Add no_std + alloc support

Author: taiki-e

.travis.yml

@@ -54,6 +54,14 @@ matrix:
         - cargo build --manifest-path futures-sink/Cargo.toml --no-default-features
         - cargo build --manifest-path futures-util/Cargo.toml --no-default-features
 
+    - name: cargo build (alloc)
+      rust: nightly
+      script:
+        - cargo build --manifest-path futures/Cargo.toml --no-default-features --features=alloc,nightly
+        - cargo build --manifest-path futures-core/Cargo.toml --no-default-features --features=alloc,nightly
+        - cargo build --manifest-path futures-sink/Cargo.toml --no-default-features --features=alloc,nightly
+        - cargo build --manifest-path futures-util/Cargo.toml --no-default-features --features=alloc,nightly
+
     - name: cargo build (default features)
       rust: nightly
       script:

futures-core/Cargo.toml

@@ -10,6 +10,7 @@ documentation = "https://rust-lang-nursery.github.io/futures-api-docs/0.3.0-alph
 description = """
 The core traits and types in for the `futures` library.
 """
+build = "build.rs"
 
 [lib]
 name = "futures_core"
@@ -18,9 +19,11 @@ name = "futures_core"
 default = ["std"]
 std = ["either/use_std"]
 nightly = []
+alloc = ["alloc-shim/alloc"]
 
 [dependencies]
 either = { version = "1.4", default-features = false, optional = true }
+alloc-shim = { version = "0.2.1", optional = true }
 
 [dev-dependencies]
 futures-preview = { path = "../futures", version = "=0.3.0-alpha.12" }

futures-core/build.rs

@@ -0,0 +1,9 @@
+use std::env;
+
+fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
+
+    if env::var_os("CARGO_FEATURE_STD").is_some() || env::var_os("CARGO_FEATURE_ALLOC").is_some() {
+        println!("cargo:rustc-cfg=alloc");
+    }
+}

futures-core/src/future/future_obj.rs

@@ -185,9 +185,10 @@ where
     unsafe fn drop(_ptr: *mut ()) {}
 }
 
-#[cfg(feature = "std")]
-mod if_std {
+#[cfg(alloc)]
+mod if_alloc {
     use super::*;
+    use std::boxed::Box;
     use std::mem;
 
     unsafe impl<'a, T, F> UnsafeFutureObj<'a, T> for Box<F>

futures-core/src/future/mod.rs

@@ -33,9 +33,11 @@ impl<F: FusedFuture + ?Sized> FusedFuture for &mut F {
     }
 }
 
-#[cfg(feature = "std")]
-mod if_std {
+#[cfg(alloc)]
+mod if_alloc {
     use super::*;
+    use std::boxed::Box;
+
     impl<F: FusedFuture + ?Sized> FusedFuture for Box<F> {
         fn is_terminated(&self) -> bool {
             <F as FusedFuture>::is_terminated(&**self)
@@ -48,6 +50,7 @@ mod if_std {
         }
     }
 
+    #[cfg(feature = "std")]
     impl<F: FusedFuture> FusedFuture for std::panic::AssertUnwindSafe<F> {
         fn is_terminated(&self) -> bool {
             <F as FusedFuture>::is_terminated(&**self)

futures-core/src/lib.rs

@@ -2,6 +2,7 @@
 
 #![feature(futures_api)]
 #![cfg_attr(feature = "nightly", feature(cfg_target_has_atomic))]
+#![cfg_attr(feature = "alloc", feature(alloc))]
 
 #![cfg_attr(not(feature = "std"), no_std)]
 
@@ -10,6 +11,12 @@
 
 #![doc(html_root_url = "https://rust-lang-nursery.github.io/futures-api-docs/0.3.0-alpha.12/futures_core")]
 
+#[cfg(all(feature = "alloc", not(feature = "nightly")))]
+compile_error!("The `alloc` feature requires the `nightly` feature active to explicitly opt-in to unstable features");
+
+#[cfg(all(feature = "alloc", not(feature = "std")))]
+extern crate alloc as std;
+
 pub mod future;
 #[doc(hidden)] pub use self::future::{Future, FusedFuture, TryFuture};
 

futures-core/src/stream/mod.rs

@@ -145,8 +145,8 @@ impl<S, T, E> TryStream for S
     }
 }
 
-#[cfg(feature = "std")]
-mod if_std {
+#[cfg(alloc)]
+mod if_alloc {
     use std::boxed::Box;
     use super::*;
 
@@ -161,6 +161,7 @@ mod if_std {
         }
     }
 
+    #[cfg(feature = "std")]
     impl<S: Stream> Stream for ::std::panic::AssertUnwindSafe<S> {
         type Item = S::Item;
 

futures-core/src/stream/stream_obj.rs

@@ -2,7 +2,6 @@ use super::Stream;
 use crate::task::{LocalWaker, Poll};
 use core::fmt;
 use core::marker::PhantomData;
-use core::mem;
 use core::pin::Pin;
 
 /// A custom trait object for polling streams, roughly akin to
@@ -188,10 +187,11 @@ where
     unsafe fn drop(_ptr: *mut ()) {}
 }
 
-#[cfg(feature = "std")]
-mod if_std {
-    use std::boxed::Box;
+#[cfg(alloc)]
+mod if_alloc {
     use super::*;
+    use std::boxed::Box;
+    use std::mem;
 
     unsafe impl<'a, T, F> UnsafeStreamObj<'a, T> for Box<F>
         where F: Stream<Item = T> + 'a

futures-core/src/task/mod.rs

@@ -6,5 +6,5 @@ pub mod __internal;
 pub use self::spawn::{Spawn, LocalSpawn, SpawnError};
 
 pub use core::task::{Poll, Waker, LocalWaker, UnsafeWake};
-#[cfg(feature = "std")]
+#[cfg(alloc)]
 pub use std::task::{Wake, local_waker, local_waker_from_nonlocal};

futures-sink/Cargo.toml

@@ -10,15 +10,19 @@ documentation = "https://rust-lang-nursery.github.io/futures-api-docs/0.3.0-alph
 description = """
 The asynchronous `Sink` trait for the futures-rs library.
 """
+build = "build.rs"
 
 [lib]
 name = "futures_sink"
 
 [features]
 std = ["either/use_std", "futures-core-preview/std", "futures-channel-preview/std"]
 default = ["std"]
+nightly = ["futures-core-preview/nightly"]
+alloc = ["futures-core-preview/alloc", "alloc-shim/alloc"]
 
 [dependencies]
 either = { version = "1.4", default-features = false, optional = true }
 futures-core-preview = { path = "../futures-core", version = "=0.3.0-alpha.12", default-features = false }
 futures-channel-preview = { path = "../futures-channel", version = "=0.3.0-alpha.12", default-features = false }
+alloc-shim = { version = "0.2.1", optional = true }