[swift-evolution] [Idea] Add blockreturn keyword

[Brent Royal-Gordon]

> I often get “return blind” when looking at a method with blocks inside of it. I don’t know whether a return returns from the method or a block within that method. It would be nice to have a totally different keyword for blocks so I could distinguish the two.

Without endorsing the idea, I'll suggest the `leave` keyword for this role.

Of course, this also kind of suggests that we ought to support non-local returns, which I don't think is realistic.

(Hmm. *Could* we support non-local returns from @noescape closures passed to rethrows functions?)

-- 
Brent Royal-Gordon
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