Accept both bare (from rlang) or string as a function input

I editing an existing function in a package. Currently, the function accepts a column name in a data frame as a string. I am updating the function to accept either a string name or a bare name. But I am running into some issues.

The general approach I'd like to take is to convert a bare to a string, so the rest of the function does not need to be updated. If the user passes a string column name, then I don't need to modify the input.

The code below converts a bare input to a string, but I can't figure out how to conditionally convert to a string or leave the string unmodified.

test_fun <- function(by) {
  # convert to enquo
  by1 <- rlang::enquo(by)

  # convert enquo to string
  by2 <- rlang::quo_text(by1)

  by2
}

# converts to string
test_fun(varname)

# not sure how to pass this unmodified
test_fun("varname")

As noted, I strongly recommend against the practice of accepting multiple types if this creates ambiguity (and it does, here).

That said, the following does it:

test_fun = function (by) {
    by = substitute(by)
    if (is.name(by)) {
        as.character(by)
    } else if (is.character(by)) {
        by
    } else {
        stop('Unexpected type')
    }
}

Using rlang, in this case, doesn’t simplify the code.