An async function compiles down a function that returns an iterable-ish state machine thing (a Future), that needs to be stepped through. The await keyword indi

An async function compiles down a function that returns an iterable-ish state machine thing (a Future), that needs to be stepped through. The await keyword indicates a yield point. It's kind of like how if you declare a Python function containing the yield keyword, then the function returns an iterable rather than simply executing top to bottom. In the same way that Python yield only makes sense from within an iterable, Rust's await keyword only makes sense inside of a Future. Outside of a Future, there'd be no concept of a yield. This is why the "outer" function must be declared async.