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: as_expression.rs
    path of file: ./repos/diesel/diesel_derives/src
    
    the code of the file until where you have to start completion: use proc_macro2::TokenStream;
use quote::quote;
use syn::parse_quote;
use syn::DeriveInput;
use syn::Result;

use crate::model::Model;
use crate::util::{ty_for_foreign_derive, wrap_in_dummy_mod};

pub fn derive(item: DeriveInput) -> Result<TokenStream> {
    let model = Model::from_item(&item, true, false)?;

    if model.sql_types.is_empty() {
        return Err(syn::Error::new(
            proc_macro2::Span::call_site(),
            "At least one `sql_type` is needed for deriving `AsExpression` on a structure.",
        ));
    }

    let struct_ty = ty_for_foreign_derive(&item, &model)?;

    // type generics are already handled by `ty_for_foreign_derive`
    let (impl_generics_plain, _, where_clause_plain) = item.generics.split_for_impl();

    let mut generics = item.generics.clone();
    generics.params.push(parse_quote!('__expr));

    let (impl_generics, _, where_clause) = generics.split_for_impl();

    let mut generics2 = generics.clone();
    generics2.params.push(parse_quote!('__expr2));
    let (impl_generics2, _, where_clause2) = generics2.split_for_impl();

    let tokens = model.sql_types.iter().map(|sql_type| {

        let mut to_sql_generics = item.generics.clone();
        to_sql_generics.params.push(parse_quote!(__DB));
        to_sql_generics.make_where_clause().predicates.push(parse_quote!(__DB: diesel::backend::Backend));
        to_sql_generics.make_where_clause().predicates.push(parse_quote!(Self: ToSql<#sql_type, __DB>));
        let (to_sql_impl_generics, _, to_sql_where_clause) = to_sql_generics.split_for_impl();

        let tokens = quote!(
            impl #impl_generics AsExpression<#sql_type>
                for &'__expr #struct_ty #where_clause
            {
                type Expression = Bound<#sql_type, Self>;

                fn as_expression(self) -> Self::Expression {
                    Bound::new(self)
                }
            }

            impl #impl_generics AsExpression<Nullable<#sql_type>>
                for &'__expr #struct_ty #where_clause
            {
                type Expression = Bound<Nullable<#sql_type>, Self>;

                fn as_expression(self) -> Self::Expression {
                    Bound::new(self)
                }
            }

            impl #impl_generics2 AsExpression<#sql_type>
                for &'__expr2 &'__expr #struct_ty #where_clause2
            {
                type Expression = Bound<#sql_type, Self>;

                fn as_expression(self) -> Self::Expression {
                    Bound::new(self)
                }
            }

            impl #impl_generics2 AsExpression<Nullable<#sql_type>>
            
    
pub fn derive(item: DeriveInput) -> Result<TokenStream> {
    let model = Model::from_item(&item, true, false)?;

    if model.sql_types.is_empty() {
        return Err(syn::Error::new(
            proc_macro2::Span::call_site(),
            "At least one `sql_type` is needed for deriving `AsExpression` on a structure.",
        ));
    }

    let struct_ty = ty_for_foreign_derive(&item, &model)?;

    // type generics are already handled by `ty_for_foreign_derive`
    let (impl_generics_plain, _, where_clause_plain) = item.generics.split_for_impl();

    let mut generics = item.generics.clone();
    generics.params.push(parse_quote!('__expr));

    let (impl_generics, _, where_clause) = generics.split_for_impl();

    let mut generics2 = generics.clone();
    generics2.params.push(parse_quote!('__expr2));
    let (impl_generics2, _, where_clause2) = generics2.split_for_impl();

    let tokens = model.sql_types.iter().map(|sql_type| {

        let mut to_sql_generics = item.generics.clone();
        to_sql_generics.params.push(parse_quote!(__DB));
        to_sql_generics.make_where_clause().predicates.push(parse_quote!(__DB: diesel::backend::Backend));
        to_sql_generics.make_where_clause().predicates.push(parse_quote!(Self: ToSql<#sql_type, __DB>));
        let (to_sql_impl_generics, _, to_sql_where_clause) = to_sql_generics.split_for_impl();

        let tokens = quote!(
            impl #impl_generics AsExpression<#sql_type>
                for &'__expr #struct_ty #where_clause
            {
                type Expression = Bound<#sql_type, Self>;

                fn as_expression(self) -> Self::Expression {
                    Bound::new(self)
                }
            }

            impl #impl_generics AsExpression<Nullable<#sql_type>>
                for &'__expr #struct_ty #where_clause
            {
                type Expression = Bound<Nullable<#sql_type>, Self>;

                fn as_expression(self) -> Self::Expression {
                    Bound::new(self)
                }
            }

            impl #impl_generics2 AsExpression<#sql_type>
                for &'__expr2 &'__expr #struct_ty #where_clause2
            {
                type Expression = Bound<#sql_type, Self>;

                fn as_expression(self) -> Self::Expression {
                    Bound::new(self)
                }
            }

            impl #impl_generics2 AsExpression<Nullable<#sql_type>>
                for &'__expr2 &'__expr #struct_ty #where_clause2
            {
                type Expression = Bound<Nullable<#sql_type>, Self>;

                fn as_expression(self) -> Self::Expression {
                    Bound::new(self)
                }
            }

            impl #to_sql_impl_generics diesel::serialize::ToSql<Nullable<#sql_type>, __DB>
                for #struct_ty #to_sql_where_clause
            {
                fn to_sql<'__b>(&'__b self, out: &mut Output<'__b, '_, __DB>) -> serialize::Result
                {
                    ToSql::<#sql_type, __DB>::to_sql(self, out)
                }
            }
        );

        if model.not_sized {
            tokens
        } else {
            quote!(
                #tokens

                impl #impl_generics_plain AsExpression<#sql_type> for #struct_ty #where_clause_plain {
                    type Expression = Bound<#sql_type, Self>;

                    fn as_expression(self) -> Self::Expression {
                        Bound::new(self)
                    }
                }

                impl #impl_generics_plain AsExpression<Nullable<#sql_type>> for #struct_ty #where_clause_plain {
                    type Expression = Bound<Nullable<#sql_type>, Self>;

                    fn as_expression(self) -> Self::Expression {
                        Bound::new(self)
                    }
                }
            )
        }
    });

    Ok(wrap_in_dummy_mod(quote! {
        use diesel::expression::AsExpression;
        use diesel::internal::derives::as_expression::Bound;
        use diesel::sql_types::Nullable;
        use diesel::serialize::{self, ToSql, Output};

        #(#tokens)*
    }))
}