Also I wouldn't start by rewriting the thing that calls do_something, I'd start by rewriting do_something. Calling into rust from c++ using something like zngur lets you define rust types in c++ and then call idiomatic rust. You can't do it in the opposite direction because you cannot safely represent all c++ types in rust, because some of them aren't safe.
I have millions of lines of C++. do_something exists and is used but a lot of those lines and works well. I have a new feature that needs to call do_something. I'm not rewriting any code. My current code base was a rewrite of previous code into C++ started before rust existed), and it costs a nearly a billion dollars! I cannot go to my bosses and say that expensive rewrite that is only now starting to pay off because of how much better our code is needs to be scrapped. Maybe in 20 years we can ask for another billion (adjust for inflation) to rewrite again, but today either I write C++, or I interoperate with existing C++ with minimal effort.
I'm working on interoperation with existing C++. It is a hard problem and so far every answer I've found means all of our new features still needs to be written in C++ but now I'm putting in a framework where that code could be used by non-C++. I hope in 5 years that framework is in place by enough that early adopters can write something other than C++ - only time will tell though.
Yeah that use case is harder, but I'm involved in a similar one. Our approach is to split off new work as a separate process when possible and do it entirely in rust. You can call into c++ from rust, it just means more unsafe code in rust wrapping the c++ that has to change when you or your great grandchild finally do get around to writing do_something in rust. I am super aware of how daunting it is, especially if your customer base isn't advocating for the switch. Which most don't care until they get pwned and then they come with lawyers. Autocxx has proven a painful way to go. The chrome team has had some input to stuff and seem to be making it better.