diesel_derives 2.4.0

You should not use this crate directly, it is internal to Diesel.
Documentation 
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use proc_macro2::TokenStream;
use quote::quote;
use syn::DeriveInput;
use syn::Result;
use syn::parse_quote;

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)?;
    let struct_ty = ty_for_foreign_derive(&item, &model)?;
    Ok(wrap_in_dummy_mod(derive_inner(struct_ty, item.generics)?))
}

pub fn derive_inner(struct_ty: syn::Type, mut generics: syn::Generics) -> Result<TokenStream> {
    {
        generics.params.push(parse_quote!(__DB));
        generics.params.push(parse_quote!(__ST));
        let where_clause = generics.make_where_clause();
        where_clause
            .predicates
            .push(parse_quote!(__DB: diesel::backend::Backend));
        where_clause
            .predicates
            .push(parse_quote!(__ST: diesel::sql_types::SingleValue));
        where_clause
            .predicates
            .push(parse_quote!(Self: diesel::deserialize::FromSql<__ST, __DB>));
    }
    let (impl_generics, _, where_clause) = generics.split_for_impl();
    let implementation = quote! {
        // Need to put __ST and __DB after lifetimes but before const params
        impl #impl_generics diesel::deserialize::Queryable<__ST, __DB> for #struct_ty
        #where_clause
        {
            type Row = Self;

            fn build(row: Self) -> diesel::deserialize::Result<Self> {
                diesel::deserialize::Result::Ok(row)
            }
        }
    };
    Ok(implementation)
}