Safety features. The committee are, perhaps unconsciously, biased against safety on the presumption (seen in many comments here on HN) that safer has to mean lower performance.
But part of the impetus for Carbon is that WG21 (the C++ Standards Committee) rejected proposals that C++ should focus on better performance and safety. So maybe performance is no longer important either. What's left?
Where they've taken things which might appear on the surface to be modelled on a safer Rust feature, usually the committee insists they be made unsafe. For example suppose I call a Rust function which might return a char or might not, it returns Option<char> and if I'm an idiot and I try to treat that as a char, it doesn't type check because it isn't one, I need to say what I'm going to do when it isn't or else that won't compile.
You can write that in modern C++... except it can automatically try to take the char (which isn't there) out of the empty optional structure and that's Undefined Behaviour. So whereas the Rust prevents programmers from making easy mistakes, the C++ turns those into unexploded bombs throughout your code.
But part of the impetus for Carbon is that WG21 (the C++ Standards Committee) rejected proposals that C++ should focus on better performance and safety. So maybe performance is no longer important either. What's left?
Where they've taken things which might appear on the surface to be modelled on a safer Rust feature, usually the committee insists they be made unsafe. For example suppose I call a Rust function which might return a char or might not, it returns Option<char> and if I'm an idiot and I try to treat that as a char, it doesn't type check because it isn't one, I need to say what I'm going to do when it isn't or else that won't compile.
You can write that in modern C++... except it can automatically try to take the char (which isn't there) out of the empty optional structure and that's Undefined Behaviour. So whereas the Rust prevents programmers from making easy mistakes, the C++ turns those into unexploded bombs throughout your code.