The benchmarks game's results are not very correlated with real-world performance.
In this context, Rust's semantics do allow C-level performance- they make it straightforward to describe the same machine code that C would. In some cases, they even enable more compiler optimizations than C does.
Java, on the other hand, makes it hard or impossible to do that (straightforwardly at least). A more mature compiler will make a lot of difference- compare early Java performance to where it is now, for example.
Do you think a 100 line ray tracer is any more "correlated with real-world performance" than 100 line benchmarks game programs?
What I questioned was Rusky's technical-sounding but apparently unsupported claim that "the benchmarks game's results are not very correlated with real-world performance." For Rusky to know he'd need some kind-of "real-world performance" measurements and a correlation coefficient - neither of which have been shown.
It looks like the worst offenders (vs C) are regex-dna (which is a young regex + unicode vs pcre + ascii) and fasta (where the rust version is still single-core). The others are at 1-2x cpu and on par for memory. I suspect most of them could be on par or better with a little effort - only regex looks hard.
I'm working on improving the regex engine now. Hopefully I can submit some improvements soon. :-) It will be hard to get near the top, but I think there's some low hanging fruit I can tackle first.
It isn't bad manners to ask Rusky or pcwalton to show measurements in support of their claims.
It isn't bad manners to trust that when steveklabnik answers "No, as optimizing performance hasn't been a focus, shipping good language semantics has been." that's what he means.
Okay, so I just tested the n-body benchmark [1] for Rust and C++ (program #8), and where they list runtimes of 24.62/9.4 (a factor of 2.62), I actually got a factor of 1.25 (5.45/4.35) on my machine [2]. I used the same version of Rust and nearly the same version of gcc (4.9.1 vs 4.9.2).
Conclusion: Don't draw conclusions from the benchmarks game.
