What problem does this feature solve?

We want to use inject in app install for recursive dependence.

What does the proposed API look like?

export default {
  install (app: App, ...options: any[]) {
    app.provide('some_token', SomeService(app.inject('another_token')));
  }
}

You can inject via install options.

How? Would you please show me the code?

app.use(Plugin, Injectables) where Injectables is an object containing all your dependencies.