Introduce manifest editing API for "add target dependency"

As a convenience for when we want to be able to add new target
dependencies, introduce a manifest editing API that adds a new target
dependency to an existing target in the manifest.

Author: DougGregor

Sources/PackageModelSyntax/CMakeLists.txt

@@ -10,6 +10,7 @@ add_library(PackageModelSyntax
   AddPackageDependency.swift
   AddProduct.swift
   AddTarget.swift
+  AddTargetDependency.swift
   ManifestEditError.swift
   ManifestSyntaxRepresentable.swift
   PackageDependency+Syntax.swift

Sources/PackageModelSyntax/ManifestEditError.swift

@@ -18,6 +18,8 @@ import SwiftSyntax
 /// package manifest programattically.
 package enum ManifestEditError: Error {
     case cannotFindPackage
+    case cannotFindTargets
+    case cannotFindTarget(targetName: String)
     case cannotFindArrayLiteralArgument(argumentName: String, node: Syntax)
     case oldManifest(ToolsVersion)
 }
@@ -33,6 +35,10 @@ extension ManifestEditError: CustomStringConvertible {
         switch self {
         case .cannotFindPackage:
             "invalid manifest: unable to find 'Package' declaration"
+        case .cannotFindTargets:
+            "unable to find package targets in manifest"
+        case .cannotFindTarget(targetName: let name):
+            "unable to find target named '\(name)' in package"
         case .cannotFindArrayLiteralArgument(argumentName: let name, node: _):
             "unable to find array literal for '\(name)' argument"
         case .oldManifest(let version):

Tests/PackageModelSyntaxTests/ManifestEditTests.swift

@@ -611,6 +611,46 @@ class ManifestEditTests: XCTestCase {
             )
         }
     }
+
+    func testAddTargetDependency() throws {
+        try assertManifestRefactor("""
+            // swift-tools-version: 5.5
+            let package = Package(
+                name: "packages",
+                dependencies: [
+                    .package(url: "https://github.com/apple/swift-testing.git", from: "0.8.0"),
+                ],
+                targets: [
+                    .testTarget(
+                        name: "MyTest"
+                    ),
+                ]
+            )
+            """,
+            expectedManifest: """
+            // swift-tools-version: 5.5
+            let package = Package(
+                name: "packages",
+                dependencies: [
+                    .package(url: "https://github.com/apple/swift-testing.git", from: "0.8.0"),
+                ],
+                targets: [
+                    .testTarget(
+                        name: "MyTest",
+                        dependencies: [
+                            .product(name: "Testing", package: "swift-testing"),
+                        ]
+                    ),
+                ]
+            )
+            """) { manifest in
+            try AddTargetDependency.addTargetDependency(
+                .product(name: "Testing", package: "swift-testing"),
+                targetName: "MyTest",
+                to: manifest
+            )
+        }
+    }
 }