Allow Starlight plugins to include other plugins

[Fevol]

What version of starlight are you using?

0.25.5

What is your idea?

Expose a addPlugin method within the setup hook for a plugin, which loads this plugin (immediately) after the current plugin.

Why is this feature necessary?

In my specific use case, I am attempting to make a theme which includes a separate component defined as a Starlight plugin. While it would technically be possible to add that plugin's integration with the addIntegration function, the same is not possible for its custom CSS, overrides, etc.

It is possible to bodge this functionality by calling the plugin's setup hook manually, but this brings additional issues with configs only being shallowly merged when both the child and parent plugin call updateConfig.

(Example of the bodge)

export default function plugin(userConfig?: object): StarlightPlugin {
  return {
    name: 'parent',
    hooks: {
      setup: async (args) => {
        const { config, logger, updateConfig } = args;
        if (config.plugins?.some(plugin => plugin.name === 'child')) {
          logger.warn("`child` is already included in the `astro.config.mjs`. Skipping integration.");
        } else {
          await (childPlugin(userConfig).hooks).setup(args);
        }
        
        updateConfig({
          // This will overwrite any component overrides defined by childPlugin
          components: { someComponent: "some-component.astro", ...config.components }
        });
      },
    },
  };
}

Additional links of discussions:

• Previous discussion for adding an addPlugin method
• Discord discussion

Do you have examples of this feature in other projects?

No response

Participation

• I am willing to submit a pull request for this feature.

[delucis]

Thanks for the discussion and helpful links @Fevol!

I think having a dedicated addPlugin() method in the plugin setup hook could make sense for this as discussed in the original implementation PR you linked.