Both Rust and C++ have the same low-level capabilities: both can use inline assembly, or call external functions via a C API. At the language level, both have comparable performance characteristics: they both use manual memory management, both can work directly with memory via raw pointers and bytewise transmutes, both have the same access to SIMD intrinsics, and can precisely control memory layout of data. Finally, both can use the same LLVM backend for optimizations and code generation.
This implies that, in the limit of code maximally optimized at the expense of everything else (including readability and development effort), the performance of two languages should be the same. In fact, it's not uncommon to produce exactly the same assembly output (at least for sufficiently small programs).
Thus the question isn't "which is faster", but "which is faster given 'idiomatic' code with 'reasonable' development effort, readability and maintainability". In this form the question becomes fundamentally subjective, since one can always optimize more at the expense of other code properties.