C++ is a huge and complex language compared to C. [Compared size of the standard reference document for each.]

C++ is complex to parse. Clang and Gcc use back-tracking to parse C++ and there can be long compilation times.

Tiobe language rankings for May 2019:

Rust is ranked 34th at 0.335% below Lisp and Prolog

Go is ranked 19th at 1.114%.

In complexity and size, Go language is closer to C programming language than either Rust or C++. [Rust and C++ are much larger languages than Go.]

Lastly, Rust does not appear to have a standard BNF grammar specification and I counted around 33 FIXME unspecified sections in the reference document for Rust. I haven't even discussed the number of unstable features in Rust. in the Ref