[Swift Build] Correct -parse-as-library behavior for executable targets

Fixtures/Miscellaneous/ParseAsLibrary/Package.swift

@@ -0,0 +1,15 @@
+// swift-tools-version: 6.0
+
+import PackageDescription
+
+let package = Package(
+    name: "ParseAsLibrary",
+    products: [],
+    targets: [
+        .executableTarget(name: "ExecutableTargetOneFileNamedMainMainAttr"),
+        .executableTarget(name: "ExecutableTargetOneFileNamedMainNoMainAttr"),
+        .executableTarget(name: "ExecutableTargetOneFileNotNamedMainMainAttr"),
+        .executableTarget(name: "ExecutableTargetOneFileNotNamedMainNoMainAttr"),
+        .executableTarget(name: "ExecutableTargetTwoFiles"),
+    ]
+)

Fixtures/Miscellaneous/ParseAsLibrary/Sources/ExecutableTargetOneFileNamedMainMainAttr/main.swift

@@ -0,0 +1,3 @@
+@main struct Entry {
+    static func main() {}
+}

Fixtures/Miscellaneous/ParseAsLibrary/Sources/ExecutableTargetOneFileNamedMainNoMainAttr/main.swift

@@ -0,0 +1 @@
+print(42)

Fixtures/Miscellaneous/ParseAsLibrary/Sources/ExecutableTargetOneFileNotNamedMainMainAttr/othername.swift

@@ -0,0 +1,3 @@
+@main struct Entry {
+    static func main() {}
+}

Fixtures/Miscellaneous/ParseAsLibrary/Sources/ExecutableTargetOneFileNotNamedMainNoMainAttr/othername.swift

@@ -0,0 +1 @@
+print(42)

Fixtures/Miscellaneous/ParseAsLibrary/Sources/ExecutableTargetTwoFiles/one.swift

@@ -0,0 +1,3 @@
+@main struct Entry {
+    static func main() {}
+}

Fixtures/Miscellaneous/ParseAsLibrary/Sources/ExecutableTargetTwoFiles/two.swift

@@ -0,0 +1 @@
+func foo() {}

Package.swift

@@ -534,7 +534,6 @@ let package = Package(
         .target(
             name: "SwiftBuildSupport",
             dependencies: [
-                "Build",
                 "SPMBuildCore",
                 "PackageGraph",
             ],

Sources/Build/BuildDescription/SwiftModuleBuildDescription.swift

@@ -34,34 +34,6 @@ import struct TSCBasic.ByteString
 @available(*, deprecated, renamed: "SwiftModuleBuildDescription")
 public typealias SwiftTargetBuildDescription = SwiftModuleBuildDescription
 
-// looking into the file content to see if it is using the @main annotation
-// this is not bullet-proof since theoretically the file can contain the @main string for other reasons
-// but it is the closest to accurate we can do at this point
-package func containsAtMain(fileSystem: FileSystem, path: AbsolutePath) throws -> Bool {
-    let content: String = try fileSystem.readFileContents(path)
-    let lines = content.split(whereSeparator: { $0.isNewline }).map { $0.trimmingCharacters(in: .whitespaces) }
-
-    var multilineComment = false
-    for line in lines {
-        if line.hasPrefix("//") {
-            continue
-        }
-        if line.hasPrefix("/*") {
-            multilineComment = true
-        }
-        if line.hasSuffix("*/") {
-            multilineComment = false
-        }
-        if multilineComment {
-            continue
-        }
-        if line.hasPrefix("@main") {
-            return true
-        }
-    }
-    return false
-}
-
 /// Build description for a Swift module.
 public final class SwiftModuleBuildDescription {
     /// The package this target belongs to.

Sources/SPMBuildCore/CMakeLists.txt

@@ -26,6 +26,7 @@ add_library(SPMBuildCore
   Plugins/PluginMessages.swift
   Plugins/PluginScriptRunner.swift
   CommandPluginResult.swift
+  MainAttrDetection.swift
   ResolvedPackage+Extensions.swift
   Triple+Extensions.swift
   XCFrameworkMetadata.swift

Sources/SPMBuildCore/MainAttrDetection.swift

@@ -0,0 +1,42 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift open source project
+//
+// Copyright (c) 2014-2022 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See http://swift.org/LICENSE.txt for license information
+// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import Basics
+import Foundation
+
+// looking into the file content to see if it is using the @main annotation
+// this is not bullet-proof since theoretically the file can contain the @main string for other reasons
+// but it is the closest to accurate we can do at this point
+package func containsAtMain(fileSystem: FileSystem, path: AbsolutePath) throws -> Bool {
+    let content: String = try fileSystem.readFileContents(path)
+    let lines = content.split(whereSeparator: { $0.isNewline }).map { $0.trimmingCharacters(in: .whitespaces) }
+
+    var multilineComment = false
+    for line in lines {
+        if line.hasPrefix("//") {
+            continue
+        }
+        if line.hasPrefix("/*") {
+            multilineComment = true
+        }
+        if line.hasSuffix("*/") {
+            multilineComment = false
+        }
+        if multilineComment {
+            continue
+        }
+        if line.hasPrefix("@main") {
+            return true
+        }
+    }
+    return false
+}

