[5.5] Pass compiler flags to alllow use of @main in single-source-file executable modules #3421
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… a single source file that isn't named `main.swift` (swiftlang#3410) The Swift compiler has certain special behaviors regarding main source files: - if a module has just a single source file of any name, it's treated as the main source file - if a module has a source file named `main.swift`, it's treated as the main source file If a source file is considered the main source file, it can have top level code. But a source file that has top level code can't also have `@main`. This means that a single source file executable module can't use `@main`, regardless of the name of that source file. A second empty source file can be added as a workaround, but we can employ some countermeasures in SwiftPM. Specifically, if the executable module consists of a single source file and it is not named `main.swift`, we pass `-parse-as-library` so that a single-source file module will work. This matches what can be seen in the build logs in Xcode, meaning that packages will build the same in SwiftPM and in Xcode. Note that this still does not allow use of `@main` in source files named `main.swift`, but that will require compiler support to address. Since this has the potential to break existing packages that use top-level code in a single source file that isn't named `main.swift`, this behavior is gated by a 5.5 tools version. See https://bugs.swift.org/browse/SR-14488 for discussion about improvements so that SwiftPM can convey the intent to build an executable module to the compiler regardless of the number of files in the module or their names. rdar://76746150 (cherry picked from commit 41b97c8)
abertelrud
requested review from
friedbunny,
neonichu and
tomerd
as code owners
abertelrud
added
5.5
ready
|
@swift-ci please test |
tomerd
approved these changes
Apr 21, 2021
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This is the 5.5 nomination of #3410 which makes it possible to have single-file source modules with a
@mainby taking the same approach as Xcode does when building such modules (also increasing compatibility of packages between SwiftPM and Xcode).The original PR is #3410 and has a lot more details.