[SR-14488] Can't use @main in executable target with single file

[natecook1000]

Previous ID	SR-14488
Radar	rdar://problem/76728168
Original Reporter	@natecook1000
Type	Bug
Status	Closed
Resolution	Duplicate

Attachment: Download

Environment

This is using the swift-DEVELOPMENT-SNAPSHOT-2021-04-10-a toolchain from swift.org:

Apple Swift version 5.5-dev (LLVM bcf6482631963a8, Swift 37aa203b753b23b)
Target: x86_64-apple-macosx11.0

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug
Assignee	None
Priority	Medium

md5: 5673e04166479b5dfbd94eced5e04ace

duplicates:

• SR-12683 @main is no longer usable due to misdetection of top level code

Issue Description:

When trying to build a package that has an executable target with only one file, I get the following error message, even when that file isn't named "main.swift":

.../spm-onefile/Sources/spm-onefile/OneFile.swift:1:1: error: 'main' attribute cannot be used in a module that contains top-level code
@main
^
.../spm-onefile/Sources/spm-onefile/OneFile.swift:1:1: note: top-level code defined in this source file
@main

The source of OneFile.swift:

@main
struct OneFile {
  static func main() {
    print("Hello, world!")
  }
}

[typesanitizer]

@swift-ci create

[abertelrud]

This seems to be compiler behavior, with special-casing for a single source file vs multiple, and also special-casing of the file name "main.swift". There is a `-parse-as-library` flag that SwiftPM can pass but then "main.swift" will stop being recognized as an entry point:

❯ swiftc -parse-as-library -c main.swift

main.swift:1:1: error: expressions are not allowed at the top level

I'm not sure what SwiftPM can do here without some help from the frontend.

[abertelrud]

Perhaps the compiler can have a flag that SwiftPM and other build systems can use to convey the intent that "this is intended to be an executable module, i.e. there should at the end be a single entry point from either a main.swift or a @main, please emit an error if that's not the case".

Perhaps something like `-parse-as-executable`? Leaving off the flag would have today's heuristics, but passing either `-parse-as-library` (with today's semantics) or `-parse-as-executable` (with new semantics roughly like above) would specifically convey intent, allowing the compiler to have better diagnostics than just leaving it to the linker to report the absence of a `_main` symbol?

In the meantime, I think SwiftPM will need to apply some heuristics to counteract the compiler's. We can't just pass `-parse-as-executable`, because then `main.swift` will stop working, but maybe we can look for the last path component of all of the source files, and unless one of them is `main.swift`, we pass `-parse-as-executable`.

[abertelrud]

I put up at PR at swiftlang/swift-package-manager#3410 for SwiftPM to mitigate this by passing `-parse-as-library` when there is just one source file in the module and it isn't named `main.swift`. For better ergonomics, I suggest what I mentioned above, so that SwiftPM and other build systems can convey their intent to the compiler, and it can disable its heuristics and use that intent for diagnostics (e.g. emit an error if there is no entry point at all when a single entry point was expected, etc).

[tbkka]

Merging with SR-12683