I think this could be a decent interface with one addition, a way to comment on the completion being suggested. You could ask it for a different completion or to extend the completion, do something different, do a specific thing, whatever. An active way to "explain my intent" with the AI (besides leaving comments hinting at what you want) in addition to the passive completion system.

To be honest, I'm not quite sure what the ideal UX looks like yet. The AI assisted autocomplete is too little, but the idea of saying "Build X for purpose Y" is too high-level. Maintaining Markdown documents that the AI implements, also feels too high-level, but letting the human fully drive the implementation probably again too low-level.

I'm guessing the direction I'd prefer, would be tooling built to accept and be driven by humans, but allowed to be extended/corrected by AI, or something like that, maybe.

Maybe a slight contradiction, and very wish-washy/hand-wavey, but I haven't personally quite figured out what I think would be best yet either, what the right level actually is, so probably the best I could say right now :) Sorry!

Still magical a few years in?

>Imagine if I had written a comment to explain my intent for the function before I started writing the function body.

This in particular is not dissimilar from opening a chat with a model and giving it a prompt as usual but then adding at the end:

Begin your response below:

  { func

Which editor?

> Imagine if I had written a comment to explain my intent for the function before I started writing the function body.

The loon programming language (a Lisp) has "semantic functions", where the body is just the doc comment.