📖 Add docs: Creating custom markers for unsupported file extensions

Shows how to use Kubebuilder as a library to create custom marker support
for external plugins with non-Go file extensions like .rs, .java, .py

- Adds comprehensive guide for implementing custom markers
- Provides working code examples for Rust-based external plugins
- Includes integration patterns for external plugin communication
- Addresses all PR review feedback and Copilot suggestions
- Fixes trailing whitespace and code quality issues

Author: nerdeveloper

docs/book/src/SUMMARY.md

@@ -128,6 +128,7 @@
   - [Extending](./plugins/extending.md)
     - [CLI and Plugins](./plugins/extending/extending_cli_features_and_plugins.md)
     - [External Plugins](./plugins/extending/external-plugins.md)
+    - [Custom Markers](./plugins/extending/custom-markers.md)
     - [E2E Tests](./plugins/extending/testing-plugins.md)
   - [Plugins Versioning](./plugins/plugins-versioning.md)
 

docs/book/src/plugins/extending/custom-markers.md

@@ -0,0 +1,238 @@
+# Creating Custom Markers
+
+## Overview
+
+When using Kubebuilder as a library, you may need to scaffold files with extensions that aren't natively supported by Kubebuilder's marker system. This guide shows you how to create custom marker support for any file extension.
+
+## When to Use Custom Markers
+
+Custom markers are useful when:
+
+- You're building an external plugin for languages not natively supported by Kubebuilder
+- You want to scaffold files with custom extensions (`.rs`, `.java`, `.py`, `.tpl`, etc.)
+- You need scaffolding markers in non-Go files for your own use cases
+- Your file extensions aren't (and shouldn't be) part of the core `commentsByExt` map
+
+## Understanding Markers
+
+Markers are special comments used by Kubebuilder for scaffolding purposes. They indicate where code can be inserted or modified. The core Kubebuilder marker system only supports `.go`, `.yaml`, and `.yml` files by default.
+
+Example of a marker in a Go file:
+```go
+// +kubebuilder:scaffold:imports
+```
+
+## Implementation Example
+
+Here's how to implement custom markers for Rust files (`.rs`). This same pattern can be applied to any file extension.
+
+### Define Your Marker Type
+
+```go
+// pkg/markers/rust.go
+package markers
+
+import (
+    "fmt"
+    "path/filepath"
+    "strings"
+)
+
+const RustPluginPrefix = "+rust:scaffold:"
+
+type RustMarker struct {
+    prefix  string
+    comment string
+    value   string
+}
+
+func NewRustMarker(path string, value string) (RustMarker, error) {
+    ext := filepath.Ext(path)
+    if ext != ".rs" {
+        return RustMarker{}, fmt.Errorf("expected .rs file, got %s", ext)
+    }
+
+    return RustMarker{
+        prefix:  formatPrefix(RustPluginPrefix),
+        comment: "//",
+        value:   value,
+    }, nil
+}
+
+func (m RustMarker) String() string {
+    return m.comment + " " + m.prefix + m.value
+}
+
+func formatPrefix(prefix string) string {
+    trimmed := strings.TrimSpace(prefix)
+    var builder strings.Builder
+    if !strings.HasPrefix(trimmed, "+") {
+        builder.WriteString("+")
+    }
+    builder.WriteString(trimmed)
+    if !strings.HasSuffix(trimmed, ":") {
+        builder.WriteString(":")
+    }
+    return builder.String()
+}
+```
+
+<aside class="note">
+<h1>Implementation Reference</h1>
+
+The `formatPrefix` implementation shown above is adapted from Kubebuilder's internal
+[markerPrefix function](https://github.com/kubernetes-sigs/kubebuilder/blob/master/pkg/machinery/marker.go).
+
+</aside>
+
+### Use in Template Generation
+
+```go
+package templates
+
+import (
+    "fmt"
+    "github.com/yourorg/yourplugin/pkg/markers"
+)
+
+func GenerateRustFile(projectName string) (string, error) {
