Token list, not token trees. There are official libraries for parsing token stream as rust code but you can parse it as anything (eg json, html) if you want to.
I think you meant TokenStream. They are trees, behind the scenes, because matching delimiters happens early on between lexing and parsing. By the time the rustc_proc_macro::TokenStream is exposed, the rustc_ast::tokenstream::TokenTree is hidden to the proc macro API.
https://doc.rust-lang.org/stable/nightly-rustc/rustc_ast/tok...
https://doc.rust-lang.org/stable/nightly-rustc/rustc_proc_ma...