Funny string question

[Stefan_Salewski]

    import strutils
    proc test =
      var s: string
      s = "test" & "Nim"
      s[0] = toUpperAscii(s[0])
      echo s
    
    
    Run

I wonder if the buffer for this short string is really allocated on the heap. 
Because, I think it don't has to, it should be possible for the initial small 
buffer to live on the stack.