Once you understand traits, those trait bounds really aren't bad. Now if this were some macro-heavy library, I could understand the complaints, but this is just constraining the type of the generic type.

I find it readable, it's saying that the "inner" function returns a reference to the value the State object is containing (which must be of a type that implements Send and Sync), and that reference cannot outlive the State object the value was returned from.

There are many languages I don't/didn't have to learn before I could read and/or patch code written in it. I hacked on C and C++ code long before I understood their basics. Go would've been the same if I hadn't bothered to spend a couple days learning it shortly after first exposure.