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: field.rs
    path of file: ./repos/darling/core/src/codegen
    
    the code of the file until where you have to start completion: use std::borrow::Cow;

use proc_macro2::TokenStream;
use quote::{quote, quote_spanned, ToTokens, TokenStreamExt};
use syn::{spanned::Spanned, Ident, Path, Type};

use crate::codegen::{DefaultExpression, PostfixTransform};
use crate::usage::{self, IdentRefSet, IdentSet, UsesTypeParams};

/// Properties need
    
    fn to_tokens(&self, tokens: &mut TokenStream) {
        let field: &Field = self.0;
        let ident = field.ident;
        tokens.append_all(if field.multiple {
            if let Some(ref expr) = field.default_expression {
                quote_spanned!(expr.span()=> #ident: if !#ident.is_empty() {
                    #ident
                } else {
                    #expr
                })
            } else {
                quote!(#ident: #ident)
            }
        } else if let Some(ref expr) = field.default_expression {
            quote_spanned!(expr.span()=> #ident: if let Some(__val) = #ident.1 {
                __val
            } else {
                #expr
            })
        } else {
            quote!(#ident: #ident.1.expect("Uninitialized fields without defaults were already checked"))
        });
    }