Tree-sitter is just a parser, so it can’t do any evaulation, but it can generate either an AST or CST for you. I think Toros would be the program that aspires to eventually evaluate Nix code.
Actually, I was thinking it might be useful to use NixEL (the grammar used by Toros) to generate the tree-sitter grammar, so that various tooling (an LSP, editor highlighing, etc) can have a single source of truth for a Nix grammar. Not sure if that’s feasible though. cc @kamadorueda