> Isn't it kind of a poor design choice that Rust will not actually begin execution of the function until `.await` is called? Begin execution where? If every

> Isn't it kind of a poor design choice that Rust will not actually begin execution of the function until `.await` is called? Begin execution where? If every future started executing immediately on a global event loop, that event loop would need to allocate space for every future on the heap. A heap allocation for every future is exactly the sort of overhead that Rust is trying so carefully to avoid. With Rust futures, you can have a large call tree of async functions calling other async functions. Each one of those returns a future, and those futures get cobbled together by the compiler into a single giant future of a statically known size. Once that state machine object is assembled, you can make a single heap allocation to put it on your event loop. Or if you're going to block the current thread waiting on it, depending on what runtime library you're using, you might even get away with zero heap allocations. This sort of thing is also why Rust's futures are poll-based, rather than using callbacks. Callbacks would force everything to be heap allocated, and would work poorly with lifetimes and ownership in general.