Use types to represent a select strategy instead of a boolean

futures-util/src/future/mod.rs

@@ -82,6 +82,9 @@ mod join_all;
 #[cfg(feature = "alloc")]
 pub use self::join_all::{join_all, JoinAll};
 
+mod select_strategy;
+pub use select_strategy::{Biased, Fair, IsBiased};
+
 mod select;
 pub use self::select::{select, select_biased, Select};
 

futures-util/src/future/select.rs

@@ -1,18 +1,19 @@
-use super::assert_future;
+use super::{assert_future, Biased, Fair, IsBiased};
 use crate::future::{Either, FutureExt};
+use core::marker::PhantomData;
 use core::pin::Pin;
 use futures_core::future::{FusedFuture, Future};
 use futures_core::task::{Context, Poll};
 
 /// Future for the [`select()`] function.
 #[must_use = "futures do nothing unless you `.await` or poll them"]
 #[derive(Debug)]
-pub struct Select<A, B> {
+pub struct Select<A, B, BIASED = Fair> {
     inner: Option<(A, B)>,
-    _biased: bool,
+    _phantom: PhantomData<BIASED>,
 }
 
-impl<A: Unpin, B: Unpin> Unpin for Select<A, B> {}
+impl<A: Unpin, B: Unpin, BIASED> Unpin for Select<A, B, BIASED> {}
 
 /// Waits for either one of two differently-typed futures to complete.
 ///
@@ -86,14 +87,14 @@ impl<A: Unpin, B: Unpin> Unpin for Select<A, B> {}
 ///     })
 /// }
 /// ```
-pub fn select<A, B>(future1: A, future2: B) -> Select<A, B>
+pub fn select<A, B>(future1: A, future2: B) -> Select<A, B, Fair>
 where
     A: Future + Unpin,
     B: Future + Unpin,
 {
     assert_future::<Either<(A::Output, B), (B::Output, A)>, _>(Select {
         inner: Some((future1, future2)),
-        _biased: false,
+        _phantom: PhantomData,
     })
 }
 
@@ -167,21 +168,22 @@ where
 ///     })
 /// }
 /// ```
-pub fn select_biased<A, B>(future1: A, future2: B) -> Select<A, B>
+pub fn select_biased<A, B>(future1: A, future2: B) -> Select<A, B, Biased>
 where
     A: Future + Unpin,
     B: Future + Unpin,
 {
     assert_future::<Either<(A::Output, B), (B::Output, A)>, _>(Select {
         inner: Some((future1, future2)),
-        _biased: true,
+        _phantom: PhantomData,
     })
 }
 
-impl<A, B> Future for Select<A, B>
+impl<A, B, BIASED> Future for Select<A, B, BIASED>
 where
     A: Future + Unpin,
     B: Future + Unpin,
+    BIASED: IsBiased,
 {
     type Output = Either<(A::Output, B), (B::Output, A)>;
 
@@ -195,7 +197,6 @@ where
                 Some(value) => value,
             }
         }
-        let _biased = self._biased;
 
         let (a, b) = self.inner.as_mut().expect("cannot poll Select twice");
 
@@ -208,7 +209,7 @@ where
         }
 
         #[cfg(feature = "std")]
-        if _biased || crate::gen_index(2) == 0 {
+        if BIASED::IS_BIASED || crate::gen_index(2) == 0 {
             poll_wrap!(a, unwrap_option(self.inner.take()).1, Either::Left);
             poll_wrap!(b, unwrap_option(self.inner.take()).0, Either::Right);
         } else {
@@ -225,10 +226,11 @@ where
     }
 }
 
-impl<A, B> FusedFuture for Select<A, B>
+impl<A, B, BIASED> FusedFuture for Select<A, B, BIASED>
 where
     A: Future + Unpin,
     B: Future + Unpin,
+    BIASED: IsBiased,
 {
     fn is_terminated(&self) -> bool {
         self.inner.is_none()

futures-util/src/future/select_ok.rs

@@ -1,7 +1,9 @@
 use super::assert_future;
+use super::{Biased, Fair, IsBiased};
 use crate::future::TryFutureExt;
 use alloc::vec::Vec;
 use core::iter::FromIterator;
+use core::marker::PhantomData;
 use core::mem;
 use core::pin::Pin;
 use futures_core::future::{Future, TryFuture};
@@ -10,12 +12,12 @@ use futures_core::task::{Context, Poll};
 /// Future for the [`select_ok`] function.
 #[derive(Debug)]
 #[must_use = "futures do nothing unless you `.await` or poll them"]
-pub struct SelectOk<Fut> {
+pub struct SelectOk<Fut, BIASED = Fair> {
     inner: Vec<Fut>,
-    _biased: bool,
+    _phantom: PhantomData<BIASED>,
 }
 
-impl<Fut: Unpin> Unpin for SelectOk<Fut> {}
+impl<Fut: Unpin, BIASED> Unpin for SelectOk<Fut, BIASED> {}
 
