Allow expressions in facet specs even when some variables are not available on some layers - #3757

(Cherry-picked version of #3735 for v3.3.0-rc branch)

Fix #2963

Background

Not all variables in a facet spec are available on all the layers. A solution suggested on r-lib/rlang#888 (comment) is:

bind the same symbols inside each panel, and when the symbols are undefined use an active binding to throw a typed error? And then you'd only catch these particular errors in the tryCatch()

Main change

This PR injects such an active bindings on the column names of all the plot data so that we can let eval_facet() fail gracefully.

eval_facet() evaluates facet specs in these two steps:

1. When the spec is a symbol (the most common case), check if the layer data contains the same name of column. If it does, subset the column and return it. If it doesn't, return NULL. In this case, no error happens even if the symbol is not available on all the layers.
2. When the spec is a expression, evaluate it. In this case, an error is raised on the layer where the symbol is not available.

This PR changes only step 2, so I expect this doesn't affect most of the existing code. Note that, while step 2 can also handle symbols, we still need step 1 to issue a friendly error ("At least one layer must contain all faceting variables") when no layer contain the variable.

devtools::load_all("~/repo/ggplot2/")
#> Loading ggplot2

# works fine with expressions
ggplot(mtcars, aes(mpg, cyl)) +
  geom_point() +
  geom_vline(xintercept = 20) +
  facet_wrap(vars(2 * am))

# works fine with external variables
two <- 2
ggplot(mtcars, aes(mpg, cyl)) +
  geom_point() +
  geom_vline(xintercept = 20) +
  facet_wrap(vars(two * am))

# raises an error when the expression refers to some non-existent variable
ggplot(mtcars, aes(mpg, cyl)) +
  geom_point() +
  geom_vline(xintercept = 20) +
  facet_wrap(vars(no_such_variable * am))
#> Error in eval_tidy(facet, mask): object 'no_such_variable' not found

# special case: raises a friendlier error when the expression is a symbol
ggplot(mtcars, aes(mpg, cyl)) +
  geom_point() +
  geom_vline(xintercept = 20) +
  facet_wrap(vars(no_such_variable))
#> Error: At least one layer must contain all faceting variables: `no_such_variable`.
#> * Plot is missing `no_such_variable`
#> * Layer 1 is missing `no_such_variable`
#> * Layer 2 is missing `no_such_variable`

# a data pronoun can be handled
ggplot(mtcars, aes(mpg, cyl)) +
  geom_point() +
  geom_vline(xintercept = 20) +
  facet_wrap(vars(.data$am))

^{Created on 2020-01-23 by the reprex package (v0.3.0)}

Minor change

• This PR removes env argument of eval_facet(). This is not necessary, but done in order to avoid confusion. I believe all facet specs are converted to quosures and bare expressions are not allowed, which means env is always ignored on eval_tidy().