across() and c_across() when used out of dplyr context (tidyverse#5194)

* Better error messages for across() and c_across() when used out of dplyr context.

closes tidyverse#5180

* no need for fun =

* simplify condition

R/across.R

@@ -158,7 +158,7 @@ c_across <- function(cols = everything()) {
 # to do any required "set up" work (like the `eval_select()` call) a single
 # time per top-level call, rather than once per group.
 across_setup <- function(cols, fns, names, key) {
-  mask <- peek_mask()
+  mask <- peek_mask("across()")
 
   value <- mask$across_cache_get(key)
   if (!is.null(value)) {
@@ -220,7 +220,7 @@ across_setup <- function(cols, fns, names, key) {
 }
 
 c_across_setup <- function(cols, key) {
-  mask <- peek_mask()
+  mask <- peek_mask("c_across()")
 
   value <- mask$across_cache_get(key)
   if (!is.null(value)) {

tests/testthat/test-across-errors.txt

@@ -3,3 +3,13 @@ Error: Problem with `summarise()` input `res`.
 x `.fns` must be NULL, a function, a formula, or a list of functions/formulas
 i Input `res` is `across(is.numeric, 42)`.
 
+
+outside context
+===============
+
+> across()
+Error: across() must only be used inside dplyr verbs
+
+> c_across()
+Error: c_across() must only be used inside dplyr verbs
+

tests/testthat/test-across.R

@@ -111,6 +111,10 @@ test_that("across() gives meaningful messages", {
   verify_output(test_path("test-across-errors.txt"), {
     tibble(x = 1) %>%
       summarise(res = across(is.numeric, 42))
+
+    "# outside context"
+    across()
+    c_across()
   })
 })