[swift-evolution] Make non-void functions @warn_unused_result by default

[David Sweeris]

What about @warnUnusedResult being implied for @pure functions, and leaving it off by default for everything else? 

- Dave Sweeris

> On Feb 24, 2016, at 16:47, Gwendal Roué via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Should @warn_unused_result become the default, the quality of the code of the users of those libraries risks a brutal degradation, full of poor `let _ = ...`.