Support data masking pronouns in slice_*() (#1302)

* Support data masking pronouns in `slice_*()`

* Add test

NEWS.md

@@ -1,5 +1,7 @@
 # dbplyr (development version)
 
+* `slice_*()` now supports the data masking pronouns `.env` and `.data` (@mgirlich, #1294).
+
 * `tbl()` now informs when the user probably forgot to wrap the table identifier
   with `in_schema()` or `sql()` (@mgirlich, #1287).
 

R/verb-slice.R

@@ -163,7 +163,8 @@ slice_by <- function(.data, order_by, size, .by, with_ties = FALSE) {
 
   # must use `add_order()` as `window_order()` only allows variables
   # this is only okay to do because the previous, legal window order is restored
-  .data$lazy_query <- add_order(.data, quos({{order_by}}))
+  dots <- partial_eval_dots(.data, !!!quos({{order_by}}), .named = FALSE)
+  .data$lazy_query <- add_order(.data, dots)
 
   out <- filter(.data, !!window_fun) %>%
     window_order(!!!old_frame)

tests/testthat/_snaps/verb-slice.md

@@ -66,6 +66,39 @@
       ! `na_rm = FALSE` isn't supported on database backends.
       i It must be TRUE instead.
 
+# slice_* can use data masking pronouns
+
+    Code
+      lf %>% slice_max(x)
+    Output
+      <SQL>
+      SELECT `x`, `id`
+      FROM (
+        SELECT `df`.*, RANK() OVER (ORDER BY `x` DESC) AS `q01`
+        FROM `df`
+      ) AS `q01`
+      WHERE (`q01` <= 1)
+    Code
+      lf %>% slice_max(.data$x)
+    Output
+      <SQL>
+      SELECT `x`, `id`
+      FROM (
+        SELECT `df`.*, RANK() OVER (ORDER BY `x` DESC) AS `q01`
+        FROM `df`
+      ) AS `q01`
+      WHERE (`q01` <= 1)
+    Code
+      lf %>% slice_max(.data$x * .env$x)
+    Output
+      <SQL>
+      SELECT `x`, `id`
+      FROM (
+        SELECT `df`.*, RANK() OVER (ORDER BY `x` * -1 DESC) AS `q01`
+        FROM `df`
+      ) AS `q01`
+      WHERE (`q01` <= 1)
+
 # slice_sample errors when expected
 
     Code

tests/testthat/test-verb-slice.R

@@ -28,6 +28,17 @@ test_that("slice_max orders in opposite order", {
   expect_snapshot(error = TRUE, db %>% slice_max(id, n = 1, na_rm = FALSE))
 })
 
+test_that("slice_* can use data masking pronouns", {
+  lf <- lazy_frame(x = c(1, 1, 2), id = c(1, 2, 3))
+  x <- -1L
+
+  expect_snapshot({
+    lf %>% slice_max(x)
+    lf %>% slice_max(.data$x)
+    lf %>% slice_max(.data$x * .env$x)
+  })
+})
+
 test_that("slice_sample errors when expected", {
   db <- memdb_frame(x = c(1, 1, 2), id = c(1, 2, 3))