Conflict with rlang's new {{ operator

[wlandau]

Prework

• Read and abide by drake's code of conduct.
• Search for duplicates among the existing issues, both open and closed.
• Advanced users: verify that the bug still persists in the current development version (i.e. remotes::install_github("ropensci/drake")) and mention the SHA-1 hash of the Git commit you install.

Description

Development rlang has a new interpolation operator {{. This operator conflicts with drake commands with nested curly braces. For example, this plan

drake_plan(
  y = {
    dir.create(file_out("xyz"))
    writeLines("123", "xyz/e.txt")
  }
)

creates this command

rlang::expr(local({
  {
    dir.create(file_out("xyz"))
    writeLines("123", "xyz/e.txt")
  }
}))

which generates the error in r-lib/rlang#772.

[wlandau]

To be clear, this only affects multiline commands. It does not affect single-line commands, e.g. small function calls.