The current setup is brittle and tends to break very easily in unexpected ways (#18886 (comment) being an example). The system is also somewhat difficult to follow (the way we partition the VFS into sourceroots is not easily understandable). We should simplify this and clean it up. One idea we've had is to first and foremost move the VFS state into salsa (once we have migrated over to the new one), as currently we have a weird middle ground between the two (some info is outside salsa, some is inside). The other thing is that we need to likely add support for lazily loading file contents (which likely means incurring i/o within salsa queries).