I think what you mean is that current Rust _implementations_ optimize better than current C _implementations_. There's no reason a C99 compiler can't do those optimizations.
>There's no reason a C99 compiler can't do those optimizations.
No, there actually is, the C standard says structs have to be laid out in memory in the same order they're written and C has much weaker aliasing rules.
C99 has the restrict keyword, which guarantees that the pointer won't be aliased. As far as reordering struct members, there's no reason why an implementation can't provide that as an optional optimization you must explicitly turn on. Providing such an optimization and corresponding compiler flag would not disqualify it from being a conforming implementation.
There have been compilers that did this optimization.
179.art, one of the SPEC2000 benchmarks, has some poorly laid out structs. Sun Microsystems was the first company to introduce targeted optimizations for this benchmark.
GCC also had an optimization pass[0] for this. It may have been removed.
