Writing a Plugin in Rust

This is a short reference on how to write a plugin for Pruner in Rust. For this we will reimplement the trim-newlines plugin which makes for a perfect, simple example. Head over to that repository to see the end result of this document.

If you haven't yet, read Writing Plugins for a quick overview of the plugin API and how plugins are loaded.

Create a WASM component crate

First we need to setup the project and for that we need to create a Cargo.toml which can be compiled as a WASM component. Really this just means it needs to be defined as a cdylib crate.

toml

# Cargo.toml
[package]
name = "trim-newlines"
version = "0.0.1"
edition = "2024"
resolver = "2"

[dependencies]
pruner-plugin-api = "1"

[lib]
crate-type = ["cdylib"]

Add initial plugin stub

Next we should create a lib.rs file which defines and exports a Component struct that implements the PluginApi trait from pruner-plugin-api.

rust

// lib.rs
use pruner_plugin_api::{FormatError, FormatOpts, PluginApi};

struct Component;

impl PluginApi for Component {
    fn format(source: Vec<u8>, _opts: FormatOpts) -> Result<Vec<u8>, FormatError> {
        Ok(source)
    }
}

pruner_plugin_api::bindings::export!(Component);

And that's it for a functioning plugin!

Compile

You can now compile this directly with cargo, making sure to target wasm32-wasip2:

bash

cargo build --release --target wasm32-wasip2

Which will output a compiled plugin to target/wasm32-wasip2/release/trim_newlines.wasm that is ready to be loaded up and called by Pruner. Let's add it into our Pruner config:

Configure Pruner to use it

Add the plugin to pruner.toml, then reference it from a language pipeline:

toml

# pruner.toml
[plugins]
trim_newlines = "file:///path/to/target/wasm32-wasip2/release/trim_newlines.wasm"

[languages]
markdown = ["trim_newlines"]

Running Pruner with --log-level=debug should show your plugin being loaded and instantiated:

bash

cat README.md | pruner format --lang markdown --log-level debug

It's not doing very much right now, so let's add some actual behaviour to this plugin:

Added plugin behaviour

Update format to strip leading/trailing newlines and add a single trailing newline:

rust

// lib.rs
use pruner_plugin_api::{FormatError, FormatOpts, PluginApi};

struct Component;

impl PluginApi for Component {
    fn format(source: Vec<u8>, _opts: FormatOpts) -> Result<Vec<u8>, FormatError> {
        let mut start = 0;
        let mut end = source.len();

        while start < end && (source[start] == b'\n' || source[start] == b'\r') {
            start += 1;
        }

        while end > start && (source[end - 1] == b'\n' || source[end - 1] == b'\r') {
            end -= 1;
        }

        let mut result = source[start..end].to_vec();
        result.push(b'\n');
        Ok(result)
    }
}

pruner_plugin_api::bindings::export!(Component);

Recompile with cargo build --release --target wasm32-wasip2 and we are done. Newlines will now be trimmed from our Markdown files!

Writing a Plugin in Rust ​

Create a WASM component crate ​

Add initial plugin stub ​

Compile ​