Consider allowing `.env$` in `join_by()`

[DavisVaughan]

Motivated by https://github.com/tidyverse/dplyr/issues/6821

library(dplyr, warn.conflicts = FALSE)

df1 <- tibble(
  X1 = c("chr1", "chr2"),
  a = c(1, 2)
)

df2 <- tibble(
  X2 = c("chr2", "chr3"),
  b = c(3, 4)
)

by.1 <- "X1"
by.2 <- "X2"

# The "old" way
# inner_join(df1, df2, by = setNames(by.2, by.1))

# This works but we don't typically encourage using `!!` in high level APIs
inner_join(df1, df2, by = join_by(!!by.1 == !!by.2))
#> # A tibble: 1 × 3
#>   X1        a     b
#>   <chr> <dbl> <dbl>
#> 1 chr2      2     3
 
# It might be nice to be able to do this, but it currently errors
inner_join(df1, df2, by = join_by(.env$by.1 == .env$by.2))
#> Error in `join_by()`:
#> ! The left-hand side of a `$` expression must be either `x$` or `y$`.
#> ℹ Expression 1 contains `.env$by.1`.

Some notes from talking with @lionel-:

• We have an equivalent-ish to .data$ already with x$ and y$. They don't have a . prefix because I attempted to match the original argument names
• So it seems like we are missing an equivalent to the .env$ pronoun, and we may as well call it .env$ here too
• It would look up in the caller_env() of join_by(), and I don't think join_by() would get an env = caller_env() argument to "pass the env through" unlike what is done in glue().
• It would look up using inherits = TRUE, because it seems like tidy eval does this too, and we generally think that makes sense
• It would require the result to be a single string, or die trying. i.e. you can only use this to inject a single name from the environment, not to inject an entire expression

[DavisVaughan]

Or, since we don't really promote .data$ and .env$ that much, maybe we can take inspiration from any_of() / all_of() and have a custom helper that join_by() knows about that can look up variables in the surrounding env

The idea is something like this, likely with a different name:

join_by(all_of(by.1) == all_of(by.2))

It seems like tidyselect itself needs a helper that aligns with this idea of selecting exactly one thing from the environment, so if it adds that then we should use the same name here.