Global typechecking or Merlin vs Dune watch mode

[nojb]

Hello,

I have been wondering for some time how difficult it would be to have Merlin perform whole-project typechecking instead of doing it one file at a time, as today. This is in my view the main difference between Dune watch mode and Merlin, but if Merlin could do whole-project analysis, there would be less of a need to use Dune watch mode (which is much, much slower than typechecking via Merlin in large projects). This is the way Rust analyzer https://rust-analyzer.github.io/ works and it is pretty great.

I don't know how much sense it makes in Merlin's present architecture, but, naïvely, I see Merlin maintaining a mapping from .cmi files to signatures. Whenever Merlin typechecks a file it would update the mapping with the new signature. And whenever it needs to load a persistent signature it would use the one present in the mapping, if any, rather than the one on disk.

Anyway, I thought I would bring up the question in case it triggers any interesting discussion. Thank you for your work on Merlin!