[dbaupp]

You can use lifetimes to enforce that a certain thing is static data, and combined with an enum, you get the best of both worlds: compile-time constants require no allocations, but still flexible enough to allow run-time construction:

  enum StringRef {
      Static(&'static str),
      Owned(~str)
  }

and then `error` would take `StringRef` and be called like:

  self.error(Static("trailing characters"))
  // or
  self.error(Owned(fmt!("%u trailing characters", count)))

(There was even a pull request that added this and the corresponding one for vectors to the stdlib, but it didn't landed (yet): https://github.com/mozilla/rust/pull/7599)