Swift Vision: Improving the Approachability of Data-Race Safety

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2024-11-26 23:00:10

This document lays out several potential paths for improving the usability of Swift 6, especially in simple situations where users aren’t intending to use concurrency at all.

A key tenet of our thinking in this vision is that we want to drastically reduce the number of explicit concurrency annotations necessary in projects that aren’t trying to leverage parallelism for performance. This is important for many kinds of programming, such as UI programming and scripts, where concurrency is often localized and large swathes of the code are generally expected to be constrained to the main actor. At the same time, we want to maintain a smooth path for experienced programmers to opt in to concurrency and maintain the safety of complete data-race checking.

We believe that the right solution to these problems is to allow code to opt in to being “single-threaded” by default, on a module-by-module basis. This would change the default isolation rule for unannotated code in the module: rather than being non-isolated, and therefore having to deal with the presumption of concurrency, the code would instead be implicitly isolated to @MainActor. Code imported from other modules would be unaffected by the current module’s choice of default.