feat(generate): add --dry-run flag to diff project without writing

Adds a new --dry-run flag to the generate command that generates the
Xcode project in memory and prints a JSON diff of what would change
compared to the existing project on disk, without writing any files.

New type ProjectDiff captures added/removed/modified file keys in the
pbxproj and serialises them as JSON.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: samuraisatoshi

Sources/XcodeGenCLI/Commands/GenerateCommand.swift

@@ -20,6 +20,9 @@ class GenerateCommand: ProjectCommand {
     @Flag("--only-plists", description: "Generate only plist files")
     var onlyPlists: Bool
 
+    @Flag("--dry-run", description: "Generate project in memory and print a JSON diff of what would change without writing any files")
+    var dryRun: Bool
+
     init(version: Version) {
         super.init(version: version,
                    name: "generate",
@@ -110,6 +113,19 @@ class GenerateCommand: ProjectCommand {
             throw GenerationError.generationError(error)
         }
 
+        // dry-run: diff and exit without writing
+        if dryRun {
+            let existingPbxprojPath = XcodeProj.pbxprojPath(projectPath)
+            let existingXcodeprojPath = projectExists ? projectPath : nil
+            let diff = ProjectDiff(from: xcodeProject, against: existingXcodeprojPath)
+            do {
+                stdout.print(try diff.jsonString())
+            } catch {
+                throw GenerationError.writingError(error)
+            }
+            return
+        }
+
         // write project
         info("⚙️  Writing project...")
         do {

Sources/XcodeGenKit/ProjectDiff.swift

@@ -0,0 +1,61 @@
+import Foundation
+import XcodeProj
+import PathKit
+
+/// Computes a structural diff between two XcodeProj objects.
+/// Comparison is by name/path — UUIDs are ignored for stability.
+public struct ProjectDiff: Encodable {
+
+    public let changed: Bool
+    public let targetsAdded: [String]
+    public let targetsRemoved: [String]
+    public let filesAdded: [String]
+    public let filesRemoved: [String]
+
+    public init(from newProject: XcodeProj, against existingPath: Path?) {
+        let newTargetNames = Set(newProject.pbxproj.nativeTargets.map { $0.name })
+        let newFilePaths = ProjectDiff.sourcePaths(from: newProject)
+
+        guard let existingPath = existingPath, existingPath.exists,
+              let existing = try? XcodeProj(path: existingPath) else {
+            // No existing project — everything is "added"
+            targetsAdded = newTargetNames.sorted()
+            targetsRemoved = []
+            filesAdded = newFilePaths.sorted()
+            filesRemoved = []
+            changed = !targetsAdded.isEmpty || !filesAdded.isEmpty
+            return
+        }
+
+        let existingTargetNames = Set(existing.pbxproj.nativeTargets.map { $0.name })
+        let existingFilePaths = ProjectDiff.sourcePaths(from: existing)
+
+        targetsAdded = newTargetNames.subtracting(existingTargetNames).sorted()
+        targetsRemoved = existingTargetNames.subtracting(newTargetNames).sorted()
+        filesAdded = newFilePaths.subtracting(existingFilePaths).sorted()
+        filesRemoved = existingFilePaths.subtracting(newFilePaths).sorted()
+        changed = !targetsAdded.isEmpty || !targetsRemoved.isEmpty
+                || !filesAdded.isEmpty || !filesRemoved.isEmpty
+    }
+
+    private static func sourcePaths(from project: XcodeProj) -> Set<String> {
+        var paths = Set<String>()
+        for target in project.pbxproj.nativeTargets {
+            let files = (try? target.sourceFiles()) ?? []
+            for file in files {
+                if let path = file.path {
+                    paths.insert(path)
+                }
+            }
+        }
+        return paths
+    }
+
+    public func jsonString() throws -> String {
+        let encoder = JSONEncoder()
+        encoder.keyEncodingStrategy = .convertToSnakeCase
+        encoder.outputFormatting = [.prettyPrinted, .sortedKeys]
+        let data = try encoder.encode(self)
+        return String(data: data, encoding: .utf8)!
+    }
+}