I suspect once you get your head wrapped around Rust's type system idioms things will get better. I am interested to see how this turns out for people in general, though.
You can get sum types in C++17 and with boost::variant in early versions.
What specifically do you mean by "the way the borrow checker handles callbacks"?
https://github.com/nikomatsakis/nll-rfc/blob/master/0000-non...
Basically if you have a struct to represent, lets say a Window, and inside of it try to reference instance variables on a closure that would handle a button click for example, self will be moved.
There are a few workarounds, but all of them are less ergonomic than any mainstream GUI toolkit.
There was an initial hard slog around the borrow checker, but once it all clicked it's been very smooth sailing. The only thing I've had trouble with in the months since is when I went to write some macros, but that was more me not reading the docs closely enough. I'd say I am very productive in it now and I don't view any of Rust's features as being a productivity problem.
If you're struggling with Rust, I'd say keep going until you get that "click."
The danger to Rust is that for many projects it wins by default because the alternative languages are immature, rather than because its performance/security capabilities are actually worth the productivity trade-off compared to other languages.
Once Swift matures a bit, it will take a lot of the lustre from Rust.