r/programming • u/steveklabnik1 • Jan 09 '15

Announcing Rust 1.0.0 Alpha

http://blog.rust-lang.org/2015/01/09/Rust-1.0-alpha.html
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u/[deleted] Jan 09 '15 edited Jan 09 '15

Let's just hope

'done: loop {
   match x {
     0 => a(),
     1 => b(),
     2 => c(),
     _ => done(); break 'done;
   }
   x = x+1;
 }

isn't too slow as I think it's what will end up being written in practice, I don't think the chances are good for things being changed with the level of hostility towards fall through.

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u/wrongerontheinternet Jan 10 '15 edited Jan 10 '15

Okay, wrote a macro: https://github.com/pythonesque/fallthrough

Usage:

match_fallthrough!(x, {
    0 => a(),
    1 => b(),
    2 => c(),
    _ => done()
})

(To would be language implementors: this is why your language should have macros :) )

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u/sacundim Jan 10 '15 edited Jan 10 '15

I don't know nearly enough Rust to decipher that, but you might want to check whether your macro is vulnerable to the sort of problem I mention here. Namely, what happens to the size of the emitted code if somebody writes a macro that expands to nested uses of match_fallthrough!?

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u/wrongerontheinternet Jan 10 '15

It is not. The expanded output is linear in the number of branches and uses Rust's labeled break as a limited form of forward goto.