This is one of those ideas that I've seen kicking around for at least a decade now, but manifesting it in real code is easier said than done. And that isn't even the real challenge, the real challeng is keeping it working over time.

I've seen some stuff based on treesitter that seems to be prompting a revival of the idea, but it still has fundamental issues, e.g., if I'm embedding in python:

    sql = "SELECT * FROM table "
    if arbitrarilyComplicatedCondition:
        sql += "INNER JOIN a AS joined ON table.thing = a.id "
    else:
        sql += "INNER JOIN b AS joined ON table.thing = b.id "
    sql += "WHERE joined.

and if you imagine trying to write something to autocomplete at the point I leave off, you're fundamentally stuck on not knowing which table to autocomplete with. It doesn't matter what tech you swing at the problem, since trying to analyze "arbitrarilyComplicatedCondition" is basically Turing Complete (which I will prove by vigorous handwave here because turning that into a really solid statement would be much larger than this entire post, but, it can be done). And that's just a simple and quick example, it's not just "autocomplete", it's any analysis you may want to do on the embedded content.

This is just a simple example; they get arbitrarily complicated, quickly. This is one of those things that when you think of the simple case it seems so easy but when you try to bring it into the real world it immediately explodes with all the complexity your mind's eye was ignoring.