[tialaramex]

In Rust if you have a &Thing right here, you can't give that to some new thread unless you can promise the thread has a shorter lifespan than the &Thing does - otherwise the thread could access it after it ceases to exist, and you've introduced Undefined Behaviour.

Without scoped threads this means all references you want to give to a thread must be allowed to live forever, the lifetime 'static, since your threads might live forever (the mechanism to kill them could be discarded unused somehow). Since scoped threads always cease to exist when they go out of scope, we can give them references that live any time longer than that scope.