Recommend using all_of() for looking up variables

NEWS.md

@@ -1,6 +1,28 @@
 
 # tidyselect (development)
 
+* Selection helpers like `all_of()` and `starts_with()` are now
+  available in all selection contexts, even when they haven't been
+  attached to the search path. The most visible consequence of this
+  change is that it is now easier to use selection functions without
+  attaching the host package:
+
+  ```r
+  # Before
+  dplyr::select(mtcars, dplyr::starts_with("c"))
+
+  # After
+  dplyr::select(mtcars, starts_with("c"))
+  ```
+
+  It is still recommended to export the helpers from your package so
+  that users can easily look up the documentation with `?`.
+
+* Selecting non-column variables with bare names now triggers an
+  informative message suggesting to use `all_of()` instead. Referring
+  to contextual objects with a bare name is brittle because it might
+  be masked by a data frame column. Using `all_of()` is safe (#76).
+
 * Using arithmetic operators in selection context now fails more
   informatively (#84).
 

R/vars-select.R

@@ -407,7 +407,7 @@ vars_select_eval <- function(vars, quos) {
     # Add `.data` pronoun in the context mask even though it doesn't
     # contain data. This way the pronoun can be used in any parts of the
     # expression.
-    context_mask <- new_data_mask(env())
+    context_mask <- new_data_mask(env(!!!vars_select_helpers))
     context_mask$.data <- data_mask$.data
   }
 
@@ -628,6 +628,11 @@ eval_sym <- function(name, data_mask, context_mask, colon = FALSE) {
   )
 
   if (!is_missing(value)) {
+    inform(glue_c(
+      "Note: Selecting non-column variables is brittle.",
+      i = "If the data contains `{name}` it will be selected instead.",
+      i = "Use `all_of({name})` to silence this message."
+    ))
     return(value)
   }
 

tests/testthat/outputs/vars-select-context-lookup.txt

@@ -0,0 +1,8 @@
+> vars_select(letters, vars)
+Message: Note: Selecting non-column variables is brittle.
+i If the data contains `vars` it will be selected instead.
+i Use `all_of(vars)` to silence this message.
+
+  a   b 
+"a" "b" 
+

tests/testthat/test-vars-select-eval.R

@@ -124,3 +124,22 @@ test_that("can't use arithmetic operators in data context", {
 test_that("can use arithmetic operators in non-data context", {
   expect_identical(vars_select(letters, identity(2 * 2 + 2 ^ 2 / 2)), c(f = "f"))
 })
+
+test_that("symbol lookup outside data informs caller about better practice", {
+  vars <- c("a", "b")
+  expect_message(
+    vars_select(letters, vars),
+    "Use `all_of(vars)` to silence",
+    fixed = TRUE
+  )
+  verify_output(test_path("outputs", "vars-select-context-lookup.txt"), {
+    vars_select(letters, vars)
+  })
+})
+
+test_that("selection helpers are in the context mask", {
+  out <- local(envir = baseenv(), {
+    tidyselect::vars_select(letters, all_of("a"))
+  })
+  expect_identical(out, c(a = "a"))
+})