Stream select macro

futures-macro/src/lib.rs

@@ -19,6 +19,7 @@ use proc_macro::TokenStream;
 mod executor;
 mod join;
 mod select;
+mod stream_select;
 
 /// The `join!` macro.
 #[cfg_attr(fn_like_proc_macro, proc_macro)]
@@ -54,3 +55,12 @@ pub fn select_biased_internal(input: TokenStream) -> TokenStream {
 pub fn test_internal(input: TokenStream, item: TokenStream) -> TokenStream {
     crate::executor::test(input, item)
 }
+
+/// The `stream_select!` macro.
+#[cfg_attr(fn_like_proc_macro, proc_macro)]
+#[cfg_attr(not(fn_like_proc_macro), proc_macro_hack::proc_macro_hack)]
+pub fn stream_select_internal(input: TokenStream) -> TokenStream {
+    crate::stream_select::stream_select(input.into())
+        .unwrap_or_else(syn::Error::into_compile_error)
+        .into()
+}

futures-macro/src/stream_select.rs

@@ -0,0 +1,108 @@
+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)?;
+    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)),*)
+
+        }
+    })
+}

futures-util/src/async_await/mod.rs

@@ -30,6 +30,13 @@ mod select_mod;
 #[cfg(feature = "async-await-macro")]
 pub use self::select_mod::*;
 
+// Primary export is a macro
+#[cfg(feature = "async-await-macro")]
+mod stream_select_mod;
+#[allow(unreachable_pub)] // https://github.com/rust-lang/rust/issues/64762
+#[cfg(feature = "async-await-macro")]
+pub use self::stream_select_mod::*;
+
 #[cfg(feature = "std")]
 #[cfg(feature = "async-await-macro")]
 mod random;

futures-util/src/async_await/stream_select_mod.rs

@@ -0,0 +1,46 @@
+//! The `stream_select` macro.
+
+#[cfg(feature = "std")]
+#[allow(unreachable_pub)]
+#[doc(hidden)]
+#[cfg_attr(not(fn_like_proc_macro), proc_macro_hack::proc_macro_hack(support_nested))]
+pub use futures_macro::stream_select_internal;
+
+/// Combines several streams, all producing the same `Item` type, into one stream.
+/// This is similar to `select_all` but does not require the streams to all be the same type.
+/// It also keeps the streams inline, and does not require `Box<dyn Stream>`s to be allocated.
+/// Streams passed to this macro must be `Unpin`.
+///
+/// Fairness for this stream is implemented in terms of the futures `select` macro. If multiple
+/// streams are ready, one will be pseudo randomly selected at runtime.
+///
+/// This macro is gated behind the `async-await` feature of this library, which is activated by default.
+/// Note that `stream_select!` relies on `proc-macro-hack`, and may require to set the compiler's recursion
+/// limit very high, e.g. `#![recursion_limit="1024"]`.
+///
+/// # Examples
+///
+/// ```
+/// # futures::executor::block_on(async {
+/// use futures::{stream, StreamExt, stream_select};
+/// let endless_ints = |i| stream::iter(vec![i].into_iter().cycle()).fuse();
+///
+/// let mut endless_numbers = stream_select!(endless_ints(1i32), endless_ints(2), endless_ints(3));
+/// match endless_numbers.next().await {
+///     Some(1) => println!("Got a 1"),
+///     Some(2) => println!("Got a 2"),
+///     Some(3) => println!("Got a 3"),
+///     _ => unreachable!(),
+/// }
+/// # });
+/// ```
+#[cfg(feature = "std")]
+#[macro_export]
+macro_rules! stream_select {
+    ($($tokens:tt)*) => {{
+        use $crate::__private as __futures_crate;
+        $crate::stream_select_internal! {
+            $( $tokens )*
+        }
+    }}
+}

futures/src/lib.rs

@@ -137,6 +137,11 @@ pub use futures_util::{join, pending, poll, select_biased, try_join}; // Async-a
 #[doc(inline)]
 pub use futures_util::{future, sink, stream, task};
 
+#[cfg(feature = "std")]
+#[cfg(feature = "async-await")]
+pub use futures_util::stream_select;
+
+#[cfg_attr(feature = "cfg-target-has-atomic", cfg(target_has_atomic = "ptr"))]
 #[cfg(feature = "alloc")]
 #[doc(inline)]
 pub use futures_channel as channel;

futures/tests/async_await_macros.rs

@@ -4,7 +4,9 @@ use futures::future::{self, poll_fn, FutureExt};
 use futures::sink::SinkExt;
 use futures::stream::StreamExt;
 use futures::task::{Context, Poll};
-use futures::{join, pending, pin_mut, poll, select, select_biased, try_join};
+use futures::{
+    join, pending, pin_mut, poll, select, select_biased, stream, stream_select, try_join,
+};
 use std::mem;
 
 #[test]
@@ -308,6 +310,42 @@ fn select_on_mutable_borrowing_future_with_same_borrow_in_block_and_default() {
     });
 }
 
+#[test]
+#[allow(unused_assignments)]
+fn stream_select() {
+    // stream_select! macro
+    block_on(async {
+        let endless_ints = |i| stream::iter(vec![i].into_iter().cycle());
+
+        let mut endless_ones = stream_select!(endless_ints(1i32), stream::pending());
+        assert_eq!(endless_ones.next().await, Some(1));
+        assert_eq!(endless_ones.next().await, Some(1));
+
+        let mut finite_list = stream_select!(stream::iter(vec![1, 2, 3].into_iter()));
+        assert_eq!(finite_list.next().await, Some(1));
+        assert_eq!(finite_list.next().await, Some(2));
+        assert_eq!(finite_list.next().await, Some(3));
+        assert_eq!(finite_list.next().await, None);
+
+        let endless_mixed = stream_select!(endless_ints(1i32), endless_ints(2), endless_ints(3));
+        // Take 1000, and assert a somewhat even distribution of values.
+        // The fairness is randomized, but over 1000 samples we should be pretty close to even.
+        // This test may be a bit flaky. Feel free to adjust the margins as you see fit.
+        let mut count = 0;
+        let results = endless_mixed
+            .take_while(move |_| {
+                count += 1;
+                let ret = count < 1000;
+                async move { ret }
+            })
+            .collect::<Vec<_>>()
+            .await;
+        assert!(results.iter().filter(|x| **x == 1).count() >= 299);
+        assert!(results.iter().filter(|x| **x == 2).count() >= 299);
+        assert!(results.iter().filter(|x| **x == 3).count() >= 299);
+    });
+}
+
 #[test]
 fn join_size() {
     let fut = async {