[RFC] Metadata API

[kdy1]

While working on cache-related things, I found that it will be much more easier if we have a way to store some metadata in AST.
But metadata increase the size of the type, meaning that it will be less performant.

So I want to attach a type map (which uses downcasting using Any) to

• Module
• Script
• Function
• BlockStmt

I think it's the sweet spot. I can't explain the exact reason, but it's my guess based on my experience.

If you think another type also should have the metadata field or those should not, please tell me.
Also, I want to hear more about (potential) usecase.

Usecase

Caching

collect_decls was quite costly when I profiled it. But with this typed metadata API, it can cache data for each node instead of recalculating it every time.

Unresolved question

rkyv

I'm not sure how should we pass this, or preserve it while calling plugins.
(Note: We may not have any metadata prior to plugin invocation, and it's true in current model)