WIP: Add linkage of SwiftSyntax, to set the stage for future PRs that need it.

Package.swift

@@ -19,7 +19,7 @@ This allows some clients (such as IDEs) that use SwiftPM's data model but not it
 to not have to depend on SwiftDriver, SwiftLLBuild, etc. We should probably have better names here,
 though that could break some clients.
 */
-let swiftPMDataModelProduct = (
+var swiftPMDataModelProduct = (
     name: "SwiftPMDataModel",
     targets: [
         "SourceControl",
@@ -33,6 +33,10 @@ let swiftPMDataModelProduct = (
     ]
 )
 
+#if compiler(>=5.5)
+swiftPMDataModelProduct.targets.append("PackageSyntax")
+#endif
+
 /** The `libSwiftPM` set of interfaces to programmatically work with Swift
  packages.  `libSwiftPM` includes all of the SwiftPM code except the
  command line tools, while `libSwiftPMDataModel` includes only the data model.
@@ -53,6 +57,14 @@ automatic linking type with `-auto` suffix appended to product's name.
 */
 let autoProducts = [swiftPMProduct, swiftPMDataModelProduct]
 
+var commandsDependencies: [Target.Dependency] = ["SwiftToolsSupport-auto", "Basics", "Build", "PackageGraph", "SourceControl", "Xcodeproj", "Workspace", "XCBuildSupport", "ArgumentParser", "PackageCollections"]
+var commandsSwiftSettings: [SwiftSetting]? = nil
+
+#if compiler(>=5.5)
+commandsDependencies.append("PackageSyntax")
+commandsSwiftSettings = [.define("BUILD_PACKAGE_SYNTAX")]
+#endif
+
 let package = Package(
     name: "SwiftPM",
     platforms: [
@@ -219,7 +231,8 @@ let package = Package(
         .target(
             /** High-level commands */
             name: "Commands",
-            dependencies: ["SwiftToolsSupport-auto", "Basics", "Build", "PackageGraph", "SourceControl", "Xcodeproj", "Workspace", "XCBuildSupport", "ArgumentParser", "PackageCollections"]),
+            dependencies: commandsDependencies,
+            swiftSettings: commandsSwiftSettings),
         .target(
             /** The main executable provided by SwiftPM */
             name: "swift-package",
@@ -269,7 +282,8 @@ let package = Package(
             dependencies: ["Build", "SPMTestSupport"]),
         .testTarget(
             name: "CommandsTests",
-            dependencies: ["swift-build", "swift-package", "swift-test", "swift-run", "Commands", "Workspace", "SPMTestSupport", "Build", "SourceControl"]),
+            dependencies: ["swift-build", "swift-package", "swift-test", "swift-run", "Commands", "Workspace", "SPMTestSupport", "Build", "SourceControl"],
+            swiftSettings: commandsSwiftSettings),
         .testTarget(
             name: "WorkspaceTests",
             dependencies: ["Workspace", "SPMTestSupport"]),
@@ -378,3 +392,24 @@ if ProcessInfo.processInfo.environment["SWIFTCI_USE_LOCAL_DEPS"] == nil {
         .package(path: "../swift-crypto"),
     ]
 }
+
+
+#if compiler(>=5.5)
+// SwiftSyntax depends on lib_InternalSwiftSyntaxParser from the toolchain,
+// which had an ABI break in Swift 5.5. As a result, we shouldn't attempt to
+// compile PackageSyntax with an earlier compiler version. Although PackageSyntax
+// should compile with any 5.5 compiler, it will only be functional when built
+// with a toolchain that has a compatible parser library.
+if ProcessInfo.processInfo.environment["SWIFTCI_USE_LOCAL_DEPS"] == nil {
+    package.dependencies += [.package(url: "https://github.com/apple/swift-syntax.git", .branch(relatedDependenciesBranch))]
+} else {
+    package.dependencies += [.package(path: "../swift-syntax")]
+}
+
+package.targets += [
+    .target(name: "PackageSyntax",
+            dependencies: ["Workspace", "PackageModel", "PackageLoading",
+                           "SourceControl", "SwiftSyntax", "SwiftToolsSupport-auto"]),
+    .testTarget(name: "PackageSyntaxTests", dependencies: ["PackageSyntax", "SPMTestSupport", "SwiftSyntax"]),
+]
+#endif

Sources/PackageSyntax/Lib.swift

@@ -0,0 +1,7 @@
+import SwiftSyntax
+
+public func DoSomethingWithSwiftSyntax() throws -> String {
+    let parsed = try SyntaxParser.parse(source: "let abc = 42")
+    print(parsed)
+    return parsed.description
+}

Tests/PackageSyntaxTests/PackageSyntaxTest.swift

@@ -0,0 +1,22 @@
+/*
+  This source file is part of the Swift.org open source project
+
+  Copyright (c) 2021 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 Swift project authors
+  */
+
+import XCTest
+
+import PackageSyntax
+import SPMTestSupport
+
+final class PackageSyntaxTests: XCTestCase {
+
+    func testDoingSomethingWithSwiftSyntax() throws {
+        XCTAssertEqual(try DoSomethingWithSwiftSyntax(), "description")
+    }
+}
+