You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: lib.rs
    path of file: ./repos/rust-analyzer/crates/mbe/src
    
    the code of the file until where you have to start completion: //! `mbe` (short for Macro By Example) crate contains code for handling
//! `macro_rules` macros. It uses `TokenTree` (from `tt` package) as the
//! interface, although it contains some code to bridge `SyntaxNode`s and
//! `TokenTree`s as well!
//!
//! The tests for this functionality live in another crate:
//! `hir_def::macro_expansion_tests::mbe`.

#![warn(rust_2018_idioms, unused_lifetimes)]

mod expander;
mod parser;
mod syntax_bridge;
mod to_parser_input;
mod tt_iter;

#[cfg(test)]
mod benchmark;

use stdx::impl_from;
use tt::Span;

use std::fmt;

use crate::{
    parser::{MetaTemplate, MetaVarKind, Op},
    tt_iter::TtIter,
};

// FIXME: we probably should re-think  `token_tree_to_syntax_node` interfaces
pub use ::parser::TopEntryPoint;
pub use tt::{Delimiter, DelimiterKind, Punct};

pub use crate::syntax_bridge::{
    parse_exprs_with_sep, parse_to
    
fn validate<S: Span>(pattern: &MetaTemplate<S>) -> Result<(), ParseError> {
    for op in pattern.iter() {
        match op {
            Op::Subtree { tokens, .. } => validate(tokens)?,
            Op::Repeat { tokens: subtree, separator, .. } => {
                // Checks that no repetition which could match an empty token
                // https://github.com/rust-lang/rust/blob/a58b1ed44f5e06976de2bdc4d7dc81c36a96934f/src/librustc_expand/mbe/macro_rules.rs#L558
                let lsh_is_empty_seq = separator.is_none() && subtree.iter().all(|child_op| {
                    match *child_op {
                        // vis is optional
                        Op::Var { kind: Some(kind), .. } => kind == MetaVarKind::Vis,
                        Op::Repeat {
                            kind: parser::RepeatKind::ZeroOrMore | parser::RepeatKind::ZeroOrOne,
                            ..
                        } => true,
                        _ => false,
                    }
                });
                if lsh_is_empty_seq {
                    return Err(ParseError::RepetitionEmptyTokenTree);
                }
                validate(subtree)?
            }
            _ => (),
        }
    }
    Ok(())
}