+    marker, err := markers.NewRustMarker("src/main.rs", "imports")
+    if err != nil {
+        return "", err
+    }
+
+    content := fmt.Sprintf(`// Generated by Rust Plugin
+%s
+
+use std::error::Error;
+
+fn main() -> Result<(), Box<dyn Error>> {
+    println!("Hello from %s!");
+    Ok(())
+}
+`, marker.String(), projectName)
+
+    return content, nil
+}
+
+func GenerateCargoToml(projectName string) string {
+    return fmt.Sprintf(`[package]
+name = "%s"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+`, projectName)
+}
+```
+
+### Integrate with External Plugin
+
+```go
+package main
+
+import (
+    "bufio"
+    "encoding/json"
+    "fmt"
+    "io"
+    "os"
+
+    "sigs.k8s.io/kubebuilder/v4/pkg/plugin/external"
+    "github.com/yourorg/yourplugin/pkg/markers"
+)
+
+func main() {
+    // External plugins communicate via JSON over STDIN/STDOUT
+    reader := bufio.NewReader(os.Stdin)
+    input, err := io.ReadAll(reader)
+    if err != nil {
+        returnError(fmt.Errorf("error reading STDIN: %w", err))
+        return
+    }
+
+    pluginRequest := &external.PluginRequest{}
+    err = json.Unmarshal(input, pluginRequest)
+    if err != nil {
+        returnError(fmt.Errorf("error unmarshaling request: %w", err))
+        return
+    }
+
+    var response external.PluginResponse
+
+    switch pluginRequest.Command {
+    case "init":
+        response = handleInit(pluginRequest)
+    default:
+        response = external.PluginResponse{
+            Command: pluginRequest.Command,
+            Error:   true,
+            ErrorMsgs: []string{fmt.Sprintf("unknown command: %s", pluginRequest.Command)},
+        }
+    }
+
+    output, err := json.Marshal(response)
+    if err != nil {
+        fmt.Fprintf(os.Stderr, "failed to marshal response: %v\n", err)
+        os.Exit(1)
+    }
+    fmt.Printf("%s", output)
+}
+
+func handleInit(req *external.PluginRequest) external.PluginResponse {
+    // Create Rust file with custom markers
+    marker, err := markers.NewRustMarker("src/main.rs", "imports")
+    if err != nil {
+        return external.PluginResponse{
+            Command: "init",
+            Error:   true,
+            ErrorMsgs: []string{fmt.Sprintf("failed to create Rust marker: %v", err)},
+        }
+    }
+
+    fileContent := fmt.Sprintf(`// Generated by Rust Plugin
+%s
+
+use std::error::Error;
+
+fn main() -> Result<(), Box<dyn Error>> {
+    println!("Hello from Rust!");
+    Ok(())
+}
+`, marker.String())
+
+    // External plugins use "universe" to represent file changes.
+    // "universe" is a map from file paths to their file contents,
+    // passed through the plugin chain to coordinate file generation.
+    universe := make(map[string]string)
+    universe["src/main.rs"] = fileContent
+
+    return external.PluginResponse{
+        Command:  "init",
+        Universe: universe,
+    }
+}
+
+func returnError(err error) {
+    response := external.PluginResponse{
+        Error:     true,
+        ErrorMsgs: []string{err.Error()},
+    }
+    output, marshalErr := json.Marshal(response)
+    if marshalErr != nil {
+        fmt.Fprintf(os.Stderr, "failed to marshal error response: %v\n", marshalErr)
+        os.Exit(1)
+    }
+    fmt.Printf("%s", output)
+}
+```
+
+## Adapting for Other Languages
+
+To support other file extensions, modify the marker implementation by changing:
+
+- The comment syntax (e.g., `//` for Java, `#` for Python, `{{/* ... */}}` for templates)
+- The file extension check (e.g., `.java`, `.py`, `.tpl`)
+- The marker prefix (e.g., `+java:scaffold:`, `+python:scaffold:`)
+
+For more information on creating external plugins, see [External Plugins](external-plugins.md).