pub const Error = struct {
    tag: Tag,
    is_note: bool = false,
    /// True if `token` points to the token before the token causing an issue.
    token_is_prev: bool = false,
    token: TokenIndex,
    extra: union {
        none: void,
        expected_tag: Token.Tag,
        offset: usize,
    } = .{ .none = {} },

    pub const Tag = enum {
        asterisk_after_ptr_deref,
        chained_comparison_operators,
        decl_between_fields,
        expected_block,
        expected_block_or_assignment,
        expected_block_or_expr,
        expected_block_or_field,
        expected_container_members,
        expected_expr,
        expected_expr_or_assignment,
        expected_expr_or_var_decl,
        expected_fn,
        expected_inlinable,
        expected_labelable,
        expected_param_list,
        expected_prefix_expr,
        expected_primary_type_expr,
        expected_pub_item,
        expected_return_type,
        expected_semi_or_else,
        expected_semi_or_lbrace,
        expected_statement,
        expected_suffix_op,
        expected_type_expr,
        expected_var_decl,
        expected_var_decl_or_fn,
        expected_loop_payload,
        expected_container,
        extern_fn_body,
        extra_addrspace_qualifier,
        extra_align_qualifier,
        extra_allowzero_qualifier,
        extra_const_qualifier,
        extra_volatile_qualifier,
        ptr_mod_on_array_child_type,
        invalid_bit_range,
        same_line_doc_comment,
        unattached_doc_comment,
        test_doc_comment,
        comptime_doc_comment,
        varargs_nonfinal,
        expected_continue_expr,
        expected_semi_after_decl,
        expected_semi_after_stmt,
        expected_comma_after_field,
        expected_comma_after_arg,
        expected_comma_after_param,
        expected_comma_after_initializer,
        expected_comma_after_switch_prong,
        expected_comma_after_for_operand,
        expected_comma_after_capture,
        expected_initializer,
        mismatched_binary_op_whitespace,
        invalid_ampersand_ampersand,
        c_style_container,
        expected_var_const,
        wrong_equal_var_decl,
        var_const_decl,
        extra_for_capture,
        for_input_not_captured,

        zig_style_container,
        previous_field,
        next_field,

        /// `expected_tag` is populated.
        expected_token,

        /// `offset` is populated
        invalid_byte,
    };
}