Fix PackageToJS plugin wasm-opt fallback when output file exists

When wasm-opt is not installed, DefaultPackagingSystem falls back to copying
the input Wasm file directly to output. However, FileManager.copyItem
fails if the destination file already exists, while wasm-opt overwrites
existing files.

Changes:
- Remove existing output file before copying in wasm-opt fallback
- Add focused tests for DefaultPackagingSystem fallback behavior
- Test both new and existing output file scenarios

Fixes issue where builds fail with 'File exists' error when wasm-opt
is unavailable and output file already exists from previous builds.

Author: kateinoigakukun

Plugins/PackageToJS/Sources/PackageToJS.swift

@@ -309,6 +309,10 @@ final class DefaultPackagingSystem: PackagingSystem {
     func wasmOpt(_ arguments: [String], input: String, output: String) throws {
         guard let wasmOpt = try? which("wasm-opt") else {
             _ = warnMissingWasmOpt
+            // Remove existing output file if it exists (to match wasm-opt behavior)
+            if FileManager.default.fileExists(atPath: output) {
+                try FileManager.default.removeItem(atPath: output)
+            }
             try FileManager.default.copyItem(atPath: input, toPath: output)
             return
         }

Plugins/PackageToJS/Tests/DefaultPackagingSystemTests.swift

@@ -0,0 +1,86 @@
+import Foundation
+import Testing
+
+@testable import PackageToJS
+
+@Suite struct DefaultPackagingSystemTests {
+
+    @Test func wasmOptFallbackHandlesNewOutputFile() throws {
+        try withTemporaryDirectory { tempDir, _ in
+            let inputFile = tempDir.appendingPathComponent("input.wasm")
+            let outputFile = tempDir.appendingPathComponent("output.wasm")
+            let inputContent = Data("input wasm content".utf8)
+            try inputContent.write(to: inputFile)
+
+            var warnings: [String] = []
+            let system = DefaultPackagingSystem { warning in
+                warnings.append(warning)
+            }
+
+            // Temporarily remove wasm-opt to force fallback behavior
+            let wasmOptPath = "/opt/homebrew/bin/wasm-opt"
+            let backupPath = "/tmp/wasm-opt-backup-test"
+            let wasmOptExists = FileManager.default.fileExists(atPath: wasmOptPath)
+
+            if wasmOptExists {
+                try? FileManager.default.moveItem(atPath: wasmOptPath, toPath: backupPath)
+            }
+
+            defer {
+                if wasmOptExists {
+                    try? FileManager.default.moveItem(atPath: backupPath, toPath: wasmOptPath)
+                }
+            }
+
+            // This should work - fallback should copy file
+            try system.wasmOpt(["-Os"], input: inputFile.path, output: outputFile.path)
+
+            // Verify the output file was created with input content
+            let finalContent = try Data(contentsOf: outputFile)
+            #expect(finalContent == inputContent)
+            #expect(warnings.contains { $0.contains("wasm-opt is not installed") })
+        }
+    }
+
+    @Test func wasmOptFallbackHandlesExistingOutputFile() throws {
+        try withTemporaryDirectory { tempDir, _ in
+            let inputFile = tempDir.appendingPathComponent("input.wasm")
+            let outputFile = tempDir.appendingPathComponent("output.wasm")
+            let inputContent = Data("input wasm content".utf8)
+            let existingContent = Data("existing output content".utf8)
+
+            // Create input file and existing output file
+            try inputContent.write(to: inputFile)
+            try existingContent.write(to: outputFile)
+
+            var warnings: [String] = []
+            let system = DefaultPackagingSystem { warning in
+                warnings.append(warning)
+            }
+
+            // Temporarily remove wasm-opt to force fallback behavior
+            let wasmOptPath = "/opt/homebrew/bin/wasm-opt"
+            let backupPath = "/tmp/wasm-opt-backup-test"
+            let wasmOptExists = FileManager.default.fileExists(atPath: wasmOptPath)
+
+            if wasmOptExists {
+                try? FileManager.default.moveItem(atPath: wasmOptPath, toPath: backupPath)
+            }
+
+            defer {
+                if wasmOptExists {
+                    try? FileManager.default.moveItem(atPath: backupPath, toPath: wasmOptPath)
+                }
+            }
+
+            // This should work - fallback should overwrite existing file
+            try system.wasmOpt(["-Os"], input: inputFile.path, output: outputFile.path)
+
+            // Verify the output file was overwritten with input content
+            let finalContent = try Data(contentsOf: outputFile)
+            #expect(finalContent == inputContent)
+            #expect(finalContent != existingContent)
+            #expect(warnings.contains { $0.contains("wasm-opt is not installed") })
+        }
+    }
+}