[isido]

A good, but difficult question - it's difficult to quantize complexity. Both languages have aim that their users can have much as much control as possible, even if it entails complexity in the language desing.

Perhaps one answer could be, that much of Rust's complexity arises from the concepts of 'borrowing' and 'lifetimes' and how they are encoded in the language.

In C++, complexity much of the complexity arises from the copy/reference semantics, and the possibility of overriding standard behaviour. You need to have wider context to understand local code. So you need to understand how the C++ works on quite a low level and you might need to know more specifics on your C++ codebase than might be the case in Rust.