Pass -parse-as-library when compiling an executable module that has a single source file that isn't named main.swift

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.

This does not allow use of `@main` in source files named `main.swift`, but that will require compiler support to address.

rdar://76746150

Author: abertelrud

Sources/Build/BuildPlan.swift

@@ -564,6 +564,13 @@ public final class SwiftTargetBuildDescription {
 
     /// True if this is the test discovery target.
     public let testDiscoveryTarget: Bool
+    
+    /// True if this module needs to be parsed as a library based on the configuration of the source code
+    /// (for example because it has a single source file whose name isn't "main.swift").
+    var needsToBeParsedAsLibrary: Bool {
+        let sources = self.sources
+        return sources.count == 1 && sources.first?.basename != "main.swift"
+    }
 
     /// The filesystem to operate on.
     let fs: FileSystem
@@ -773,12 +780,8 @@ public final class SwiftTargetBuildDescription {
         // FIXME: Eliminate side effect.
         result.append(try writeOutputFileMap().pathString)
 
-        switch target.type {
-        case .library, .test:
+        if target.type == .library || target.type == .test || (target.type == .executable && self.needsToBeParsedAsLibrary) {
             result.append("-parse-as-library")
-
-        case .executable, .systemModule, .binary, .plugin:
-            do { }
         }
 
         if buildParameters.useWholeModuleOptimization {
@@ -817,7 +820,7 @@ public final class SwiftTargetBuildDescription {
         result.append("-experimental-skip-non-inlinable-function-bodies")
         result.append("-force-single-frontend-invocation")
 
-        if target.type == .library || target.type == .test {
+        if target.type == .library || target.type == .test || (target.type == .executable && self.needsToBeParsedAsLibrary) {
             result.append("-parse-as-library")
         }
 
@@ -864,7 +867,7 @@ public final class SwiftTargetBuildDescription {
         // FIXME: Eliminate side effect.
         result.append(try writeOutputFileMap().pathString)
 
-        if target.type == .library || target.type == .test {
+        if target.type == .library || target.type == .test || (target.type == .executable && self.needsToBeParsedAsLibrary) {
             result.append("-parse-as-library")
         }
         // FIXME: Handle WMO

Tests/BuildTests/BuildPlanTests.swift

@@ -950,6 +950,57 @@ final class BuildPlanTests: XCTestCase {
       #endif
     }
 
+    func testParseAsLibraryFlagForExe() throws {
+        let fs = InMemoryFileSystem(emptyFiles:
+            // First executable has a single source file not named `main.swift`.
+            "/Pkg/Sources/exe1/foo.swift",
+            // Second executable has a single source file named `main.swift`.
+            "/Pkg/Sources/exe2/main.swift",
+            // Third executable has multiple source files.
+            "/Pkg/Sources/exe3/bar.swift",
+            "/Pkg/Sources/exe3/main.swift"
+        )
+
+        let diagnostics = DiagnosticsEngine()
+        let graph = try loadPackageGraph(fs: fs, diagnostics: diagnostics,
+            manifests: [
+                Manifest.createV4Manifest(
+                    name: "Pkg",
+                    path: "/Pkg",
+                    packageKind: .root,
+                    packageLocation: "/Pkg",
+                    targets: [
+                        TargetDescription(name: "exe1", type: .executable),
+                        TargetDescription(name: "exe2", type: .executable),
+                        TargetDescription(name: "exe3", type: .executable),
+                    ]),
+            ]
+        )
+        XCTAssertNoDiagnostics(diagnostics)
+
+        let result = BuildPlanResult(plan: try BuildPlan(
+            buildParameters: mockBuildParameters(shouldLinkStaticSwiftStdlib: true),
+            graph: graph, diagnostics: diagnostics, fileSystem: fs)
+        )
+
+        result.checkProductsCount(3)
+        result.checkTargetsCount(3)
+
+        XCTAssertNoDiagnostics(diagnostics)
+
+        // Check that the first target (single source file not named main) has -parse-as-library.
+        let exe1 = try result.target(for: "exe1").swiftTarget().emitCommandLine()
+        XCTAssertMatch(exe1, ["-parse-as-library", .anySequence])
+
+        // Check that the second target (single source file named main) does not have -parse-as-library.
+        let exe2 = try result.target(for: "exe2").swiftTarget().emitCommandLine()
+        XCTAssertNoMatch(exe2, ["-parse-as-library", .anySequence])
+
+        // Check that the third target (multiple source files) does not have -parse-as-library.
+        let exe3 = try result.target(for: "exe3").swiftTarget().emitCommandLine()
+        XCTAssertNoMatch(exe3, ["-parse-as-library", .anySequence])
+    }
+
     func testCModule() throws {
         let fs = InMemoryFileSystem(emptyFiles:
             "/Pkg/Sources/exe/main.swift",