> Will have to see if Toasty solves it and whether it lets one query for arbitrary structs and add fields to structs from query results (e.g., in the README example for Toasty, explicitly joining on TODOs and mapping them to a simple field on the User objects as opposed to using ORM `has_many`)

I'm not 100% following. Feel free to ping me in discord (https://discord.gg/tokio) or open an issue on Toasty and we can dig into it. Toasty is also pretty new, but maturing fast.

The other points are valid. There are challenges with extracting params and a type system. This is how topcoat does it: https://docs.rs/topcoat/latest/topcoat/router/attr.path_para...