fn prec(&mut self, tree: &Spanned<TokenTree>) -> Result<Prec, Syntax> {
        let result = match &tree.item {
            // Prefix
            TokenTree::Label(_)
            | TokenTree::Iden(_)
            | TokenTree::Lit(_)
            | TokenTree::Block(_)
            | TokenTree::List(_)
            | TokenTree::Form(_) => Prec::Call,

            // Infix ops
            TokenTree::Op(name) => Parser::op_prec(Parser::to_op(name, &tree.span)?),
        };

        Ok(result)
    }
Returns the precedence of the current non-sep token
being parsed.
passerine/src/compiler/parse.rs