Command plugin should log output to stderr, not stdout

Author: simonjbeaumont

Plugins/OpenAPIGeneratorCommand/plugin.swift

@@ -65,27 +65,27 @@ extension SwiftOpenAPIGeneratorPlugin: CommandPlugin {
         var hadASuccessfulRun = false
 
         for target in targets {
-            print("Considering target '\(target.name)':")
+            log("Considering target '\(target.name)':")
             guard let swiftTarget = target as? SwiftSourceModuleTarget else {
-                print("- Not a swift source module. Can't generate OpenAPI code.")
+                log("- Not a swift source module. Can't generate OpenAPI code.")
                 continue
             }
             do {
-                print("- Trying OpenAPI code generation.")
+                log("- Trying OpenAPI code generation.")
                 try runCommand(
                     targetWorkingDirectory: target.directory,
                     tool: context.tool,
                     sourceFiles: swiftTarget.sourceFiles,
                     targetName: target.name
                 )
-                print("- ✅ OpenAPI code generation for target '\(target.name)' successfully completed.")
+                log("- ✅ OpenAPI code generation for target '\(target.name)' successfully completed.")
                 hadASuccessfulRun = true
             } catch let error as PluginError {
                 if error.isMisconfigurationError {
-                    print("- Stopping because target isn't configured for OpenAPI code generation.")
+                    log("- Stopping because target is misconfigured for OpenAPI code generation.")
                     throw error
                 } else {
-                    print("- OpenAPI code generation failed with error.")
+                    log("- OpenAPI code generation failed with error.")
                     throw error
                 }
             }
@@ -94,3 +94,7 @@ extension SwiftOpenAPIGeneratorPlugin: CommandPlugin {
         guard hadASuccessfulRun else { throw PluginError.noTargetsWithExpectedFiles(targetNames: targets.map(\.name)) }
     }
 }
+
+private func log(_ message: @autoclosure () -> String) {
+    FileHandle.standardError.write(Data(message().appending("\n").utf8))
+}