[swift-evolution] [Review] SE-0047 Defaulting non-Void functions so they warn on unused results

[Erica Sadun]

This is the last official day of SE-0047 review. I wanted to throw this in here before things wrapped up.
Hopefully this addresses concerns brought up during the review period.

Thanks, -- Erica


Migration and Impact On Existing Code

Flipping Swift's default behavior to automatically warn on unused results should expose overlooked errors in existing code. Community review has convinced us that simply removing the existing attributes from Swift code will not be the best solution for all developers. 

For many developers, removing @warn_unused_result will be sufficient, enabling warnings to drive code audits. Deprecating @warn_unused_result instead of remove-on-migration, would better allow inspection of any code that is not @warn_unused_result.

Some developers have requested an on-demand inversion (marking un-marked functions) for code they believe is well audited and will resist any issues with masking inherent problems. Brent Royal-Gordan wrote, "If you have done a reasonably thorough audit, deleting the annotations instead of inverting them is a destructive change. When Swift 2 changed the documentation format, the migrator didn't delete all the old doc comments. Deleting @warn_unused_result in audited codebases is akin to that."

Concerns were raised over imported functions written in discardable-by-default environments, especially for C-language sources where community members felt imported functions should match the default behavior of the C-language. Global functions that return scalar values from Objective-C sources might be treated in the same way. We believe any Objective-C method or function that returns an NSObject should warn on ignored result unless explicitly marked.  removeLast() and other pop()-like functions are examples of functions that will most probably be marked with @discartableResult attribute. 


 <https://gist.github.com/erica/05f7815cdb7e644062f5#non-migration-concerns>Non-migration concerns:

In Swift Evolution discussions, the term @discardable was mildly preferred over @discardableResult.

Some community members requested a new attribute enabling exceptional imported functions to be properly annotated from Objective-C source.

Dany St-Amant requested two levels of compiler response: @discardableResult(warn) for simple warnings and @discardableResult(critical) which generates an error with unused results, such as when returning allocated resources.


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