The exact shape is TBD (I target proc macros support for 0.3.0, whereas 0.2.0 will be about completeness/editors support), but in short -- I'm thinking about "plugin"/DSL architecture where proc macro _effects_ can be described beforehand and applied by the LSP without actual code execution. Executing proc macros is a lot of effort, but LSP mostly cares about the externally observable effects, which is a much smaller subset.