No, there is an earlier "credible attempt to displace C++": D. D was created by Walter Bright, who previously was "the main developer of the first C++ compiler to translate source code directly to object code without using C as an intermediate language" (quoting Wikipedia), and so is clearly "credible" by your criteria.
The thing about Rust is that the ownership/borrowing system makes it better than C++ in important ways. Programmers have to specify variable usage details, but this (1) makes the code easier to maintain, (2) gives you a much more powerful form of RAII, (3) makes reference counting work so well that you don't need a tracing garbage collecter, and (4) makes the resulting code significantly faster in many cases. That is why Rust should compete successfully with C++, whereas a "C++ without the warts" like D could not.