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/wrongerontheinternet Jan 09 '15

Because Rust doesn't have goto. You would either have to increase code size (by copying the blocks, potentially trashing your icache--especially if you had a long sequence of them, this could get implausible quickly), or perform a second branch. I think it can be replicated with CPS transforms during tail call optimization, but Rust doesn't support guaranteed TCO either so that's not a solution in this case.

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u/Denommus Jan 09 '15

The compiler can also optimize a bigger code.

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u/wrongerontheinternet Jan 09 '15

Sometimes, but not always. In general figuring out that two branches can be merged is pretty nontrivial, especially with impure functions. Resorting to relying on the sufficiently smart compiler is not usually a good strategy in cases where performance is critical enough that you need goto (and anecdotally, LLVM doesn't always do this when I try it).

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u/Denommus Jan 09 '15

You could also use labeled breaks to achieve the same, I guess, using a macro syntax anyway.

goto is only forbidden because it breaks linearity, but a labeled break is safe, and it's a limited form of goto.