That sounds like an interesting project! I wonder if the soundness is part of it, see nicoburns' sibling comment. But error messages are quite hard. If I recall

That sounds like an interesting project! I wonder if the soundness is part of it, see nicoburns' sibling comment. But error messages are quite hard. If I recall correctly, in Rust's case, error messages got better with NLL, but that may have been because a tremendous amount of work was put into it by people who were concerned that it was going to become harder to understand, rather than some property of NLL itself. It certainly was an active concern at the time, though the exact timelines and steps are a bit fuzzy to me these days.