I was wondering if perhaps this was actually measuring a difference between LLVM and GCC, but they also provide a set of benchmarks of C Clang vs C GCC (1) and Clang is generally slower in those test. Although there is some correlation between the ones Clang wins in C And Rust.

1. https://benchmarksgame-team.pages.debian.net/benchmarksgame/...

Rust can be faster than C because in general C compilers have to assume that pointers to memory locations can overlap (unless you mark them __restrict). Rust forbids aliasing pointers. This opens up a whole world of optimizations in the Rust compiler. Broadly speaking this is why Rust can genuinely be faster than C. Same is true in FORTRAN, for what it's worth.

Well you are saying that even in C you can use the restrict keyword to tell the compiler that 2 memory locations can overlap. Of course is in the hand of the programmer to tell the compiler to do so.

I don't think there is a fair comparison between Rust and C: C is just an higher level assembler, if the programmer knows what he's doing he can use the hardware 100% of its potential. That is the reason why C is still used in all the embedded applications where you have ridiculous low power microcontrollers and you must squeeze out the best performance.

That is the difference between C and Rust to me: for each fast Rust program you are guaranteed that you can write an equivalent performant program in C (or assembly). Worst case scenario you use inline assembly in C and you get that.

Thus the contrary cannot be true for Rust: if I give you a heavily optimized C program not always you can produce an equivalent version in Rust.

Also not always these optimizations are what you want. In C you can choose the level of optimizations, and most of the time, at least on the program that I write, I choose a low level of optimization. The reason is that a lot of time performance is not the only thing that matters, but it maybe matters most the stability of the code (and a code compiler with optimizations is more likely to contain bugs) or the ability to debug (and thus the readability of the assembly output of the compiler).

Rust gives out an horrible assembly code, that is impossible to debug, or to check for correctness. You just have to hope that the compiler doesn't contains bugs. For the same reason Rust is the ideal language to write viruses, since it's difficult to reverse engineer.

So probably you can give me any C program and I'll be able to give you an equivalent Rust program. It'll probably perform about the same, too.