Comment by the_benno

They actually dropped support for diffing and incremental parsing a few months back, when they changed up some internal structures to what they call "precise AST datatypes" (which made diffing quite tricky in the type system).

My impression (from the outside looking in) is that there weren't many people making use of those capabilities, so it wasn't a high priority feature. A bit of a shame, though -- it's a really cool tool and I enjoyed building some things on top of it. I switched to a different front-end, though, when I saw that they had removed those features and all mention of incrementality/differencing from the documentations.

I'd be happy to be wrong about this, if anyone has more up-to-date information about semantic!