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: stream_select.rs
    path of file: ./repos/phaser/phaser/vendor/futures-macro/src
    
    the code of the file until where you have to start completion: use proc_macro2::TokenStream;
use quote::{format_ident, quote, ToTokens};
use syn::{parse::Parser, punctuated::Punctuated, Expr, Index, Token};

/// The `stream_select!` macro.
pub(crate) fn stream_select(input: TokenStream) -> Result<TokenStream, syn::Error> {
    let args = Punctuated::<Expr, Token![,]>::parse_terminated.parse2(input)?;
    if args.len() < 2 {
        return Ok(quote! {
           compile_error!("stream select macro needs at least two arguments.")
        });
    }
    let generic_idents = (0..args.len()).map(|i| format_ident!("_{}", i)).collect::<Vec<_>>();
    let field_idents = (0..args.len()).map(|i| format_ident!("__{}", i)).collect::<Vec<_>>();
    let field_idents_2 = (0..args.len()).map(|i| format_ident!("___{}", i)).collect::<Vec<_>>();
    let field_indices = (0..args.len()).map(Index::from).collect::<Vec<_>>();
    let args = args.iter().map(|e| e.to_token_stream());

    Ok(quote! {
        {
            #[derive(Debug)]
            struct StreamSelect<#(#generic_idents),*> (#(Option<#generic_idents>),*);

            enum StreamEnum<#(#generic_idents),*> {
                #(
                    #generic_idents(#generic_idents)
                ),*,
                None,
            }

            impl<ITEM, #(#generic_idents),*> __futures_crate::stream::Stream for StreamEnum<#(#generic_idents),*>
            where #(#generic_idents: __futures_crate::stream::Stream<Item=ITEM> + ::std::marker::Unpin,)*
            {
                type Item = ITEM;

                fn poll_next(mut self: ::std::pin::Pin<&mut Self>, cx: &mut __futures_crate::task::Context<'_>) -> __futures_crate::task::Poll<Option<Self::Item>> {
                    match self.get_mut() {
                        #(
                            Self::#generic_idents(#generic_idents) => ::std::pin::Pin::new(#generic_idents).poll_next(cx)
                        ),*,
                        Self::None => panic!("StreamEnum::None should never be polled!"),
                    }
                }
            }

            impl<ITEM, #(#generic_idents),*> __futures_crate::stream::Stream for StreamSelect<#(#generic_idents),*>
            where #(#generic_idents: __futures_crate::stream::Stream<Item=ITEM> + ::std::marker::Unpin,)*
            {
                type Item = ITEM;

                fn poll_next(mut self: ::std::pin::Pin<&mut Self>, cx: &mut __futures_crate::task::Context<'_>) -> __futures_crate::task::Poll<Option<Self::Item>> {
                    let Self(#(ref mut #field_idents),*) = self.get_mut();
                    #(
                        let mut #field_idents_2 = false;
                    )*
                    let mut any_pending = false;
                    {
                        let mut stream_array = [#(#field_idents.as_mut().map(|f| StreamEnum::#generic_idents(f)).unwrap_or(StreamEnum::None)),*];
                        __futures_crate::async_await::shuffle(&mut stream_array);

                        for mut s in stream_array {
                            if let StreamEnum::None = s {
                                continue;
                            } else {
                                match __futures_crate::stream::Stream::poll_next(::std::pin::Pin::new(&mut s), cx) {
                                    r @ __futures_crate::task::Poll::Ready(Some(_)) => {
                                        return r;
                                    },
                                    __futures_crate::task::Poll::Pending => {
                                        any_pending = true;
                                    },
                                    __futures_crate::task::Poll::Ready(None) => {
                                        match s {
                                            #(
                                                StreamEnum::#generic_idents(_) => { #field_idents_2 = true; }
                                            ),*,
                                            StreamEnum::None => panic!("StreamEnum::None should never be polled!"),
                                        }
                                    },
                                }
                            }
                        }
                    }
                    #(
                        if #field_idents_2 {
                            *#field_idents = None;
                        }
                    )*
                    if any_pending {
                        __futures_crate::task::Poll::Pending
                    } else {
                        __futures_crate
    
