Or you can do your own parsing, meaning that you can handle pretty much any syntax that can be lexed as Rust tokens. Which means that e.g. the C# LINQ syntactic sugar (from ... select ... where etc) can also be implemented as a macro. Or you could handle raw SQL and do typechecking on it.
For procedural macros, what you get is not even the syntax tree but rather the token tree: https://doc.rust-lang.org/proc_macro/struct.TokenStream.html. You can then use stdlib to parse that into a Rust syntax tree (which is roughly the equivalent of Expression): https://docs.rs/syn/latest/syn/enum.Expr.html
Or you can do your own parsing, meaning that you can handle pretty much any syntax that can be lexed as Rust tokens. Which means that e.g. the C# LINQ syntactic sugar (from ... select ... where etc) can also be implemented as a macro. Or you could handle raw SQL and do typechecking on it.