Hardly it will fully replace C++ due to the pervasiveness of the language on many domains and also due to the C++ ecosystem that encompasses system programming; embedded systems; games and so on. For instance, the C++ ecosystem still outmatches rust regarding high performance libraries for several scientific computing domains; GUI graphical user interface libraries, such as Qt, MFC, wxWidget; Game engines and so on. In addition, Rust lacks openMP feature which allows parallelizing for-loops, which are widespread on scientific computing, for taking advantage of multi-core CPUs and from SIMD.
C++ has a ton of baggage and is a very difficult language to use. There are too many language features and they all have years of warts.
Imagine PHP, but not memory safe. Mistakes cause segfaults, memory leaks, and can enable attackers write memory they shouldn't be able to access. To top it off, a lot of libraries are still written in "PHP4 style" and you've got to make use of them. C++ is really hard.
Both C and C++ require consideration of memory semantics, and they are both "hard" in this sense. You have to allocate and free memory deliberately and be conscious about what memory you're passing around. C is much simpler in design than C++. It doesn't have as many features, so it's harder to trip over. It doesn't necessarily make it easier to program, though.
Rust targets the same domains that C and C++ do: systems programing (operating systems and drivers), real time programming (robotics and control software), graphics and games, etc. These are places where explicit memory management is important. Rust does so while being much easier to write. The language leverages new programming language theory and design to protect memory from being written incorrectly. The memory semantics are baked into the language design and checked at compile time. Bad code (in terms of memory and async) simply doesn't compile.
Rust does this while being just as fast as C and C++.
In addition, Rust has a ton of modern features that make it feel like Ruby. Functional idioms, zero-cost abstractions, a modern trait-based type system, generics, macros, and a beautiful package manager and package ecosystem that is better than Ruby and npm.
IMO C++ despite its warts is easier to avoid mismanaging memory than C. RAII and smart pointers are very useful. Fewer off by one errors leading to buffer overflows as well in my experience due to things like iterators.
I think looking at this as a battle is not helpful to understanding the situation. In order to supplant C/C++, Rust (and Rust programmers) will need to operate in harmony with the existing ecosystem for decades.
This interoperability means that Rust programmers will need to also be good C/C++ programmers. Its not a binary OR, its more like diffusion.
Just as an example, the primary (or earliest) use of Rust is in Firefox. But Firefox is still a C++ project.
They're actually pretty different. C is a very simple language - it's probably possible to learn most of the important parts in a few days. C++ is a super-set of C, but it adds a ton of complexity. For instance, C++ templating itself is turing complete.
Go into pointer qualifiers and const with _Generic and you'll see that it's a mess to do anything serious with it. But it's handy for type-generic math (tgmath.h style) and that's seems to be about it.
