`cli_progress_step()` : evaluate `msg_done` twice (unexpected mix of eager and lazy) ?

Start at cli_progress_step() and reproduce the supplied examples, especially count_evals() and doesnt_work(). Trace when msg_done is evaluated at call time and completion; done should establish a consistent, documented evaluation model and cover the missing-y and repeated-evaluation cases with regression tests.

Hi, and thanks for the amazing work on the cli package!

What puzzles me is that the behavior seems in between two expected paradigms:

• Either the interpolation is eager (evaluated at the moment of the function call), which would make sense to avoid surprises and ensure the message is fixed and reproducible.
• Or the interpolation is lazy, evaluated only when the message is actually displayed, which also makes sense, e.g., to reflect the state of a variable when exiting a function.

But in fact msg_done is being evaluated twice.

• Any variable use in interpolation must exist at the time the function is called (otherwise you get an error),
• yet its value is not captured at that moment and it can still be updated before the message is shown.

Is this the intended behaviour ?

Thanks a lot in advance!

library(cli)

works_if_exists_before = function() {
  x = "foo"
  y = "bar"
  
  cli_progress_step(
    msg = "I show {x} at first",
    msg_done = "And {y} at the end"
  )
  
  Sys.sleep(1)
}

works_if_exists_before()
#> ℹ I show foo at first✔ And bar at the end [1.1s]


works_with_dummy_before = function() {
  x = "foo"
  y = ""
  
  cli_progress_step(
    msg = "I show {x} at first",
    msg_done = "And {y} at the end"
  )
  
  y = "bar"
  
  Sys.sleep(1)
}

works_with_dummy_before()
#> ℹ I show foo at first✔ And bar at the end [1.1s]


doesnt_work = function() {
  x = "foo"
  
  cli_progress_step(
    msg = "I show {x} at first",
    msg_done = "And {y} at the end"
  )
  
  y = "bar"
  Sys.sleep(1)
}

doesnt_work()
#> Error in "\"id\" %in% names(args)": ! Could not evaluate cli `{}` expression: `y`.
#> Caused by error in `eval(expr, envir = envir)`:
#> ! object 'y' not found
#> Error in c("(function (e) ", "{"): ! Error in a deferred `on.exit()` clause
#> Caused by error in `"id" %in% names(args)`:
#> ! Could not evaluate cli `{}` expression: `y`.
#> Caused by error in `eval(expr, envir = envir)`:
#> ! object 'y' not found

evaluated = function() {
  value = "once"
  function() {
    if (value == "once") {
      value <<- "twice"
      return("once")
    } else if (value == "twice") {
      value <<- "more than twice"
      return("twice")
    } else {
      return("more than twice")
    }
  }
}

count_evals = function() {
  f = evaluated()
  cli_progress_step(
    msg = "msg",
    msg_done = "evaluated {f()}"
  )
}

count_evals()
#> ℹ msg✔ evaluated twice [6ms]