Sources/SwiftBuildSupport/PackagePIFBuilder+Helpers.swift

@@ -55,6 +55,8 @@ import struct PackageGraph.ResolvedProduct
 
 import func PackageLoading.pkgConfigArgs
 
+import SPMBuildCore
+
 import enum SwiftBuild.ProjectModel
 
 // MARK: - PIF GUID Helpers
@@ -769,6 +771,27 @@ extension PackageGraph.ResolvedModule {
     func recursivelyTraverseDependencies(with block: (ResolvedModule.Dependency) -> Void) {
         [self].recursivelyTraverseDependencies(with: block)
     }
+
+    func addParseAsLibrarySettings(to settings: inout BuildSettings, toolsVersion: ToolsVersion, fileSystem: FileSystem) {
+        if toolsVersion > .v5_5 && [.executable, .snippet, .macro].contains(self.type) {
+            let usesAtMainAttr = self.sources.paths.contains { sourcePath in
+                (try? containsAtMain(fileSystem: fileSystem, path: sourcePath)) ?? false
+            }
+            if usesAtMainAttr {
+                // Always pass -parse-as-library if @main is used
+                settings[.SWIFT_LIBRARIES_ONLY] = "YES"
+                settings[.SWIFT_DISABLE_PARSE_AS_LIBRARY] = "NO"
+            } else {
+                // Never pass -parse-as-library if @main isn't used, fall back to compiler heuristics
+                settings[.SWIFT_LIBRARIES_ONLY] = "NO"
+                settings[.SWIFT_DISABLE_PARSE_AS_LIBRARY] = "YES"
+            }
+        } else if [.library, .test].contains(self.type) {
+            // Always pass -parse-as-library for libraries and tests
+            settings[.SWIFT_LIBRARIES_ONLY] = "YES"
+            settings[.SWIFT_DISABLE_PARSE_AS_LIBRARY] = "NO"
+        }
+    }
 }
 
 extension Collection<PackageGraph.ResolvedModule> {

Sources/SwiftBuildSupport/PackagePIFProjectBuilder+Modules.swift

@@ -616,6 +616,8 @@ extension PackagePIFProjectBuilder {
             settings[.SKIP_BUILDING_DOCUMENTATION] = "YES"
         }
 
+        sourceModule.addParseAsLibrarySettings(to: &settings, toolsVersion: package.manifest.toolsVersion, fileSystem: pifBuilder.fileSystem)
+
         // Handle the target's dependencies (but only link against them if needed).
         let shouldLinkProduct = (desiredModuleType == .dynamicLibrary) || (desiredModuleType == .macro)
         sourceModule.recursivelyTraverseDependencies { dependency in

Sources/SwiftBuildSupport/PackagePIFProjectBuilder+Products.swift

@@ -17,7 +17,6 @@ import TSCUtility
 import struct Basics.AbsolutePath
 import class Basics.ObservabilitySystem
 import struct Basics.SourceControlURL
-import func Build.containsAtMain
 
 import class PackageModel.BinaryModule
 import class PackageModel.Manifest
@@ -139,19 +138,10 @@ extension PackagePIFProjectBuilder {
                 settings[.INSTALL_PATH] = "/usr/local/bin"
                 settings[.LD_RUNPATH_SEARCH_PATHS] = ["$(inherited)", "@executable_path/../lib"]
             }
-        } else if mainModule.type == .snippet {
-            let hasMainModule: Bool
-            if let mainModule = product.mainModule {
-                // Check if any source file in the main module contains @main
-                hasMainModule = mainModule.sources.paths.contains { (sourcePath: AbsolutePath) in
-                    (try? containsAtMain(fileSystem: pifBuilder.fileSystem, path: sourcePath)) ?? false
-                }
-            } else {
-                hasMainModule = false
-            }
-            settings[.SWIFT_DISABLE_PARSE_AS_LIBRARY] = hasMainModule ? "NO" : "YES"
         }
 
+        mainModule.addParseAsLibrarySettings(to: &settings, toolsVersion: package.manifest.toolsVersion, fileSystem: pifBuilder.fileSystem)
+
         let mainTargetDeploymentTargets = mainModule.deploymentTargets(using: pifBuilder.delegate)
 
         settings[.MACOSX_DEPLOYMENT_TARGET] = mainTargetDeploymentTargets[.macOS] ?? nil

Tests/CommandsTests/BuildCommandTests.swift

@@ -1278,6 +1278,22 @@ struct BuildCommandTestCases {
         }
     }
 
+    @Test(arguments: [BuildSystemProvider.Kind.native, .swiftbuild])
+     func parseAsLibraryCriteria(buildSystem: BuildSystemProvider.Kind) async throws {
+         try await withKnownIssue {
+             try await fixture(name: "Miscellaneous/ParseAsLibrary") { fixturePath in
+                _ =  try await executeSwiftBuild(
+                    fixturePath,
+                    buildSystem: buildSystem,
+                    throwIfCommandFails: true
+                )
+            }
+         } when: {
+             ProcessInfo.hostOperatingSystem == .windows &&
+             buildSystem == .swiftbuild
+         }
+    }
+
     @Test(
         arguments: SupportedBuildSystemOnPlatform,
     )

Tests/BuildTests/BuildDescriptionTests.swift → Tests/SPMBuildCoreTests/MainAttrDetectionTests.swift

@@ -1,5 +1,5 @@
 import Basics
-import func Build.containsAtMain
+import SPMBuildCore
 import Testing
 
 struct ContainsAtMainReturnsExpectedValueTestData: CustomStringConvertible {
@@ -14,7 +14,7 @@ struct ContainsAtMainReturnsExpectedValueTestData: CustomStringConvertible {
 }
 
 @Suite
-struct BuildDescriptionTests {
+struct MainAttrDetectionTests {
     @Test(
         .tags(
             .TestSize.small,