Yesterday, Intel announced a project combining a clang frontend with icc's backend. [1] The backend takes LLVM IR, so hopefully Rust's LLVM IR output could be run through it.
That's most probably false: the current self-hosting implementation have an LLVM backend. So it has back end optimisations covered. As for the front end, Rust's cleaner semantics will make many optimizations easier.
Plus, it's not like the historical gap couldn't be closed by reading the gazillion papers on C++ optimization out there. Copying a known technique is much faster than developing it in the first place.
Maybe Rust will end up slower than C++ anyway, but it won't be the compiler's fault.
there exist some fairly minimal "C++" like languages for gpu, but nothing to fully deal with strings, exceptions, STL, etc.
And then there is FPGA, but probably not a fair mention since these do not exist (in general) with most common hardware (pc computers, phones, pads, etc.) - while GPU's are found almost anywhere, and on most of the devices now, one can exploit extra computing power exactly by using the GPU.
That said, Rust will get close to C/C++ in ways that GC-based languages never will. GC languages mean memory bloat and soft-realtime problems related to GC cleaning. It will likely hit the sweet spot for a lot of embedded or gaming uses. Obviously you can do those tasks in other languages (great stuff is done on phones with GC-based languages, obviously), but you have to work around some challenges that would not exist in Rust.