Using modern C++ is like walking a tightrope. If you stay within the constraints, you'll do mostly fine. But it's a fine balancing act and there's a steep cliff

Using modern C++ is like walking a tightrope. If you stay within the constraints, you'll do mostly fine. But it's a fine balancing act and there's a steep cliff to fall off if you don't pay enough attention. Look at it from the perspective of someone brand new to C++. How do they know what is "modern" and the right way to do things, versus "legacy" and the wrong way to do things? As a beginner, there are thousands of resources available to you on the internet -- some new, some decades old -- with code of varying quality that may or may not be "modern". All of it is valid C++ and will compile, but some of it may have footguns that are nonobvious to the new programmer. There's no "modern only" compiler that will reject said footguns, because C++ takes the position that leaving the footguns there is a feature. This isn't the case with Rust. Maybe it will become more like C++ in the future as the language changes, only time can tell. But for now, code that compiles in Rust is guaranteed to be free from various classes of bugs that still are possible to compile in C++, despite "modern" C++ having fixed various issues. Another way of saying this is that "just use modern C++" is a variation of "you're holding it wrong" or "just don't write buggy code", which has proven not to be scalable advice that leads to safer software.