Implement janky package traits polyfill (to slim custom builder builds)

Package.swift

@@ -1,18 +1,54 @@
 // swift-tools-version:5.7
 
+import Foundation
 import PackageDescription
 
+let environment = ProcessInfo.processInfo.environment
+let slim = environment["SWIFT_BUNDLER_SLIM"] == "1"
+let requireBuilderAPI = environment["SWIFT_BUNDLER_REQUIRE_BUILDER_API"] == "1"
+
 let package = Package(
   name: "swift-bundler",
   platforms: [.macOS(.v10_15), .iOS(.v13), .tvOS(.v13), .macCatalyst(.v13)],
-  products: [
+  products: [],
+  dependencies: [],
+  targets: []
+)
+
+// Builder API
+if !slim || requireBuilderAPI {
+  package.products += [
+    .library(name: "SwiftBundlerBuilders", targets: ["SwiftBundlerBuilders"])
+  ]
+
+  package.dependencies += [
+    .package(url: "https://github.com/gregcotten/AsyncProcess", from: "0.0.5")
+  ]
+
+  package.targets += [
+    .target(
+      name: "SwiftBundlerBuilders",
+      dependencies: [
+        .product(
+          name: "ProcessSpawnSync",
+          package: "AsyncProcess",
+          condition: .when(platforms: [.linux])
+        )
+      ]
+    )
+  ]
+}
+
+// Rest of products/targets/dependencies
+if !slim {
+  package.products += [
     .executable(name: "swift-bundler", targets: ["swift-bundler"]),
     .library(name: "SwiftBundler", targets: ["SwiftBundler"]),
     .library(name: "SwiftBundlerRuntime", targets: ["SwiftBundlerRuntime"]),
-    .library(name: "SwiftBundlerBuilders", targets: ["SwiftBundlerBuilders"]),
     .plugin(name: "SwiftBundlerCommandPlugin", targets: ["SwiftBundlerCommandPlugin"]),
-  ],
-  dependencies: [
+  ]
+
+  package.dependencies += [
     .package(url: "https://github.com/apple/swift-argument-parser.git", from: "1.5.0"),
     .package(url: "https://github.com/apple/swift-log", from: "1.5.4"),
     .package(url: "https://github.com/pointfreeco/swift-parsing", .upToNextMinor(from: "0.13.0")),
@@ -32,14 +68,14 @@ let package = Package(
     .package(url: "https://github.com/apple/swift-crypto", from: "3.10.0"),
     .package(url: "https://github.com/CoreOffice/XMLCoder", from: "0.17.1"),
     .package(url: "https://github.com/adam-fowler/async-collections.git", from: "0.1.0"),
-    .package(url: "https://github.com/gregcotten/AsyncProcess", from: "0.0.5"),
 
     // File watcher dependencies
     .package(url: "https://github.com/sersoft-gmbh/swift-inotify", "0.4.0"..<"0.5.0"),
     .package(url: "https://github.com/apple/swift-system", from: "1.2.0"),
     .package(url: "https://github.com/apple/swift-async-algorithms", from: "1.0.3"),
-  ],
-  targets: [
+  ]
+
+  package.targets += [
     .executableTarget(name: "swift-bundler", dependencies: ["SwiftBundler"]),
     .target(
       name: "SwiftBundler",
@@ -125,17 +161,6 @@ let package = Package(
       ]
     ),
 
-    .target(
-      name: "SwiftBundlerBuilders",
-      dependencies: [
-        .product(
-          name: "ProcessSpawnSync",
-          package: "AsyncProcess",
-          condition: .when(platforms: [.linux])
-        )
-      ]
-    ),
-
     .target(
       name: "HotReloadingProtocol",
       dependencies: [
@@ -186,4 +211,4 @@ let package = Package(
       ]
     ),
   ]
-)
+}

Sources/SwiftBundler/Bundler/ProjectBuilder.swift

@@ -464,9 +464,21 @@ enum ProjectBuilder {
           isGUIExecutable: false
         )
 
+        // Let Swift Bundler know that we only need the builder API. This
+        // greatly reduces the length of the dependency resolution phase for
+        // clean builds (which is the bottleneck for single-file builders),
+        // and greatly improves incremental build performance on Windows where
+        // package trees with lots of files seem to result in pretty terrible
+        // incremental build performance.
+        let environment = [
+          "SWIFT_BUNDLER_SLIM": "1",
+          "SWIFT_BUNDLER_REQUIRE_BUILDER_API": "1",
+        ]
+
         return await SwiftPackageManager.build(
           product: builderProductName,
-          buildContext: buildContext
+          buildContext: buildContext,
+          additionalEnvironmentVariables: environment
         ).andThen { _ in
           await SwiftPackageManager.getProductsDirectory(buildContext)
         }.mapError { error in

Sources/SwiftBundler/Bundler/SwiftPackageManager/SwiftPackageManager.swift

@@ -79,10 +79,12 @@ enum SwiftPackageManager {
   /// - Parameters:
   ///   - product: The product to build.
   ///   - buildContext: The context to build in.
+  ///   - additionalEnvironmentVariables to add to SwiftPM CLI invocations.
   /// - Returns: If an error occurs, returns a failure.
   static func build(
     product: String,
-    buildContext: BuildContext
+    buildContext: BuildContext,
+    additionalEnvironmentVariables: [String: String] = [:]
   ) async -> Result<Void, SwiftPackageManagerError> {
     return await createBuildArguments(
       product: product,
@@ -91,6 +93,7 @@ enum SwiftPackageManager {
       let process = Process.create(
         "swift",
         arguments: arguments,
+        environment: additionalEnvironmentVariables,
         directory: buildContext.genericContext.projectDirectory,
         runSilentlyWhenNotVerbose: false
       )

Sources/SwiftBundler/Commands/BundleCommand.swift

@@ -64,7 +64,7 @@
     self.hotReloadingEnabled = hotReloadingEnabled
   }
 
   static func validateArguments(
     _ arguments: BundleArguments,
     platform: Platform,
     skipBuild: Bool,
@@ -170,7 +170,7 @@
     return true
   }
 
   static func resolveCodesigningContext(
     codesignArgument: Bool?,
     identityArgument: String?,
     provisioningProfile: URL?,
@@ -607,11 +607,13 @@
       }
 
       let dependenciesScratchDirectory = outputDirectory / "projects"
+      var dependencyContext = buildContext.genericContext
+      dependencyContext.scratchDirectory = dependenciesScratchDirectory
 
       let dependencies = try await ProjectBuilder.buildDependencies(
         appConfiguration.dependencies,
         packageConfiguration: configuration,
-        context: buildContext.genericContext,
+        context: dependencyContext,
         appName: appName,
         dryRun: skipBuild
       ).unwrap()