Yep, familiar with all of this.

That said, I would love to see a solution in my favorite class of solution: where library authors can use and benefit from this, but the average user doesn't have to notice.

I tend to think that the non-existential Scala system was _so close_, and that if you _slightly_ tweaked the scoping rules around it, you could have something great.

For example, if - as a user - I could use `.serialize(...)` from some library and it used _their_ scoped traits by default, but if I _explicitly_ (named) imported some trait(s) on my side, I could substitute my own, that'd work great.

You'd likely want to pair it with some way of e.g. allowing a per-crate prelude of explicit imports that you can ::* import within the crate to override many things at once, but... I think that with the right tweaks, you could say 'this library uses serde by default, but I can provide my own Serializer trait instead... and perhaps, if I turn off the serde Cargo feature, even their default scoped trait disappears'.