fix pin compile error

Author: laizy

src/stream/from_fn.rs

@@ -6,6 +6,8 @@ use std::task::{Context, Poll};
 use futures::ready;
 use futures::stream::Stream;
 
+use pin_utils::unsafe_unpinned;
+
 /// Creates a new stream where each iteration calls the provided closure.
 ///
 /// This allows creating a custom iterator with any behavior
@@ -52,7 +54,11 @@ pub fn from_fn<T, F>(f: F) -> FromFn<F, T>
 /// [`stream::from_fn`]: fn.from_fn.html
 pub struct FromFn<F, T> {
     closure: F,
-    fut: Option<Box<dyn Future<Output = Option<T>>>>,
+    fut: Option<Pin<Box<dyn Future<Output = Option<T>>>>>,
+}
+
+impl<F,T> FromFn<F,T> {
+    unsafe_unpinned!(closure: F);
 }
 
 impl<T, F: Unpin> Stream for FromFn<F, T>
@@ -63,10 +69,11 @@ impl<T, F: Unpin> Stream for FromFn<F, T>
     #[inline]
     fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
         if let None = self.fut {
-            self.fut = Some((self.closure)());
+            let fut = (self.as_mut().closure())();
+            self.fut = unsafe{Some(Pin::new_unchecked(fut))};
         }
 
-        let pinned = unsafe { Pin::new_unchecked(&mut self.fut.as_mut().unwrap()) };
+        let pinned = Pin::new(self.fut.as_mut().unwrap());
         let out = ready!(pinned.poll(cx));
 
         self.fut = None;