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

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.