It would be super useful if autocomplete searched all open buffers/documents for words, instead of just the one being currently edited. This is basically how Vim's Omnicomplete works (unless you deck it out with crazy options and plugins). It also seems like a much quicker option than the full-blown Intellisense-style proposals for autocomplete, and wouldn't need language-level support.
If there are people who prefer single-buffer completion, maybe it would just be a flag somewhere that could be set to enable multi-buffer autocomplete?
Ordering the suggestions would be a bit more difficult for multi-buffer completion, but I guess you could just throw all of the suggestions from other buffers beneath the distance-sorted suggestions from the current one being edited.
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