Support swift package migrate with --build-system swiftbuild (#8888)

Similar to the API digester integration, pass along a build system
delegate to capture serialized diagnostic paths from the build and pass
them to the fix-it application infra.

Builds on the changes in
#8857, only the
second commit is new

Author: owenv

Fixtures/SwiftMigrate/ExistentialAnyWithCommonPluginDependencyMigration/Package.swift

@@ -0,0 +1,13 @@
+// swift-tools-version:5.8
+
+import PackageDescription
+
+let package = Package(
+    name: "ExistentialAnyMigration",
+    targets: [
+        .target(name: "Library", dependencies: ["CommonLibrary"], plugins: [.plugin(name: "Plugin")]),
+        .plugin(name: "Plugin", capability: .buildTool, dependencies: ["Tool"]),
+        .executableTarget(name: "Tool", dependencies: ["CommonLibrary"]),
+        .target(name: "CommonLibrary"),
+    ]
+)

Fixtures/SwiftMigrate/ExistentialAnyWithCommonPluginDependencyMigration/Plugins/Plugin/Plugin.swift

@@ -0,0 +1,17 @@
+import PackagePlugin
+import Foundation
+
+@main struct Plugin: BuildToolPlugin {
+    func createBuildCommands(context: PluginContext, target: Target) throws -> [Command] {
+        let tool = try context.tool(named: "Tool")
+        let output = context.pluginWorkDirectory.appending(["generated.swift"])
+        return [
+            .buildCommand(
+                displayName: "Plugin",
+                executable: tool.path,
+                arguments: [output],
+                inputFiles: [],
+                outputFiles: [output])
+        ]
+    }
+}

Fixtures/SwiftMigrate/ExistentialAnyWithCommonPluginDependencyMigration/Sources/CommonLibrary/Common.swift

@@ -0,0 +1,6 @@
+public func common() {}
+
+
+protocol P {}
+
+func needsMigration(_ p: P) {}

Fixtures/SwiftMigrate/ExistentialAnyWithCommonPluginDependencyMigration/Sources/Library/Test.swift

@@ -0,0 +1,6 @@
+import CommonLibrary
+
+func bar() {
+    generatedFunction()
+    common()
+}

Fixtures/SwiftMigrate/ExistentialAnyWithCommonPluginDependencyMigration/Sources/Tool/tool.swift

@@ -0,0 +1,13 @@
+import Foundation
+import CommonLibrary
+
+@main struct Entry {
+    public static func main() async throws {
+        common()
+        let outputPath = CommandLine.arguments[1]
+        let contents = """
+        func generatedFunction() {}
+        """
+        FileManager.default.createFile(atPath: outputPath, contents: contents.data(using: .utf8))
+    }
+}

Fixtures/SwiftMigrate/ExistentialAnyWithPluginMigration/Package.swift

@@ -0,0 +1,12 @@
+// swift-tools-version:5.8
+
+import PackageDescription
+
+let package = Package(
+    name: "ExistentialAnyMigration",
+    targets: [
+        .target(name: "Library", plugins: [.plugin(name: "Plugin")]),
+        .plugin(name: "Plugin", capability: .buildTool, dependencies: ["Tool"]),
+        .executableTarget(name: "Tool"),
+    ]
+)

Fixtures/SwiftMigrate/ExistentialAnyWithPluginMigration/Plugins/Plugin/Plugin.swift

@@ -0,0 +1,17 @@
+import PackagePlugin
+import Foundation
+
+@main struct Plugin: BuildToolPlugin {
+    func createBuildCommands(context: PluginContext, target: Target) throws -> [Command] {
+        let tool = try context.tool(named: "Tool")
+        let output = context.pluginWorkDirectory.appending(["generated.swift"])
+        return [
+            .buildCommand(
+                displayName: "Plugin",
+                executable: tool.path,
+                arguments: [output],
+                inputFiles: [],
+                outputFiles: [output])
+        ]
+    }
+}

Fixtures/SwiftMigrate/ExistentialAnyWithPluginMigration/Sources/Library/Test.swift

@@ -0,0 +1,20 @@
+protocol P {
+}
+
+func test1(_: P) {
+}
+
+func test2(_: P.Protocol) {
+}
+
+func test3() {
+    let _: [P?] = []
+}
+
+func test4() {
+    var x = 42
+}
+
+func bar() {
+    generatedFunction()
+}

Fixtures/SwiftMigrate/ExistentialAnyWithPluginMigration/Sources/Tool/tool.swift

@@ -0,0 +1,12 @@
+import Foundation
+
+@main struct Entry {
+    public static func main() async throws {
+        let outputPath = CommandLine.arguments[1]
+        let contents = """
+        func generatedFunction() {}
+        func dontmodifyme(_: P) {}
+        """
+        FileManager.default.createFile(atPath: outputPath, contents: contents.data(using: .utf8))
+    }
+}

Sources/Build/BuildOperation.swift

@@ -396,9 +396,9 @@ public final class BuildOperation: PackageStructureDelegate, SPMBuildCore.BuildS
     }
 
     /// Perform a build using the given build description and subset.
-    public func build(subset: BuildSubset) async throws {
+    public func build(subset: BuildSubset) async throws -> BuildResult {
         guard !self.config.shouldSkipBuilding(for: .target) else {
-            return
+            return BuildResult(serializedDiagnosticPathsByTargetName: .failure(StringError("Building was skipped")))
         }
 
         let buildStartTime = DispatchTime.now()
@@ -422,7 +422,7 @@ public final class BuildOperation: PackageStructureDelegate, SPMBuildCore.BuildS
         // any errors up-front. Returns true if we should proceed with the build
         // or false if not. It will already have thrown any appropriate error.
         guard try await self.compilePlugins(in: subset) else {
-            return
+            return BuildResult(serializedDiagnosticPathsByTargetName: .failure(StringError("Plugin compilation failed")))
         }
 
         let configuration = self.config.configuration(for: .target)
@@ -452,6 +452,17 @@ public final class BuildOperation: PackageStructureDelegate, SPMBuildCore.BuildS
         )
         guard success else { throw Diagnostics.fatalError }
 
+        let serializedDiagnosticResult: Result<[String: [AbsolutePath]], Error>
+        var serializedDiagnosticPaths: [String: [AbsolutePath]] = [:]
+        do {
+            for module in try buildPlan.buildModules {
+                serializedDiagnosticPaths[module.module.name, default: []].append(contentsOf: module.diagnosticFiles)
+            }
+            serializedDiagnosticResult = .success(serializedDiagnosticPaths)
+        } catch {
+            serializedDiagnosticResult = .failure(error)
+        }
+
         // Create backwards-compatibility symlink to old build path.
         let oldBuildPath = self.config.dataPath(for: .target).parentDirectory.appending(
             component: configuration.dirname
@@ -463,7 +474,7 @@ public final class BuildOperation: PackageStructureDelegate, SPMBuildCore.BuildS
                     warning: "unable to delete \(oldBuildPath), skip creating symbolic link",
                     underlyingError: error
                 )
-                return
+                return BuildResult(serializedDiagnosticPathsByTargetName: serializedDiagnosticResult)
             }
         }
 
@@ -479,6 +490,8 @@ public final class BuildOperation: PackageStructureDelegate, SPMBuildCore.BuildS
                 underlyingError: error
             )
         }
+
+        return BuildResult(serializedDiagnosticPathsByTargetName: serializedDiagnosticResult)
     }
 
     /// Compiles any plugins specified or implied by the build subset, returning