Shortened guard statement for throwable functions that must succeed

[xwu (Xiaodi Wu)]

This is a commonly rejected change:

Shorthand for guard else empty return statement

From the Swift Evolution common proposals document: Infer return for omitted guard body: It has been proposed many times to allow omission of the guard body for the sake of brevity. However, a core principle of Swift is to make control flow explicit and visible. For example, the try keyword exists solely to indicate to the human reader where thrown errors can happen. Implicit returns would violate this principle, favoring terseness over clarity in a way that isn't typical of Swift. F…