TaintedString seems to be ignored in loops

[gradha]

The following example compiles and runs:

import unicode

proc test() =
  let input = readFile("weird.nim")
  for letter in runes(input):
    echo int(letter)

when 1 > 2:
  proc failtest() =
    let
      input = readFile("weird.nim")
      letters = runes(input)
    for letter in letters:
      echo int(letter)

when isMainModule:
  test()

However, changing the when condition to true makes the compiler display the error:

weird.nim(12, 22) Error: type mismatch: got (TaintedString)
but expected one of: 
unicode.runes(s: string): TRune

This is unexpected, why does the compiler catch the TaintedString being passed to the iterator in the let block but not in the for block? Shouldn't the compiler fail in the first case too forcing an explicit conversion from TaintedString to string?

[gradha]

The TaintedString error is misleading, actual tainted mode has to be turned on through a compiler switch. The problem here is that the runes iterator can't be assigned in a let block, the user needs to import sequtils and use toSeq on it. It would be nice if the compiler could display a more informative error, like "Can't assign iterator result in let block" or something similar which would point users directly to the problem. Maybe even suggest to import sequtils since toSeq is not available in the system module.

[Araq]

The error message is fine. With --taintedMode:on and 'letters = runes(input.string)' it produces the same error message as with --taintMode:off.