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I've been working for the past day or so to add a very minor feature to one of our UIKit based apps using SwiftUI, as an experiment. Suddenly, this morning, previews stopped working, citing a build error that only occurs when building for preview: Compiling failed: 'main' attribute cannot be used in a module that contains top-level code I was editing the view when this started happening, but no fiddling I've done with it since has fixed the issue. I have tried: Reverting all changes in Git Cleaning the build folder (repeatedly) Touching both the file with the SwiftUI view, and the AppDelegate that's annotated with @main Closing the project, and Xcode The project builds and runs just fine: this error only occurs with previews. I had been working in Xcode 12.5 beta 2 when the problem started, but it persists when I open the project in Xcode 12.3. Is there some extra build cache that I can go purge?