Fix crash in PIF generation

Fix a crash when unwrapping an optional 'mainModule' of a module.
In the case where a plugin has a dependency on a 'binaryTarget'
there is no main module.

This only presents itself when you have defined the binaryTarget as
both a product and target.

* Update method that determines plugin dependencies to handle
  binaryTarget types explicitly.
* Add a PIF generation test that captures this behavior.

Author: kcieplak

Fixtures/Miscellaneous/Plugins/BinaryTargetExePlugin/Dependency/MyBinaryTargetExeArtifactBundle.artifactbundle/info.json

@@ -0,0 +1,19 @@
+{
+    "schemaVersion": "1.0",
+    "artifacts": {
+        "mytool": {
+            "type": "executable",
+            "version": "1.2.3",
+            "variants": [
+                {
+                    "path": "mytool-macos/mytool",
+                    "supportedTriples": ["x86_64-apple-macosx", "arm64-apple-macosx"]
+                },
+                {
+                    "path": "mytool-linux/mytool",
+                    "supportedTriples": ["x86_64-unknown-linux-gnu"]
+                }
+            ]
+        }
+    }
+}

Fixtures/Miscellaneous/Plugins/BinaryTargetExePlugin/Dependency/MyBinaryTargetExeArtifactBundle.artifactbundle/mytool-linux/mytool

@@ -0,0 +1,29 @@
+#!/bin/bash
+
+print_usage() {
+    echo "usage: ${0##*/} [--verbose] <in> <out>"
+}
+
+# Parse arguments until we find '--' or an argument that isn't an option.
+until [ $# -eq 0 ]
+do
+    case "$1" in
+        --verbose) verbose=1; shift;;
+        --) shift; break;;
+        -*) echo "unknown option: ${1}"; print_usage; exit 1; shift;;
+        *) break;;
+    esac
+done
+
+# Print usage and leave if we don't have exactly two arguments.
+if [ $# -ne 2 ]; then
+    print_usage
+    exit 1
+fi
+
+# For our sample tool we just copy from one to the other.
+if [ $verbose != 0 ]; then
+    echo "[${0##*/}-linux] '$1' '$2'"
+fi
+
+cp "$1" "$2"

Fixtures/Miscellaneous/Plugins/BinaryTargetExePlugin/Dependency/MyBinaryTargetExeArtifactBundle.artifactbundle/mytool-macos/mytool

@@ -0,0 +1,29 @@
+#!/bin/bash
+
+print_usage() {
+    echo "usage: ${0##*/} [--verbose] <in> <out>"
+}
+
+# Parse arguments until we find '--' or an argument that isn't an option.
+until [ $# -eq 0 ]
+do
+    case "$1" in
+        --verbose) verbose=1; shift;;
+        --) shift; break;;
+        -*) echo "unknown option: ${1}"; print_usage; exit 1; shift;;
+        *) break;;
+    esac
+done
+
+# Print usage and leave if we don't have exactly two arguments.
+if [ $# -ne 2 ]; then
+    print_usage
+    exit 1
+fi
+
+# For our sample tool we just copy from one to the other.
+if [ $verbose != 0 ]; then
+    echo "[${0##*/}-macosx] '$1' '$2'"
+fi
+
+cp "$1" "$2"

Fixtures/Miscellaneous/Plugins/BinaryTargetExePlugin/Package.swift

@@ -0,0 +1,38 @@
+// swift-tools-version: 5.6
+import PackageDescription
+
+let package = Package(
+    name: "MyBinaryTargetExePlugin",
+
+    products: [
+        .executable(
+            name: "MyPluginExe",
+            targets: ["MyPluginExe"]
+        ),
+        .plugin(
+            name: "MyPlugin",
+            targets: ["MyPlugin"]
+        ),
+        .executable(
+            name: "MyBinaryTargetExe",
+            targets: ["MyBinaryTargetExeArtifactBundle"]
+        ),
+    ],
+    targets: [
+        .executableTarget(
+            name: "MyPluginExe",
+            dependencies: [],
+            exclude: [],
+        ),
+
+        .plugin(
+            name: "MyPlugin",
+            capability: .buildTool(),
+            dependencies: ["MyPluginExe", "MyBinaryTargetExeArtifactBundle"]
+        ),
+        .binaryTarget(
+            name: "MyBinaryTargetExeArtifactBundle",
+            path: "Dependency/MyBinaryTargetExeArtifactBundle.artifactbundle"
+        ),
+    ]
+)

