Fix for linking tests with swiftbuild on windows

- need library search path to ExecutableModules for windows

Author: daveinglis

Fixtures/PIFBuilder/BasicExecutable/Package.swift

@@ -0,0 +1,12 @@
+// swift-tools-version: 6.2
+
+import PackageDescription
+
+let package = Package(
+    name: "BasicExecutable",
+    targets: [
+        .executableTarget(
+            name: "Executable",
+        ),
+    ]
+)

Fixtures/PIFBuilder/BasicExecutable/Sources/BasicExecutable/BasicExecutable.swift

@@ -0,0 +1,9 @@
+// The Swift Programming Language
+// https://docs.swift.org/swift-book
+
+@main
+struct BasicExecutable {
+    static func main() {
+        print("Hello, world!")
+    }
+}

Sources/SwiftBuildSupport/PackagePIFProjectBuilder+Modules.swift

@@ -440,6 +440,9 @@ extension PackagePIFProjectBuilder {
                 // Redirect the built executable into a separate directory so it won't conflict with the real one.
                 settings[.TARGET_BUILD_DIR] = "$(TARGET_BUILD_DIR)/ExecutableModules"
 
+                // on windows modules are libraries, so we need to add a search path so the linker finds them
+                impartedSettings[.LIBRARY_SEARCH_PATHS, .windows] = ["$(inherited)", "$(TARGET_BUILD_DIR)/ExecutableModules"]
+
                 // Don't install the Swift module of the testable side-built artifact, lest it conflict with the regular
                 // one.
                 // The modules should have compatible contents in any case — only the entry point function name is

Tests/SwiftBuildSupportTests/PIFBuilderTests.swift

@@ -150,6 +150,26 @@ extension SwiftBuild.ProjectModel.BaseTarget {
 
 @Suite
 struct PIFBuilderTests {
+
+    @Test func platformExecutableModuleLibrarySearchPath() async throws {
+        try await withGeneratedPIF(fromFixture: "PIFBuilder/BasicExecutable") { pif, observabilitySystem in
+            let releaseConfig = try pif.workspace
+                .project(named: "BasicExecutable")
+                .target(named: "Executable")
+                .buildConfig(named: "Release")
+
+            for platform in ProjectModel.BuildSettings.Platform.allCases {
+                let search_paths = releaseConfig.impartedBuildProperties.settings[.LIBRARY_SEARCH_PATHS, platform]
+                switch platform {
+                    case .macOS, .macCatalyst, .iOS, .watchOS, .tvOS, .xrOS, .driverKit, .freebsd, .android, .linux, .wasi, .openbsd, ._iOSDevice:
+                         #expect(search_paths == nil, "for platform \(platform)")
+                    case .windows:
+                        #expect(search_paths == ["$(inherited)", "$(TARGET_BUILD_DIR)/ExecutableModules"], "for platform \(platform)")
+                }
+            }
+        }
+    }
+
     @Test func platformConditionBasics() async throws {
         try await withGeneratedPIF(fromFixture: "PIFBuilder/UnknownPlatforms") { pif, observabilitySystem in
             // We should emit a warning to the PIF log about the unknown platform
@@ -182,7 +202,7 @@ struct PIFBuilderTests {
                     case .macOS, .macCatalyst, .iOS, .watchOS, .tvOS, .xrOS, .driverKit, .freebsd:
                          #expect(ld_flags == ["-lc++", "$(inherited)"], "for platform \(platform)")
                     case .android, .linux, .wasi, .openbsd:
-                        #expect(ld_flags == ["-lstdc++", "$(inherited)"], "for platform \(platform)")                    
+                        #expect(ld_flags == ["-lstdc++", "$(inherited)"], "for platform \(platform)")
                     case .windows, ._iOSDevice:
                         #expect(ld_flags == nil, "for platform \(platform)")
                 }