in some edge cases alias parts of the inputs, so copy the
-#' outputs if needed before using mutating operations like `data.table`'s `:=`
-#' operator. Currently, there should not be any aliasing encountered except for
-#' potentially aliasing of columns in edges cases with `all_versions = TRUE`,
-#' but this may change in the future.
+#' Mutation and aliasing: `epix_slide` and `$slide` will not perform in-place
+#' mutation of the input archives on their own. In some edge cases the inputs it
+#' feeds to the slide computations may alias parts of the input archive, so copy
+#' the slide computation inputs if needed before using mutating operations like
+#' `data.table`'s `:=` operator. Similarly, in some edge cases, the output of
+#' the slide operation may alias parts of the input archive, so similarly, make
+#' sure to clone and/or copy appropriately before using in-place mutation.
 #'
 #' @examples
 #' library(dplyr)