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: format.rs
    path of file: ./repos/c2rust/c2rust-refactor/src/transform
    
    the code of the file until where you have to start completion: use std::collections::{HashMap, HashSet};
use std::str;
use std::str::FromStr;
use rustc_data_structures::sync::Lrc;
use rustc::hir::def_id::DefId;
use syntax::ast::*;
use syntax::attr;
use syntax::source_map::DUMMY_SP;
use syntax::ptr::P;
use syntax::token::{Token, TokenKind, Nonterminal};
use syntax::tokenstream::TokenTree;
use syntax_pos::{sym, Span};
use smallvec::smallvec;

use c2rust_ast_builder::mk;
use crate::ast_manip::{FlatMapNodes, MutVisitNodes, visit_nodes};
use crate::command::{CommandState, Registry};
use crate::transform::Transform;
use crate::RefactorCtxt;


/// # `convert_format_args` Command
///
/// Usage: `convert_format_args`
///
/// Marks: `target`
///
/// For each function call, if one of its argument expressions is marked `target`,
/// then parse that argument as a `printf` format string, with the subsequent arguments as the
/// format args.  Replace both the format string and the args with an invocation of the Rust
/// `format_args!` macro.
///
/// This transformation applies casts to the remaining arguments to account for differences in
/// argument conversion behavior between C-style and Rust-style string formatting.  However, it
/// does not attempt to convert the `format_args!` output into something compatible with the
/// original C function.  T
    
    fn transform(&self, krate: &mut Crate, st: &CommandState, _cx: &RefactorCtxt) {
        MutVisitNodes::visit(krate, |e: &mut P<Expr>| {
            let fmt_idx = match e.kind {
                ExprKind::Call(_, ref args) =>
                    args.iter().position(|e| st.marked(e.id, "target")),
                _ => None,
            };
            if fmt_idx.is_none() {
                return;
            }
            let fmt_idx = fmt_idx.unwrap();


            let (func, args) = expect!([e.kind] ExprKind::Call(ref f, ref a) => (f, a));

            // Find the expr for the format string.  This may not be exactly args[fmt_idx] - the
            // user can mark the actual string literal in case there are casts/conversions applied.

            let mut old_fmt_str_expr = None;
            visit_nodes(&args[fmt_idx] as &Expr, |e: &Expr| {
                info!("  look at {:?} - marked? {} - {:?}", e.id, st.marked(e.id, "fmt_str"), e);
                if st.marked(e.id, "fmt_str") {
                    if old_fmt_str_expr.is_some() {
                        warn!("multiple fmt_str marks inside argument {:?}", args[fmt_idx]);
                        return;
                    }
                    old_fmt_str_expr = Some(P(e.clone()));
                }
            });
            let mac = build_format_macro("format_args", None, old_fmt_str_expr, &args[fmt_idx..], None);
            let mut new_args = args[..fmt_idx].to_owned();
            new_args.push(mk().mac_expr(mac));

            *e = mk().id(st.transfer_marks(e.id)).call_expr(func, new_args)
        })
    }