Merge branch 'master' into feature/core_convert_id

1 files changed, 182 insertions, 0 deletions

diff --git a/src/libcore/future/future_obj.rs b/src/libcore/future/future_obj.rs
new file mode 100644
index 00000000000..6045fac2b4b
--- /dev/null
+++ b/src/libcore/future/future_obj.rs
@@ -0,0 +1,182 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![unstable(feature = "futures_api",
+            reason = "futures in libcore are unstable",
+            issue = "50547")]
+
+use fmt;
+use future::Future;
+use marker::{PhantomData, Unpin};
+use mem::PinMut;
+use task::{Context, Poll};
+
+/// A custom trait object for polling futures, roughly akin to
+/// `Box<dyn Future<Output = T> + 'a>`.
+///
+/// This custom trait object was introduced for two reasons:
+/// - Currently it is not possible to take `dyn Trait` by value and
+///   `Box<dyn Trait>` is not available in no_std contexts.
+/// - The `Future` trait is currently not object safe: The `Future::poll`
+///   method makes uses the arbitrary self types feature and traits in which
+///   this feature is used are currently not object safe due to current compiler
+///   limitations. (See tracking issue for arbitray self types for more
+///   information #44874)
+pub struct LocalFutureObj<'a, T> {
+    ptr: *mut (),
+    poll_fn: unsafe fn(*mut (), &mut Context) -> Poll<T>,
+    drop_fn: unsafe fn(*mut ()),
+    _marker: PhantomData<&'a ()>,
+}
+
+impl<'a, T> Unpin for LocalFutureObj<'a, T> {}
+
+impl<'a, T> LocalFutureObj<'a, T> {
+    /// Create a `LocalFutureObj` from a custom trait object representation.
+    #[inline]
+    pub fn new<F: UnsafeFutureObj<'a, T> + 'a>(f: F) -> LocalFutureObj<'a, T> {
+        LocalFutureObj {
+            ptr: f.into_raw(),
+            poll_fn: F::poll,
+            drop_fn: F::drop,
+            _marker: PhantomData,
+        }
+    }
+
+    /// Converts the `LocalFutureObj` into a `FutureObj`
+    /// To make this operation safe one has to ensure that the `UnsafeFutureObj`
+    /// instance from which this `LocalFutureObj` was created actually
+    /// implements `Send`.
+    #[inline]
+    pub unsafe fn into_future_obj(self) -> FutureObj<'a, T> {
+        FutureObj(self)
+    }
+}
+
+impl<'a, T> fmt::Debug for LocalFutureObj<'a, T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.debug_struct("LocalFutureObj")
+            .finish()
+    }
+}
+
+impl<'a, T> From<FutureObj<'a, T>> for LocalFutureObj<'a, T> {
+    #[inline]
+    fn from(f: FutureObj<'a, T>) -> LocalFutureObj<'a, T> {
+        f.0
+    }
+}
+
+impl<'a, T> Future for LocalFutureObj<'a, T> {
+    type Output = T;
+
+    #[inline]
+    fn poll(self: PinMut<Self>, cx: &mut Context) -> Poll<T> {
+        unsafe {
+            (self.poll_fn)(self.ptr, cx)
+        }
+    }
+}
+
+impl<'a, T> Drop for LocalFutureObj<'a, T> {
+    fn drop(&mut self) {
+        unsafe {
+            (self.drop_fn)(self.ptr)
+        }
+    }
+}
+
+/// A custom trait object for polling futures, roughly akin to
+/// `Box<dyn Future<Output = T> + Send + 'a>`.
+///
+/// This custom trait object was introduced for two reasons:
+/// - Currently it is not possible to take `dyn Trait` by value and
+///   `Box<dyn Trait>` is not available in no_std contexts.
+/// - The `Future` trait is currently not object safe: The `Future::poll`
+///   method makes uses the arbitrary self types feature and traits in which
+///   this feature is used are currently not object safe due to current compiler
+///   limitations. (See tracking issue for arbitray self types for more
+///   information #44874)
+pub struct FutureObj<'a, T>(LocalFutureObj<'a, T>);
+
+impl<'a, T> Unpin for FutureObj<'a, T> {}
+unsafe impl<'a, T> Send for FutureObj<'a, T> {}
+
+impl<'a, T> FutureObj<'a, T> {
+    /// Create a `FutureObj` from a custom trait object representation.
+    #[inline]
+    pub fn new<F: UnsafeFutureObj<'a, T> + Send>(f: F) -> FutureObj<'a, T> {
+        FutureObj(LocalFutureObj::new(f))
+    }
+}
+
+impl<'a, T> fmt::Debug for FutureObj<'a, T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.debug_struct("FutureObj")
+            .finish()
+    }
+}
+
+impl<'a, T> Future for FutureObj<'a, T> {
+    type Output = T;
+
+    #[inline]
+    fn poll(self: PinMut<Self>, cx: &mut Context) -> Poll<T> {
+        let pinned_field = unsafe { PinMut::map_unchecked(self, |x| &mut x.0) };
+        pinned_field.poll(cx)
+    }
+}
+
+/// A custom implementation of a future trait object for `FutureObj`, providing
+/// a hand-rolled vtable.
+///
+/// This custom representation is typically used only in `no_std` contexts,
+/// where the default `Box`-based implementation is not available.
+///
+/// The implementor must guarantee that it is safe to call `poll` repeatedly (in
+/// a non-concurrent fashion) with the result of `into_raw` until `drop` is
+/// called.
+pub unsafe trait UnsafeFutureObj<'a, T>: 'a {
+    /// Convert an owned instance into a (conceptually owned) void pointer.
+    fn into_raw(self) -> *mut ();
+
+    /// Poll the future represented by the given void pointer.
+    ///
+    /// # Safety
+    ///
+    /// The trait implementor must guarantee that it is safe to repeatedly call
+    /// `poll` with the result of `into_raw` until `drop` is called; such calls
+    /// are not, however, allowed to race with each other or with calls to
+    /// `drop`.
+    unsafe fn poll(ptr: *mut (), cx: &mut Context) -> Poll<T>;
+
+    /// Drops the future represented by the given void pointer.
+    ///
+    /// # Safety
+    ///
+    /// The trait implementor must guarantee that it is safe to call this
+    /// function once per `into_raw` invocation; that call cannot race with
+    /// other calls to `drop` or `poll`.
+    unsafe fn drop(ptr: *mut ());
+}
+
+unsafe impl<'a, T, F> UnsafeFutureObj<'a, T> for &'a mut F
+    where F: Future<Output = T> + Unpin + 'a
+{
+    fn into_raw(self) -> *mut () {
+        self as *mut F as *mut ()
+    }
+
+    unsafe fn poll(ptr: *mut (), cx: &mut Context) -> Poll<T> {
+        PinMut::new_unchecked(&mut *(ptr as *mut F)).poll(cx)
+    }
+
+    unsafe fn drop(_ptr: *mut ()) {}
+}