Validate mapping before data in layer(). Closes #2862

[clauswilke]

Validate mapping before data in layer(). Closes #2862

library(ggplot2)
library(magrittr)

ggplot(mtcars, aes(mpg, cyl)) %>%
  geom_point()
#> Error: `mapping` must be created by `aes()`
#> Did you use %>% instead of +?

ggplot(mtcars) %>%
  geom_point(aes(mpg, cyl))
#> Error: `mapping` must be created by `aes()`
#> Did you use %>% instead of +?

ggplot(mtcars) +
  geom_point(aes(mpg, cyl), list())
#> Error: `data` must be a data frame, or other object coercible by `fortify()`, not a list

Created on 2018-08-29 by the reprex package (v0.2.0).

[hadley]

Not sure if unit test worth it or not. Not is probably fine.

[lock]

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