[varajelle]

> This may come down to how good the language is at capturing desired semantics in a library. Rust still has a long way to go to catch up with C++ on this axis, and C++ is not waiting around.

What catch up does Rust need to do?

Rust has slice that know the size of its data built in the language, while C++ doesn't. And Rust has stricter const and mutability rules that facilitates optimizations.

As for the implementation, Rust use LLVM which is also the backend used by one of the popular C++ compiler.

[moloch-hai]

I am talking about language features that library authors can use to capture and express semantics in their libraries... but only if the language implements those features. C++ just has a lot more of them.

[xyzzyz]

Like what, for example? To the contrary, I think that, other than constness, C++ has rather few facilities to communicate semantic invariants to the compiler.

[varajelle]

And event const can't in general be used for optimizations (because there can be another reference to the same location, or one can just const_cast)

[grogers]

If the thread you are on doesn't modify the variable (e.g. by const_cast), and that variable isn't atomic or volatile, the compiler should be allowed to treat it as invariant. Whether it does in practice probably depends on a lot of things though.

[varajelle]

Yeah, but the compiler need to see through all the functions to find out that there is no const_cast and therefore makes const useless because it could as well see it is not modified. Also it needs to be good at alias analysis.