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: visit.rs
    path of file: ./repos/readyset/nom-sql/src/analysis
    
    the code of the file until where you have to start completion: //! AST walker for SQL taking shared references, inspired by [rustc's AST visitor][rustc-ast-visit].
//!
//! [rustc-ast-visit]: https://doc.rust-lang.org/stable/nightly-rustc/rustc_ast/visit/index.html
//!
//! For an equivalent AST walker trait over **mutable references**, see
//! [`nom_sql::analysis::visit_mut`].

// NOTE: remember that this file is effectively duplicated to `visit_mut.rs` - any changes made here
// should be mirrored there!

#![warn(clippy::todo, clippy::unimplemented)]

use crate::create_table_options::CreateTableOption;
use crate::rename::{RenameTableOperation, RenameTableStatement};
use crate::select::LimitClause;

    
/// fn count_placeholders(query: &str) -> usize {
///     #[derive(Default)]
///     struct PlaceholderCounter(usize);
///
///     impl<'ast> Visitor<'ast> for PlaceholderCounter {
///         type Error = !;
///
///         fn visit_literal(&mut self, literal: &'ast Literal) -> Result<(), Self::Error> {
///             if matches!(literal, Literal::Placeholder(_)) {
///                 self.0 += 1;
///             }
///             Ok(())
///         }
///     }
///
///     let mut query = match parse_query(Dialect::MySQL, query).unwrap() {
///         SqlQuery::Select(query) => query,
///         _ => panic!("unexpected query type"),
///     };
///
///     let mut counter = PlaceholderCounter::default();
///     counter.visit_select_statement(&mut query).unwrap();
///     counter.0
/// }