1 files changed, 84 insertions, 0 deletions
diff --git a/third_party/rust/futures-0.1.31/src/future/lazy.rs b/third_party/rust/futures-0.1.31/src/future/lazy.rs
new file mode 100644
index 0000000000..2f310337b6
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+++ b/third_party/rust/futures-0.1.31/src/future/lazy.rs
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+//! Definition of the Lazy combinator, deferring execution of a function until
+//! the future is polled.
+
+use core::mem;
+
+use {Future, IntoFuture, Poll};
+
+/// A future which defers creation of the actual future until a callback is
+/// scheduled.
+///
+/// This is created by the `lazy` function.
+#[derive(Debug)]
+#[must_use = "futures do nothing unless polled"]
+pub struct Lazy<F, R: IntoFuture> {
+    inner: _Lazy<F, R::Future>,
+}
+
+#[derive(Debug)]
+enum _Lazy<F, R> {
+    First(F),
+    Second(R),
+    Moved,
+}
+
+/// Creates a new future which will eventually be the same as the one created
+/// by the closure provided.
+///
+/// The provided closure is only run once the future has a callback scheduled
+/// on it, otherwise the callback never runs. Once run, however, this future is
+/// the same as the one the closure creates.
+///
+/// # Examples
+///
+/// ```
+/// use futures::future::*;
+///
+/// let a = lazy(|| ok::<u32, u32>(1));
+///
+/// let b = lazy(|| -> FutureResult<u32, u32> {
+///     panic!("oh no!")
+/// });
+/// drop(b); // closure is never run
+/// ```
+pub fn lazy<F, R>(f: F) -> Lazy<F, R>
+    where F: FnOnce() -> R,
+          R: IntoFuture
+{
+    Lazy {
+        inner: _Lazy::First(f),
+    }
+}
+
+impl<F, R> Lazy<F, R>
+    where F: FnOnce() -> R,
+          R: IntoFuture,
+{
+    fn get(&mut self) -> &mut R::Future {
+        match self.inner {
+            _Lazy::First(_) => {}
+            _Lazy::Second(ref mut f) => return f,
+            _Lazy::Moved => panic!(), // can only happen if `f()` panics
+        }
+        match mem::replace(&mut self.inner, _Lazy::Moved) {
+            _Lazy::First(f) => self.inner = _Lazy::Second(f().into_future()),
+            _ => panic!(), // we already found First
+        }
+        match self.inner {
+            _Lazy::Second(ref mut f) => f,
+            _ => panic!(), // we just stored Second
+        }
+    }
+}
+
+impl<F, R> Future for Lazy<F, R>
+    where F: FnOnce() -> R,
+          R: IntoFuture,
+{
+    type Item = R::Item;
+    type Error = R::Error;
+
+    fn poll(&mut self) -> Poll<R::Item, R::Error> {
+        self.get().poll()
+    }
+}