decode-coding-string and consing

[Jesper Harder <harder@myrealbox.com>]

,----[ C-h f decode-coding-string RET ]
| decode-coding-string is a built-in function.
| (decode-coding-string STRING CODING-SYSTEM &optional NOCOPY)
| 
| Decode STRING which is encoded in CODING-SYSTEM, and return the result.
| Optional arg NOCOPY non-nil means it is OK to return STRING itself
| if the decoding operation is trivial.
`----

Under what conditions is the decoding operation "trivial"?

I was hoping that something like:

  (decode-coding-string "abc" 'us-ascii t) or
  (decode-coding-string "abc" 'latin-1 t)

wouldn't cons a new string; but unfortunaley it does.