I hope we get some sugar here, the ? operator is a great idea. I like the trait implementation as well. I think Rust needs multiple dispatch though - a given type needs to support conversions of IOError to different custom results right? So you can use a struct in different library functions that may return a different custom error. Or maybe I am missing something here; but I do believe multiple dispatch is planned for 1.0.

I did this in Haskell using MPTC, I have used the Convertible class like:

instance Convertible IOError RedisError where safeConvert = <conversion mapping here>

Then I have combinators like:

convEither :: Convertible e1 e2 => Either e1 a -> Either e2 a

and

convEitherT :: Convertible e1 e2 => m (Either e1 a) -> EitherT m e2 a

These are quite handy in close quarters combat. For example to open a file and bail on a generic IOException with a custom application exception, I can have something like:

instance Convertible IOException CustomError

getWords :: FilePath -> IO (Either CustomError [String])

getWords fileName = runEitherT $

    do file <- tryIO (openFile fileName) >>= convEitherT

       line <- readLine file 
       return (words line)