result.messages.push({ type: 'dependency', file }) is a special API for plugin developers to tell about used files to PostCSS runner. The most popular way to use it is a watch mode.

But seems like we need to improve this message:

1. Just the file path is not enough. For instance, postcss-mixins accept glob pattern.
2. Seems like webpack’s css-loader needs more types: missing, build, context and regular dependencies. I do not like separated types like build-dependency. I think it will be better to use { type: 'dependency', build: true }

Questions:

1. @mischnic @RyanZim Can every runner support glob-pattern for file watch?
2. @alexander-akait can you explain what is build/context/missed dependency? It maybe a good idea to use these messages in other builders too.