use rlang::f_lhs() in favor of formula[[2]]

[simonpcouch]

This is the first step to address #1003.

form_a <- mpg ~ .
form_b <- ~ .

# [[2]] does not necessarily give LHS
form_a[[2]]
#> mpg
form_b[[2]]
#> .

# resulting in:
rlang::eval_tidy(form_a[[2]], mtcars)
#>  [1] 21.0 21.0 22.8 21.4 18.7 18.1 14.3 24.4 22.8 19.2 17.8 16.4 17.3 15.2 10.4
#> [16] 10.4 14.7 32.4 30.4 33.9 21.5 15.5 15.2 13.3 19.2 27.3 26.0 30.4 15.8 19.7
#> [31] 15.0 21.4
rlang::eval_tidy(form_b[[2]], mtcars)
#> Error: object '.' not found

# but:
rlang::f_lhs(form_a)
#> mpg
rlang::f_lhs(form_b)
#> NULL

# gives:
rlang::eval_tidy(rlang::f_lhs(form_a), mtcars)
#>  [1] 21.0 21.0 22.8 21.4 18.7 18.1 14.3 24.4 22.8 19.2 17.8 16.4 17.3 15.2 10.4
#> [16] 10.4 14.7 32.4 30.4 33.9 21.5 15.5 15.2 13.3 19.2 27.3 26.0 30.4 15.8 19.7
#> [31] 15.0 21.4
rlang::eval_tidy(rlang::f_lhs(form_b), mtcars)
#> NULL

^{Created on 2023-11-02 with reprex v2.0.2}

This will give us some better information to work with in check_outcome().

[EmilHvitfeldt]

Very nice!

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