Use Never type in FutureExt::never_error

Author: taiki-e

futures-util/Cargo.toml

@@ -23,7 +23,6 @@ io-compat = ["compat", "tokio-io"]
 bench = []
 nightly = ["futures-core-preview/nightly", "futures-sink-preview/nightly"]
 cfg-target-has-atomic = ["futures-core-preview/cfg-target-has-atomic"]
-never-type = []
 alloc = ["futures-core-preview/alloc", "futures-sink-preview/alloc"]
 
 [dependencies]

futures-util/src/future/mod.rs

@@ -76,9 +76,7 @@ pub use self::inspect::Inspect;
 mod unit_error;
 pub use self::unit_error::UnitError;
 
-#[cfg(feature = "never-type")]
 mod never_error;
-#[cfg(feature = "never-type")]
 pub use self::never_error::NeverError;
 
 mod either;
@@ -516,9 +514,8 @@ pub trait FutureExt: Future {
         UnitError::new(self)
     }
 
-    #[cfg(feature = "never-type")]
     /// Turns a [`Future<Output = T>`](Future) into a
-    /// [`TryFuture<Ok = T, Error = !`>](futures_core::future::TryFuture).
+    /// [`TryFuture<Ok = T, Error = Never`>](futures_core::future::TryFuture).
     fn never_error(self) -> NeverError<Self>
         where Self: Sized
     {

futures-util/src/future/never_error.rs

@@ -1,6 +1,7 @@
 use core::pin::Pin;
 use futures_core::future::{FusedFuture, Future};
 use futures_core::task::{self, Poll};
+use futures_core::never::Never;
 use pin_utils::unsafe_pinned;
 
 /// Future for the [`never_error`](super::FutureExt::never_error) combinator.
@@ -27,9 +28,9 @@ impl<Fut: FusedFuture> FusedFuture for NeverError<Fut> {
 impl<Fut, T> Future for NeverError<Fut>
     where Fut: Future<Output = T>,
 {
-    type Output = Result<T, !>;
+    type Output = Result<T, Never>;
 
-    fn poll(self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<Result<T, !>> {
+    fn poll(self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<Self::Output> {
         self.future().poll(cx).map(Ok)
     }
 }

futures-util/src/lib.rs

@@ -3,7 +3,6 @@
 
 #![cfg_attr(feature = "async-await", feature(async_await))]
 #![cfg_attr(feature = "cfg-target-has-atomic", feature(cfg_target_has_atomic))]
-#![cfg_attr(feature = "never-type", feature(never_type))]
 
 #![cfg_attr(not(feature = "std"), no_std)]
 #![warn(missing_docs, missing_debug_implementations, rust_2018_idioms, unreachable_pub)]
@@ -18,9 +17,6 @@
 #[cfg(all(feature = "cfg-target-has-atomic", not(feature = "nightly")))]
 compile_error!("The `cfg-target-has-atomic` feature requires the `nightly` feature as an explicit opt-in to unstable features");
 
-#[cfg(all(feature = "never-type", not(feature = "nightly")))]
-compile_error!("The `never-type` feature requires the `nightly` feature as an explicit opt-in to unstable features");
-
 #[cfg(all(feature = "async-await", not(feature = "nightly")))]
 compile_error!("The `async-await` feature requires the `nightly` feature as an explicit opt-in to unstable features");
 

futures/Cargo.toml

@@ -44,7 +44,6 @@ default = ["std"]
 compat = ["std", "futures-util-preview/compat"]
 io-compat = ["compat", "futures-util-preview/io-compat"]
 cfg-target-has-atomic = ["futures-core-preview/cfg-target-has-atomic", "futures-util-preview/cfg-target-has-atomic"]
-never-type = ["futures-util-preview/never-type"]
 alloc = ["futures-core-preview/alloc", "futures-sink-preview/alloc", "futures-util-preview/alloc"]
 
 [package.metadata.docs.rs]

futures/src/lib.rs

@@ -22,7 +22,6 @@
 //! completion, but *do not block* the thread running them.
 
 #![cfg_attr(feature = "cfg-target-has-atomic", feature(cfg_target_has_atomic))]
-#![cfg_attr(feature = "never-type", feature(never_type))]
 
 #![cfg_attr(not(feature = "std"), no_std)]
 
@@ -41,9 +40,6 @@ compile_error!("The `async-await` feature requires the `nightly` feature as an e
 #[cfg(all(feature = "cfg-target-has-atomic", not(feature = "nightly")))]
 compile_error!("The `cfg-target-has-atomic` feature requires the `nightly` feature as an explicit opt-in to unstable features");
 
-#[cfg(all(feature = "never-type", not(feature = "nightly")))]
-compile_error!("The `never-type` feature requires the `nightly` feature as an explicit opt-in to unstable features");
-
 #[doc(hidden)] pub use futures_core::core_reexport;
 
 #[doc(hidden)] pub use futures_core::future::Future;
@@ -227,6 +223,7 @@ pub mod future {
 
         FutureExt,
         FlattenStream, Flatten, Fuse, Inspect, IntoStream, Map, Then, UnitError,
+        NeverError,
     };
 
     #[cfg(feature = "alloc")]
@@ -261,9 +258,6 @@ pub mod future {
         InspectOk, InspectErr, TryFlattenStream, UnwrapOrElse,
     };
 
-    #[cfg(feature = "never-type")]
-    pub use futures_util::future::NeverError;
-
     #[cfg(feature = "alloc")]
     pub use futures_util::try_future::{
         try_join_all, TryJoinAll,