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: parse.rs
    path of file: ./repos/phaser/phaser/vendor/regex-syntax/src/ast
    
    the code of the file until where you have to start completion: /*!
This module provides a regular expression parser.
*/

use std::borrow::Borrow;
use std::cell::{Cell, RefCell};
use std::mem;
use std::result;

use crate::ast::{self, Ast, Position, Span};
use crate::either::Either;

use crate::is_meta_character;

type Result<T> = result::Result<T, ast::Error>;

/// A primitive is an expression with no sub-expressions. This includes
/// literals, assertions and non-set character classes. This representation

    
    fn visit_pre(&mut self, ast: &Ast) -> Result<()> {
        let span = match *ast {
            Ast::Empty(_)
            | Ast::Flags(_)
            | Ast::Literal(_)
            | Ast::Dot(_)
            | Ast::Assertion(_)
            | Ast::Class(ast::Class::Unicode(_))
            | Ast::Class(ast::Class::Perl(_)) => {
                // These are all base cases, so we don't increment depth.
                return Ok(());
            }
            Ast::Class(ast::Class::Bracketed(ref x)) => &x.span,
            Ast::Repetition(ref x) => &x.span,
            Ast::Group(ref x) => &x.span,
            Ast::Alternation(ref x) => &x.span,
            Ast::Concat(ref x) => &x.span,
        };
        self.increment_depth(span)
    }