pub(crate) fn stream_select(input: TokenStream) -> Result<TokenStream, syn::Error> {
    let args = Punctuated::<Expr, Token![,]>::parse_terminated.parse2(input)?;
    if args.len() < 2 {
        return Ok(quote! {
           compile_error!("stream select macro needs at least two arguments.")
        });
    }
    let generic_idents = (0..args.len()).map(|i| format_ident!("_{}", i)).collect::<Vec<_>>();
    let field_idents = (0..args.len()).map(|i| format_ident!("__{}", i)).collect::<Vec<_>>();
    let field_idents_2 = (0..args.len()).map(|i| format_ident!("___{}", i)).collect::<Vec<_>>();
    let field_indices = (0..args.len()).map(Index::from).collect::<Vec<_>>();
    let args = args.iter().map(|e| e.to_token_stream());

    Ok(quote! {
        {
            #[derive(Debug)]
            struct StreamSelect<#(#generic_idents),*> (#(Option<#generic_idents>),*);

            enum StreamEnum<#(#generic_idents),*> {
                #(
                    #generic_idents(#generic_idents)
                ),*,
                None,
            }

            impl<ITEM, #(#generic_idents),*> __futures_crate::stream::Stream for StreamEnum<#(#generic_idents),*>
            where #(#generic_idents: __futures_crate::stream::Stream<Item=ITEM> + ::std::marker::Unpin,)*
            {
                type Item = ITEM;

                fn poll_next(mut self: ::std::pin::Pin<&mut Self>, cx: &mut __futures_crate::task::Context<'_>) -> __futures_crate::task::Poll<Option<Self::Item>> {
                    match self.get_mut() {
                        #(
                            Self::#generic_idents(#generic_idents) => ::std::pin::Pin::new(#generic_idents).poll_next(cx)
                        ),*,
                        Self::None => panic!("StreamEnum::None should never be polled!"),
                    }
                }
            }

            impl<ITEM, #(#generic_idents),*> __futures_crate::stream::Stream for StreamSelect<#(#generic_idents),*>
            where #(#generic_idents: __futures_crate::stream::Stream<Item=ITEM> + ::std::marker::Unpin,)*
            {
                type Item = ITEM;

                fn poll_next(mut self: ::std::pin::Pin<&mut Self>, cx: &mut __futures_crate::task::Context<'_>) -> __futures_crate::task::Poll<Option<Self::Item>> {
                    let Self(#(ref mut #field_idents),*) = self.get_mut();
                    #(
                        let mut #field_idents_2 = false;
                    )*
                    let mut any_pending = false;
                    {
                        let mut stream_array = [#(#field_idents.as_mut().map(|f| StreamEnum::#generic_idents(f)).unwrap_or(StreamEnum::None)),*];
                        __futures_crate::async_await::shuffle(&mut stream_array);

                        for mut s in stream_array {
                            if let StreamEnum::None = s {
                                continue;
                            } else {
                                match __futures_crate::stream::Stream::poll_next(::std::pin::Pin::new(&mut s), cx) {
                                    r @ __futures_crate::task::Poll::Ready(Some(_)) => {
                                        return r;
                                    },
                                    __futures_crate::task::Poll::Pending => {
                                        any_pending = true;
                                    },
                                    __futures_crate::task::Poll::Ready(None) => {
                                        match s {
                                            #(
                                                StreamEnum::#generic_idents(_) => { #field_idents_2 = true; }
                                            ),*,
                                            StreamEnum::None => panic!("StreamEnum::None should never be polled!"),
                                        }
                                    },
                                }
                            }
                        }
                    }
                    #(
                        if #field_idents_2 {
                            *#field_idents = None;
                        }
                    )*
                    if any_pending {
                        __futures_crate::task::Poll::Pending
                    } else {
                        __futures_crate::task::Poll::Ready(None)
                    }
                }

                fn size_hint(&self) -> (usize, Option<usize>) {
                    let mut s = (0, Some(0));
                    #(
                        if let Some(new_hint) = self.#field_indices.as_ref().map(|s| s.size_hint()) {
                            s.0 += new_hint.0;
                            // We can change this out for `.zip` when the MSRV is 1.46.0 or higher.
                            s.1 = s.1.and_then(|a| new_hint.1.map(|b| a + b));
                        }
                    )*
                    s
                }
            }

            StreamSelect(#(Some(#args)),*)

        }
    })
}