Fixtures/Miscellaneous/Plugins/BinaryTargetExePlugin/Plugins/MyPlugin/plugin.swift

@@ -0,0 +1,9 @@
+import PackagePlugin
+
+@main
+struct MyPlugin: BuildToolPlugin {
+    
+    func createBuildCommands(context: PluginContext, target: Target) throws -> [Command] {
+        print("Hello from MyPlugin!")
+    }
+}

Fixtures/Miscellaneous/Plugins/BinaryTargetExePlugin/Sources/MyPluginExe/main.swift

@@ -0,0 +1 @@
+print("It's Me MyPluginExe\n")

Sources/SwiftBuildSupport/PackagePIFBuilder+Helpers.swift

@@ -513,6 +513,11 @@ extension PackageGraph.ResolvedModule {
 
     func productRepresentingDependencyOfBuildPlugin(in mainModuleProducts: [ResolvedProduct]) -> ResolvedProduct? {
         mainModuleProducts.only { (mainModuleProduct: ResolvedProduct) -> Bool in
+            // Handle binary-only executable products that don't have a main module, i.e. binaryTarget
+            guard let mainModule = mainModuleProduct.mainModule else {
+                return mainModuleProduct.type == .executable &&
+                    mainModuleProduct.modules.only?.type == .binary
+            }
             // NOTE: We can't use the 'id' here as we need to explicitly ignore the build triple because our build
             // triple will be '.tools' while the target we want to depend on will have a build triple of '.destination'.
             // See for more details:

Tests/SwiftBuildSupportTests/PIFBuilderTests.swift

@@ -135,4 +135,32 @@ struct PIFBuilderTests {
             #expect(releaseConfig.settings.platformSpecificSettings[.windows]?[.SWIFT_ACTIVE_COMPILATION_CONDITIONS] == ["$(inherited)"])
         }
     }
+
+    @Test func pluginWithBinaryTargetDependency() async throws {
+        try await withGeneratedPIF(fromFixture: "Miscellaneous/Plugins/BinaryTargetExePlugin") { pif, observabilitySystem in
+            // Verify that PIF generation succeeds for a package with a plugin that depends on a binary target
+            #expect(pif.workspace.projects.count > 0)
+
+            let project = try pif.workspace.project(named: "MyBinaryTargetExePlugin")
+
+            // Verify the plugin target exists
+            let pluginTarget = try project.target(named: "MyPlugin")
+            #expect(pluginTarget.common.name == "MyPlugin")
+
+            // Verify the executable target that uses the plugin exists
+            let executableTarget = try project.target(named: "MyPluginExe")
+            #expect(executableTarget.common.name == "MyPluginExe")
+
+            // Verify no errors were emitted during PIF generation
+            let errors = observabilitySystem.diagnostics.filter { $0.severity == .error }
+            #expect(errors.isEmpty, "Expected no errors during PIF generation, but got: \(errors)")
+
+            // Verify that the plugin target has a dependency (binary targets are handled differently in PIF)
+            // The key test is that PIF generation succeeds without errors when a plugin depends on a binary target
+            let binaryArtifactMessages = observabilitySystem.diagnostics.filter {
+                $0.message.contains("found binary artifact")
+            }
+            #expect(binaryArtifactMessages.count > 0, "Expected to find binary artifact processing messages")
+        }
+    }
 }