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

[Andrey Tarantsov]

Whoa. Everyone seems to be in favor of this. Don't you guys always
return something ‘just in case’, in the best traditions of C API design?
I tend to have lots of discardable results and only a few where I might
legitimately forget to use it.

IMO, in most cases ignoring a return value is “safe”:

a) If a method is called only for its result, forgetting to use the
   result won't really introduce any new bugs — if you're not using that
   missing result elsewhere, storing it into a variable won't help.

b) If you're calling a method for its side effect, normally the result
   may be discarded.

The only case where this matters is when the method establishes a long-
term relationship (like a subscription) that needs to be stored to
survive. That seems like a rare case.

Any other practical cases?

A.
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