Allow proc macros to expose their grammar to rust analyzer for syntax error understanding

My understanding of rust-analyzer - correct me if I'm wrong - is that it can partially analyze code with syntax errors as long as it doesn't use proc macros, and that this is core functionality as part of providing completions for partially written lines such as let x = . If that is correct, what would it take for proc macros to expose their grammar to get the same level of support as the rest of rust when code is incomplete? I'm most interested in getting tab completion for identifiers in proc macros, because I have several different proc macros I'd like to use very heavily and don't want to lose completion functionality in them.

The issue names no files, tests, or entry points. Begin by determining how rust-analyzer handles incomplete syntax and proc macros, then define how proc macros could expose grammar for identifier completion. Done means proc-macro code retains useful completion and syntax-error understanding while incomplete.