[Diagnostics] Warn when the result of a Void-returning function is ignored (by assigning into '_')

[theblixguy]

Emit a warning if the user ignores the result of a Void-returning function by assigning into a discard expression, because it is redundant.

func returnsVoid() {}
_ = returnsVoid() // warning

Resolves SR-1553

[theblixguy]

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[theblixguy]

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[theblixguy]

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[theblixguy]

Ok I need to update a test in SourceKit-LSP too...

[theblixguy]

swiftlang/sourcekit-lsp#221

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[xedin]

I wonder if this warning is really worse it since in warnings-as-errors setup everybody would get a bunch of new errors about this, which is seems so minor it doesn't really matter?...

[owenv]

@xedin I don’t think the compiler test suite is close enough to real-world code to determine the impact of this change since it uses _ = far more often than the average project. Maybe the source compatibility suite could better estimate the impact this would have on -warnings-as-errors users.

[xedin]

I'm not sure if there is way to run source compatibility in such mode. What I really question is whether such warning is useful or it's just noise and potential project failure source when compiled with warnings-as-errors.

[theblixguy]

I think it’s a pretty minor warning since it only affects situations where the function returns Void, which I would assume is not that common. The bug report talks about a real scenario where this could’ve been useful. Personally I think this is similar to other redundancy warnings we have - it doesn’t affect anything, just warns about the redundancy... might be useful in some scenarios like the one in the SR.

[xedin]

Alright, fair enough.

[theblixguy]

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[theblixguy]

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