add a macro rule try_or!

[electricface]

macro_rules! try_or(
    ($e:expr, $action:expr ) => (
        match $e {
            Ok(o) => o,
            Err(e) => $action
        }
    );

    ($e:expr,$fmt:expr , $($arg:tt)* ) => (
        match $e {
            Ok(o) => o, 
            Err(e) =>  { fail!($fmt, $($arg)*)}
        }
    );

)

    let mut file = try_or!(
        File::open_mode(p, Open, ReadWrite ),
        "can't open file `{}` : [{}]",p.display(),e
        );

or

    let mut file = try_or!( 
        File::open_mode( &Path::new("message.txt"), Open, ReadWrite),
        {
            error!("can't open file");
            return
        }
    );

[lifthrasiir]

As with your other issues, I think you need some rationales to suggest those macros. Try! got included in the standard library since the same pattern pops up here and there; you have to show the same thing for try_or!, possibly by pointing the recurring patterns in the standard library.

[alexcrichton]

Closing for reasons found here. Additionally, the try! macro is designed to be as lightweight as possible. To customize its functionality, I recommend using a match statement.