What about if someone wants to use a plugin that doesn't match the auto-load pattern? e.g. prettier --plugin=@company/prettier-plugin-bespoke?

Please note that I'm keeping requirePath: resolve.sync(pluginName, { basedir: resolvedPluginSearchDir }) for plugins found in pluginSearchDirs. That's because there we get a list of node modules there and we do want to resolve them as modules.

@azz good point. I guess we should decide between allowing people to use --plugin=@company/prettier-plugin-bespoke instead of --plugin=node_modules/@company/prettier-plugin-bespoke and saving others from confusion when whey type --plugin=src/index.js instead of --plugin=./src/index.js. I personally struggled in the second scenario, which brought me to proposing this change.

Practically speaking, how likely would people use @company/prettier-plugin-bespoke for Prettier plugin names given that these are not auto-loadable and so more difficult to setup in projects? Use of --plugin option seems to me like a hook to mainly use in development.

Maybe we can just offer a better error message if you forget the ./?

That can be a good option too. So do I just try/catch and then throw a custom error message if the original one starts with Cannot find module? Let's see what others think as well.

We could search locally if the provided string ends in .js, and assume node modules otherwise

We may also want to support the string ending in .:
--plugin=.

What if we do this?

let requirePath;
try {
  // try local files
  requirePath = resolve.sync(path.resolve(process.cwd(), pluginName));
} catch (e) {
  // try node modules
  requirePath = resolve.sync(pluginName, { basedir: process.cwd() });
}

If the second attempt fails, Prettier will crash. This can make things smooth for both types of uses IMO. The reason why we may want resolve.sync(pluginName, { basedir: process.cwd() }) to be second is because we don't want the final error message to be confusing because of the absolute path in it.

As of --plugin=., I guess it's already supported. I can add a test with plugins/automatic/node_modules/prettier-plugin-bar as cwd.

That's a clean solution, I like it 👍

@suchipi that's done now, just waiting for the tests to pass (Travis builds are a bit bumpy today).