data-mask -> selection, via transmute

• transmute() creates a data frame that consists of columns in ...
• names() yields a character vector of column names from ...
• all_of() accepts a character vector of names and performs tidy selection

my_pivot_longer <- function(data, ...) {
  # Forward `...` in data-mask context with `transmute()`
  # and save the inputs names
  inputs <- dplyr::transmute(data, ...)
  names <- names(inputs)
  
  # Update the data with the inputs
  data <- dplyr::mutate(data, !!!inputs)

  # Select the inputs by name with `all_of()`
  tidyr::pivot_longer(data, cols = all_of(names))
}

mtcars %>% my_pivot_longer(cyl, am = am * 100)