 /// Creates a new future which will select the first successful future over a list of futures.
 ///
@@ -41,12 +43,12 @@ impl<Fut: Unpin> Unpin for SelectOk<Fut> {}
 /// # Panics
 ///
 /// This function will panic if the iterator specified contains no items.
-pub fn select_ok<I>(iter: I) -> SelectOk<I::Item>
+pub fn select_ok<I>(iter: I) -> SelectOk<I::Item, Fair>
 where
     I: IntoIterator,
     I::Item: TryFuture + Unpin,
 {
-    let ret = SelectOk { inner: iter.into_iter().collect(), _biased: false };
+    let ret = SelectOk { inner: iter.into_iter().collect(), _phantom: PhantomData };
     assert!(!ret.inner.is_empty(), "iterator provided to select_ok was empty");
     assert_future::<
         Result<(<I::Item as TryFuture>::Ok, Vec<I::Item>), <I::Item as TryFuture>::Error>,
@@ -70,27 +72,27 @@ where
 /// # Panics
 ///
 /// This function will panic if the iterator specified contains no items.
-pub fn select_ok_biased<I>(iter: I) -> SelectOk<I::Item>
+pub fn select_ok_biased<I>(iter: I) -> SelectOk<I::Item, Biased>
 where
     I: IntoIterator,
     I::Item: TryFuture + Unpin,
 {
-    let ret = SelectOk { inner: iter.into_iter().collect(), _biased: true };
+    let ret = SelectOk { inner: iter.into_iter().collect(), _phantom: PhantomData };
     assert!(!ret.inner.is_empty(), "iterator provided to select_ok was empty");
     assert_future::<
         Result<(<I::Item as TryFuture>::Ok, Vec<I::Item>), <I::Item as TryFuture>::Error>,
         _,
     >(ret)
 }
 
-impl<Fut: TryFuture + Unpin> Future for SelectOk<Fut> {
+impl<Fut: TryFuture + Unpin, BIASED: IsBiased> Future for SelectOk<Fut, BIASED> {
     type Output = Result<(Fut::Ok, Vec<Fut>), Fut::Error>;
 
     fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
-        let Self { inner, _biased } = &mut *self;
+        let Self { inner, _phantom } = &mut *self;
         #[cfg(feature = "std")]
         {
-            if !*_biased {
+            if !BIASED::IS_BIASED {
                 crate::shuffle(inner);
             }
         }

futures-util/src/future/select_strategy.rs

@@ -0,0 +1,25 @@
+/// When used with a select future, this will make the future biased.
+/// When multiple futures are ready, the winner will be the first one
+/// specified.
+#[derive(Debug)]
+pub struct Biased;
+
+/// When used with a select future, this will make the future fair.
+/// When multiple futures are ready, the winner will be pseudo-randomly
+/// selected. This is the default behavior.
+#[derive(Debug)]
+pub struct Fair;
+
+/// Reports whether the type is an instance of [`Biased`] or not.
+pub trait IsBiased {
+    /// Contains the answer to our question: is this biased?
+    const IS_BIASED: bool;
+}
+
+impl IsBiased for Biased {
+    const IS_BIASED: bool = true;
+}
+
+impl IsBiased for Fair {
+    const IS_BIASED: bool = false;
+}

futures-util/src/future/try_select.rs

@@ -1,17 +1,19 @@
+use super::{Biased, Fair, IsBiased};
 use crate::future::{Either, TryFutureExt};
+use core::marker::PhantomData;
 use core::pin::Pin;
 use futures_core::future::{Future, TryFuture};
 use futures_core::task::{Context, Poll};
 
 /// Future for the [`try_select()`] function.
 #[must_use = "futures do nothing unless you `.await` or poll them"]
 #[derive(Debug)]
-pub struct TrySelect<A, B> {
+pub struct TrySelect<A, B, BIASED = Fair> {
     inner: Option<(A, B)>,
-    _biased: bool,
+    _phantom: PhantomData<BIASED>,
 }
 
-impl<A: Unpin, B: Unpin> Unpin for TrySelect<A, B> {}
+impl<A: Unpin, B: Unpin, BIASED> Unpin for TrySelect<A, B, BIASED> {}
 
 type EitherOk<A, B> = Either<(<A as TryFuture>::Ok, B), (<B as TryFuture>::Ok, A)>;
 type EitherErr<A, B> = Either<(<A as TryFuture>::Error, B), (<B as TryFuture>::Error, A)>;
@@ -55,14 +57,14 @@ type EitherErr<A, B> = Either<(<A as TryFuture>::Error, B), (<B as TryFuture>::E
 ///     })
 /// }
 /// ```
-pub fn try_select<A, B>(future1: A, future2: B) -> TrySelect<A, B>
+pub fn try_select<A, B>(future1: A, future2: B) -> TrySelect<A, B, Fair>
 where
     A: TryFuture + Unpin,
     B: TryFuture + Unpin,
 {
     super::assert_future::<Result<EitherOk<A, B>, EitherErr<A, B>>, _>(TrySelect {
         inner: Some((future1, future2)),
-        _biased: false,
+        _phantom: PhantomData,
     })
 }
 
@@ -103,21 +105,22 @@ where
 ///     })
 /// }
 /// ```
-pub fn try_select_biased<A, B>(future1: A, future2: B) -> TrySelect<A, B>
+pub fn try_select_biased<A, B>(future1: A, future2: B) -> TrySelect<A, B, Biased>
 where
     A: TryFuture + Unpin,
     B: TryFuture + Unpin,
 {
     super::assert_future::<Result<EitherOk<A, B>, EitherErr<A, B>>, _>(TrySelect {
         inner: Some((future1, future2)),
-        _biased: true,
+        _phantom: PhantomData,
     })
 }
 
-impl<A: Unpin, B: Unpin> Future for TrySelect<A, B>
+impl<A: Unpin, B: Unpin, BIASED> Future for TrySelect<A, B, BIASED>
 where
     A: TryFuture,
     B: TryFuture,
+    BIASED: IsBiased,
 {
     type Output = Result<EitherOk<A, B>, EitherErr<A, B>>;
 
@@ -142,7 +145,7 @@ where
 
         #[cfg(feature = "std")]
         {
-            if self._biased || crate::gen_index(2) == 0 {
+            if BIASED::IS_BIASED || crate::gen_index(2) == 0 {
                 poll_wrap!(a, b, Either::Left, Either::Right)
             } else {
                 poll_wrap!(b, a, Either::Right, Either::Left)