Please add binding.expression back to custom directives

[jcupps]

I am attempting to migrate a Vue 2 app to Vue 3 and am mystified by the decision to remove binding.expression from the custom directive API. I couldn't find any explanation given for what seems like an arbitrary and unnecessary regression.

Without it, it is impossible to create a directive like v-model. To use the example in the docs of a v-focus directive, of course setting the focus from the value of a data property is trivial:

<input v-focus="hasFocus" />
...
data: {
  hasFocus: true
}

However, making this more useful by adding two-way binding that updates the data property on focus and blur events is now impossible. In Vue 2, it could be done using binding.expression as follows:

bind(el, binding, vnode) {
  el.addEventListener('focus', () => vnode.componentInstance[binding.expression] = true);
  el.addEventListener('blur', () => vnode.componentInstance[binding.expression] = false);
}

Thanks.