Detection of variable usage prior to assignment in function scope

[immanuelazn]

I've noticed that when a variable is being used in a function without assignment, we can expect that object_usage_linter can detect it. For instance variable bar here:

foo <- function() {
  # bar gets tagged with object_usage_linter
  if (bar == "5") return(TRUE)
  return(FALSE)
}

However, if we declare this variable after usage, either independently or as a name within an iterator, the object_usage_linter no longer thinks it is an issue.

foo <- function() {
  # bar doesn't get tagged with object_usage_linter
  if (bar == "5") return(TRUE)
  bar <- "1"
  return(FALSE)
}

foo <- function(test_vec) {
  # bar doesn't get tagged with object_usage_linter
  if (bar == "5") return(TRUE)
  for (bar in test_vec) next
  return(FALSE)
}

I have all linters on and I can't seem to get something that would flag variable usage before declaration. Would it be possible to build something that does this?

[AshesITR]

This is doable in principle using static code analysis and assuming well behaved R code. However, object_usage_linter() wraps codetools::checkUsage, so that would be a better place for this enhancement.