Allow using functions that return NULL in aes() (#2997)

Author: yutannihilation

NEWS.md

@@ -6,6 +6,9 @@
   get backticks, and long expressions are abbreviated with `...`
   (@yutannihilation, #2981).
 
+* Aesthetic mappings now accept functions that return `NULL` (@yutannihilation,
+  #2997)
+
 # ggplot2 3.1.0
 
 ## Breaking changes

R/layer.r

@@ -218,8 +218,8 @@ Layer <- ggproto("Layer", NULL,
     scales_add_defaults(plot$scales, data, aesthetics, plot$plot_env)
 
     # Evaluate and check aesthetics
-    aesthetics <- compact(aesthetics)
     evaled <- lapply(aesthetics, rlang::eval_tidy, data = data)
+    evaled <- compact(evaled)
 
     n <- nrow(data)
     if (n == 0) {

tests/testthat/test-layer.r

@@ -38,6 +38,13 @@ test_that("missing aesthetics trigger informative error", {
   )
 })
 
+test_that("if an aes is mapped to a function that returns NULL, it is removed", {
+  df <- data.frame(x = 1:10)
+  null <- function(...) NULL
+  p <- cdata(ggplot(df, aes(x, null())))
+  expect_identical(names(p[[1]]), c("x", "PANEL", "group"))
+})
+
 # Data extraction ---------------------------------------------------------
 
 test_that("layer_data returns a data.frame", {