swift: Add swift_implicit_main target kwarg #14298
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The '-parse-as-library' compiler option forbids emitting top-level statements as part of an implicitly generated main function. This happens either in Swift modules consisting of only one file, or modules containing a 'main.swift' file. As a consequence, library targets should usually have this argument applied in order to not cause duplicate main symbols when linking the final executable. This is the new default behavior. Sometimes it is necessary to override this behavior, for example in case your Swift executable has a struct annotated with @main, or you have an executable Swift target that gets its main function from elsewhere (for example, linked C code). For this, targets can be passed the 'swift_implicit_main' kwarg, either with 'auto', which specifies Swift's default behavior, or 'never', which passes the '-parse-as-library' flag.
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This is a more customizable approach to #14267. (Closes #14267.)
The '-parse-as-library' compiler option forbids emitting top-level statements as part of an implicitly generated main function. This happens either in Swift modules consisting of only one file, or modules containing a 'main.swift' file. As a consequence, library targets should usually have this argument applied in order to not cause duplicate main symbols when linking the final executable. This is the new default behavior. Sometimes it is necessary to override this behavior, for example in case your Swift program has a struct annotated with @main, or you have an executable Swift target that gets its main function from elsewhere (for example, linked C code). For this, targets can be passed the 'swift_implicit_main' kwarg, either with 'auto', which specifies Swift's default behavior, or 'never', which passes the '-parse-as-library' flag.