futures_macro/
stream_select.rs

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1use proc_macro2::TokenStream;
2use quote::{format_ident, quote, ToTokens};
3use syn::{parse::Parser, punctuated::Punctuated, Expr, Index, Token};
4
5/// The `stream_select!` macro.
6pub(crate) fn stream_select(input: TokenStream) -> Result<TokenStream, syn::Error> {
7    let args = Punctuated::<Expr, Token![,]>::parse_terminated.parse2(input)?;
8    if args.len() < 2 {
9        return Ok(quote! {
10           compile_error!("stream select macro needs at least two arguments.")
11        });
12    }
13    let generic_idents = (0..args.len()).map(|i| format_ident!("_{}", i)).collect::<Vec<_>>();
14    let field_idents = (0..args.len()).map(|i| format_ident!("__{}", i)).collect::<Vec<_>>();
15    let field_idents_2 = (0..args.len()).map(|i| format_ident!("___{}", i)).collect::<Vec<_>>();
16    let field_indices = (0..args.len()).map(Index::from).collect::<Vec<_>>();
17    let args = args.iter().map(|e| e.to_token_stream());
18
19    Ok(quote! {
20        {
21            #[derive(Debug)]
22            struct StreamSelect<#(#generic_idents),*> (#(Option<#generic_idents>),*);
23
24            enum StreamEnum<#(#generic_idents),*> {
25                #(
26                    #generic_idents(#generic_idents)
27                ),*,
28                None,
29            }
30
31            impl<ITEM, #(#generic_idents),*> __futures_crate::stream::Stream for StreamEnum<#(#generic_idents),*>
32            where #(#generic_idents: __futures_crate::stream::Stream<Item=ITEM> + ::std::marker::Unpin,)*
33            {
34                type Item = ITEM;
35
36                fn poll_next(mut self: ::std::pin::Pin<&mut Self>, cx: &mut __futures_crate::task::Context<'_>) -> __futures_crate::task::Poll<Option<Self::Item>> {
37                    match self.get_mut() {
38                        #(
39                            Self::#generic_idents(#generic_idents) => ::std::pin::Pin::new(#generic_idents).poll_next(cx)
40                        ),*,
41                        Self::None => panic!("StreamEnum::None should never be polled!"),
42                    }
43                }
44            }
45
46            impl<ITEM, #(#generic_idents),*> __futures_crate::stream::Stream for StreamSelect<#(#generic_idents),*>
47            where #(#generic_idents: __futures_crate::stream::Stream<Item=ITEM> + ::std::marker::Unpin,)*
48            {
49                type Item = ITEM;
50
51                fn poll_next(mut self: ::std::pin::Pin<&mut Self>, cx: &mut __futures_crate::task::Context<'_>) -> __futures_crate::task::Poll<Option<Self::Item>> {
52                    let Self(#(ref mut #field_idents),*) = self.get_mut();
53                    #(
54                        let mut #field_idents_2 = false;
55                    )*
56                    let mut any_pending = false;
57                    {
58                        let mut stream_array = [#(#field_idents.as_mut().map(|f| StreamEnum::#generic_idents(f)).unwrap_or(StreamEnum::None)),*];
59                        __futures_crate::async_await::shuffle(&mut stream_array);
60
61                        for mut s in stream_array {
62                            if let StreamEnum::None = s {
63                                continue;
64                            } else {
65                                match __futures_crate::stream::Stream::poll_next(::std::pin::Pin::new(&mut s), cx) {
66                                    r @ __futures_crate::task::Poll::Ready(Some(_)) => {
67                                        return r;
68                                    },
69                                    __futures_crate::task::Poll::Pending => {
70                                        any_pending = true;
71                                    },
72                                    __futures_crate::task::Poll::Ready(None) => {
73                                        match s {
74                                            #(
75                                                StreamEnum::#generic_idents(_) => { #field_idents_2 = true; }
76                                            ),*,
77                                            StreamEnum::None => panic!("StreamEnum::None should never be polled!"),
78                                        }
79                                    },
80                                }
81                            }
82                        }
83                    }
84                    #(
85                        if #field_idents_2 {
86                            *#field_idents = None;
87                        }
88                    )*
89                    if any_pending {
90                        __futures_crate::task::Poll::Pending
91                    } else {
92                        __futures_crate::task::Poll::Ready(None)
93                    }
94                }
95
96                fn size_hint(&self) -> (usize, Option<usize>) {
97                    let mut s = (0, Some(0));
98                    #(
99                        if let Some(new_hint) = self.#field_indices.as_ref().map(|s| s.size_hint()) {
100                            s.0 += new_hint.0;
101                            // We can change this out for `.zip` when the MSRV is 1.46.0 or higher.
102                            s.1 = s.1.and_then(|a| new_hint.1.map(|b| a + b));
103                        }
104                    )*
105                    s
106                }
107            }
108
109            StreamSelect(#(Some(#args)),*)
110
111        }
112    })
113}
futures_macro/stream_select.rs

futures_macro/
stream_select.rs
