✨ Marker support for Rust file extension .rs

[mabulgu]

I have a new Operator SDK plugin for Rust*, and I use kubebuilder markers for scaffolding. This PR adds the .rs extension support to the marker.

Here is the plugin repo: https://github.com/SystemCraftsman/rust-operator-plugins

I would like to suggest this to be included in the Operator SDK org later on (gonna do that via the issue that I will open there).

Also I see the message When adding additional file extensions, update also the NewMarkerFor documentation and error, but no idea where I can update these. If anyone can show me the way, I can also update them to include Rust file ext.

(*) I recently presented the plugin in a KCD event: https://speakerdeck.com/mabulgu/building-kubernetes-operators-in-rust-a-new-operator-sdk-plugin

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• ✅login: mabulgu / (bf546ba, 9d553a9, 5a6de81)

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[camilamacedo86]

Hi @mabulgu 👋

It is VERY cool that you are creating an external plugin!!! 🎉
Thank you so much for building in the ecosystem and even presenting it at KCD — that's amazing!

We might even start tracking all external plugins and promoting them somewhere in the Kubebuilder docs. I’ll create a task for that — because your work deserves to be seen! 💡

---

🧭 About the PR to Add .rs Marker Support

However, in this case, Kubebuilder itself does not scaffold or use .rs files directly.

So for now, we won’t add .rs to the core marker support. This helps us keep the core focused only on what it uses internally.

But don’t worry! You can absolutely use Kubebuilder as a library and create your own custom markers to support .rs, or any other extension you want 🚀

---

📚 We should definitely create a doc for this:

“How to create my own markers for my own extensions not supported in Kubebuilder itself”

Here’s an example to help you (and others!) get started right away:

---

✅ Example: External Plugin With Custom .rs Marker Support

Using sigs.k8s.io/kubebuilder/v4/pkg/machinery

// plugin/marker.go
package plugin

import (
	"fmt"
	"path/filepath"
	"strings"

	"sigs.k8s.io/kubebuilder/v4/pkg/machinery"
)

const myPrefix = "+myplugin:scaffold:"

var rustCommentsByExt = map[string]string{
	".rs": "//",
}

type Marker struct {
	machinery.Marker
}

func NewRustMarker(path string, value string) Marker {
	ext := filepath.Ext(path)
	if comment, ok := rustCommentsByExt[ext]; ok {
		return Marker{
			machinery.Marker{
				prefix:  markerPrefix(myPrefix),
				comment: comment,
				value:   value,
			},
		}
	}
	panic(fmt.Errorf("unsupported file extension: %s", ext))
}

func markerPrefix(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()
}

// plugin/scaffold.go
package plugin

import (
	"strings"

	"sigs.k8s.io/kubebuilder/v4/pkg/machinery"
)

type RustMainFile struct {
	machinery.TemplateMixin
}

var _ machinery.Template = &RustMainFile{}

func (f *RustMainFile) SetTemplateDefaults() error {
	if f.Path == "" {
		f.Path = "main.rs"
	}

	f.TemplateBody = `{{ .ScaffoldMarker }}
fn main() {
    println!("Hello, operator!");
}
`

	m := NewRustMarker(f.Path, "mainFunc")
	f.IfExistsAction = machinery.OverwriteFile
	f.TemplateBody = strings.ReplaceAll(f.TemplateBody, "{{ .ScaffoldMarker }}", m.String())

	return nil
}

// main.go
package main

import (
	"sigs.k8s.io/kubebuilder/v4/pkg/plugin"
	"sigs.k8s.io/kubebuilder/v4/pkg/plugin/external"

	"myplugin/plugin"
)

func main() {
	external.Run(&external.Plugin{
		Name:    "my-rust.plugin",
		Version: plugin.Version{Number: 1},
		SupportedProjectVersions: []string{"3"},
		DevPhase: plugin.Alpha,

		GetScaffolder: func(c plugin.Context, args []string) (plugin.Scaffolder, error) {
			return &external.Scaffolder{
				Templates: []machinery.Template{
					&plugin.RustMainFile{},
				},
			}, nil
		},
	})
}

---

Again, HUGE thanks for contributing — we love seeing people build new ideas like this.
Keep going, and let’s grow the ecosystem together 🙌 — and yes, we will absolutely help promote these external plugins! 💖

[mabulgu]

@camilamacedo86 thanks for your quick response, and I appreciate your comments! I was actually using sth similar to what you suggested as a custom marker. Let me reevrt to that one then and see if that aligns with your suggestion above. Then I will close this PR once everything works as expected.

[camilamacedo86]

We cannot add a file that we do not support have officially
Could the extension not be added in your code using the lib?

[mabulgu]

Yeah no problem, I got that:) What I meant: I will try what you suggest above, and actually the way I had implemented it was like what you suggested above. Let me revert to what I had before and compare with your suggestion and see if that works. I dont remember why I avoided that approach, maybe just not to duplicate things (thinking I can have the rs extension definition in kubebuilder)

[mabulgu]

@camilamacedo86 ok I've tried and remembered the reason as I tried to use the same approach. Here it is:

[mabulgu]

So in other words, if we can make the marker struct use public fields instead of private ones, any other plugins can use their own custom markers.

I can create a PR for this if this sounds good to you?

[mabulgu]

Making the fields public worked for me: mabulgu@4a61855

I can create a PR for this (not via the above branch of course, with a new one) because without this change I dont think anyone can use a custom marker.

Here is the implementation on the plugin: https://github.com/SystemCraftsman/rust-operator-plugins/blob/main/pkg/plugins/rust/marker.go

This might help us create the documentation for the custom marker. I can help with that as well.

[camilamacedo86]

Yes, sure. Thank you a lot !!!
Yes, we need to allow people does that and create their own markers for their own extensions.
Please, feel free to raise the PR.