Fix: helpful error from need() when message and label are missing (#4386)

* Fix: helpful error from `need()` when `message` and `label` are missing

Previously, calling `need(expr)` without a `message` or `label` errored
with the cryptic `argument "label" is missing, with no default`. Now it
fails with a clear message naming the missing arguments.

Fixes #2509

* refactor: address PR review feedback on `need()` error patch

- Use `cli::cli_abort()` with inline markup instead of `stop(call. = FALSE)`
  to match the newer pattern used elsewhere in this dev cycle (e.g.
  `downloadButton()` in R/bootstrap.R).
- Use `fixed = TRUE` in the `expect_error()` matchers instead of escaping
  parens in a regex.
- Drop the stray blank line inside the original `need() works as expected`
  test_that block.

Author: samuelbharti

NEWS.md

@@ -34,6 +34,8 @@
 
 * Loading shiny no longer creates `.Random.seed` in the global environment as a side effect. (#4382)
 
+* `need()` now gives a clearer error when called without either a `message` or `label` argument, instead of the cryptic "argument \"label\" is missing, with no default". (thanks @chasemc, #2509)
+
 # shiny 1.13.0
 
 ## New features

R/utils.R

@@ -1063,7 +1063,12 @@ validate <- function(..., errorClass = character(0)) {
 #' @rdname validate
 need <- function(expr, message = paste(label, "must be provided"), label) {
 
-  force(message) # Fail fast on message/label both being missing
+  if (missing(message) && missing(label)) {
+    cli::cli_abort(
+      "{.fn need} requires either a {.arg message} or {.arg label} argument."
+    )
+  }
+  force(message) # Fail fast in case `label` is missing but referenced via `message`
 
   if (!isTruthy(expr))
     return(message)

tests/testthat/test-utils.R

@@ -108,6 +108,39 @@ test_that("need() works as expected", {
   expect_null(need(c(FALSE, FALSE, TRUE), FALSE))
 })
 
+test_that("need() message/label handling (#2509)", {
+  # Errors clearly when both `message` and `label` are missing, regardless of
+  # whether `expr` is truthy or falsy (the message default is evaluated eagerly).
+  expect_error(
+    need(1 + 1),
+    "`need()` requires either a `message` or `label` argument",
+    fixed = TRUE
+  )
+  expect_error(
+    need(NULL),
+    "`need()` requires either a `message` or `label` argument",
+    fixed = TRUE
+  )
+
+  # `label` alone: default message is built from `label`
+  expect_identical(need(NULL, label = "input$x"), "input$x must be provided")
+  expect_null(need(1, label = "input$x"))
+
+  # `message` alone: used verbatim, no `label` needed
+  expect_identical(need(NULL, message = "custom message"), "custom message")
+  expect_null(need(1, message = "custom message"))
+
+  # `message = FALSE` is a documented "fail with no message" mode, and must
+  # work without a `label`
+  expect_false(need(NULL, FALSE))
+
+  # When both are provided, `message` wins (no reference to `label`)
+  expect_identical(
+    need(NULL, message = "explicit", label = "ignored"),
+    "explicit"
+  )
+})
+
 test_that("req works", {
   expect_error(req(TRUE, FALSE))
   expect_error(req(TRUE, stop("boom")))