@pmmeurcatpics @adotinthevoid @imperio rustc_resolve and rustc_expand are very tightly tied to rustc's internal representation. they output HirIds into an arena, "freeze" them, and once those arenas are full you cannot ever modify them again. so for example things like "suggest extracting a function into a separate module and check that it name resolves after" are simply impossible, you have to completely rerun compilation with a new Session rather than calling back into the library.

in general lots of rustc is like this ... it has so much global state.