The signature for scoped threads is both simpler and more complicated depending on how you look at it:

https://doc.rust-lang.org/stable/std/thread/fn.scope.html

But really, that first type signature is not very complex. It can get far, far, far worse. That’s just what happens when you encode things in types.

(It reads as “spawn is a function that accepts a closure that returns a type T. It returns a JoinHandle that also wraps a T. Both the closure and the T must be able to be sent to another thread and have a static lifetime.”)