Warn when using constant aesthetics (#5256)

* Add constant check

* Exclude geom_{ab/h/v}line from check

* Add test

* Pass constructor as call in warning

R/geom-abline.R

@@ -147,5 +147,7 @@ GeomAbline <- ggproto("GeomAbline", Geom,
 
   draw_key = draw_key_abline,
 
-  rename_size = TRUE
+  rename_size = TRUE,
+
+  check_constant_aes = FALSE
 )

R/geom-hline.R

@@ -60,5 +60,7 @@ GeomHline <- ggproto("GeomHline", Geom,
 
   draw_key = draw_key_path,
 
-  rename_size = TRUE
+  rename_size = TRUE,
+
+  check_constant_aes = FALSE
 )

R/geom-vline.R

@@ -60,5 +60,7 @@ GeomVline <- ggproto("GeomVline", Geom,
 
   draw_key = draw_key_vline,
 
-  rename_size = TRUE
+  rename_size = TRUE,
+
+  check_constant_aes = FALSE
 )

R/layer.R

@@ -292,15 +292,22 @@ Layer <- ggproto("Layer", NULL,
     }
 
     n <- nrow(data)
+    aes_n <- lengths(evaled)
     if (n == 0) {
       # No data, so look at longest evaluated aesthetic
       if (length(evaled) == 0) {
         n <- 0
       } else {
-        aes_n <- lengths(evaled)
         n <- if (min(aes_n) == 0) 0L else max(aes_n)
       }
     }
+    if ((self$geom$check_constant_aes %||% TRUE)
+        && length(aes_n) > 0 && all(aes_n == 1) && n > 1) {
+      cli::cli_warn(c(
+        "All aesthetics have length 1, but the data has {n} rows.",
+        i = "Did you mean to use {.fn annotate}?"
+      ), call = self$constructor)
+    }
     check_aesthetics(evaled, n)
 
     # Set special group and panel vars

tests/testthat/_snaps/layer.md

@@ -84,6 +84,11 @@
     ! Can only draw one boxplot per group
     i Did you forget `aes(group = ...)`?
 
+# layer warns for constant aesthetics
+
+    All aesthetics have length 1, but the data has 32 rows.
+    i Did you mean to use `annotate()`?
+
 # layer_data returns a data.frame
 
     `layer_data()` must return a <data.frame>

tests/testthat/test-layer.R

@@ -123,6 +123,14 @@ test_that("layer reports the error with correct index etc", {
   expect_snapshot_error(ggplotGrob(p))
 })
 
+test_that("layer warns for constant aesthetics", {
+  p <- ggplot(mtcars, aes(x = seq_along(mpg))) + geom_point(aes(y = 2))
+  expect_silent(ggplot_build(p))
+
+  p <- ggplot(mtcars, aes(x = 1)) + geom_point(aes(y = 2))
+  expect_snapshot_warning(ggplot_build(p))
+})
+
 # Data extraction ---------------------------------------------------------
 
 test_that("layer_data returns a data.frame", {