[lytigas]

There's really no evidence that C++ has a higher performance ceiling. From a broad view, both are compiled with no runtime. More specifically, Rust and C/C++ trade the lead constantly in the language benchmarks game. In some cases, Rust can be faster because it makes stack allocating easier. In others, C++ can be faster because of specialization (which Rust doesn't have yet).

[skohan]

Oh really? It's hard to believe that C++ could not achieve better performance with hand-tailored memory management than safe rust. I can understand that unsafe rust should perform about on par with C++.

[roca]

Rust has some extra costs, but it also has advantages over C and C++.

For example, the Rust compiler can and does reorder fields in structs to improve packing. In C and C++ you have do it manually, and for C++ templated types you sometimes can't pick an order that's optimal for all type parameters. (Rust can pick different field orders for different monomorphized types.)

The Rust compiler does other nice representation optimizations, e.g. Option<bool> is represented as a single byte with three possible values. Not just a hack for Options, but general.

Maybe even more importantly, Rust is really strict about aliasing and this can improve optimization. E.g. a variable that's a mutable reference to T ("&mut T") cannot alias any other reference to T in scope. An immutable reference to T ("&T") can alias other immutable references to T, but the data is truly immutable (unlike a C or C++ const reference). This completely subsumes "type-based alias analysis" and also C/C++ "restrict", and is stronger than both. This information is potentially really useful for optimizers, but unfortunately, since LLVM is mainly for C/C++, the Rust compiler can't take full advantage of this aliasing information yet :-(.

[vardump]

> For example, the Rust compiler can and does reorder fields in structs to improve packing.

Yikes! That can be disabled, right?

[dthul]

Yeah, if you need "C" style struct representation (which guarantees order), you can enable that on a per-struct basis.

[roca]

Use #[repr(C)] to force C-compatible layout.

[Filligree]

Real-world C++ code often copies data far more than is really necessary, because that's the easy way to be sure you aren't mutating a string that someone else expects to remain immutable.

Rust's borrow-checker, obviously, prevents that problem. Certainly it's possible to write C++ code which is as efficient -- and in some cases more efficient -- but will it actually happen?

[rcxdude]

Rust's memory safety rarely comes with a significant performance penalty, especially compared with normal C++ practice.