Adds a from_fn stream implementation (#277)

* Adds a from_fn stream implementation

* Update src/stream/from_fn.rs

Co-Authored-By: Yoshua Wuyts <yoshuawuyts+github@gmail.com>

* Fix review nits

* Use async_std Mutex

src/stream/from_fn.rs

@@ -0,0 +1,100 @@
+use std::marker::PhantomData;
+use std::pin::Pin;
+
+use crate::future::Future;
+use crate::stream::Stream;
+use crate::task::{Context, Poll};
+
+/// A stream that yields elements by calling a closure.
+///
+/// This stream is constructed by [`from_fn`] function.
+///
+/// [`from_fn`]: fn.from_fn.html
+#[derive(Debug)]
+pub struct FromFn<F, Fut, T> {
+    f: F,
+    future: Option<Fut>,
+    __t: PhantomData<T>,
+}
+
+/// Creates a new stream where to produce each new element a provided closure is called.
+///
+/// This allows creating a custom stream with any behaviour without using the more verbose
+/// syntax of creating a dedicated type and implementing a `Stream` trait for it.
+///
+/// # Examples
+///
+/// ```
+/// # fn main() { async_std::task::block_on(async {
+/// #
+/// use async_std::prelude::*;
+/// use async_std::sync::Mutex;
+/// use std::sync::Arc;
+/// use async_std::stream;
+///
+/// let count = Arc::new(Mutex::new(0u8));
+/// let s = stream::from_fn(|| {
+///     let count = Arc::clone(&count);
+///
+///     async move {
+///         *count.lock().await += 1;
+///
+///         if *count.lock().await > 3 {
+///             None
+///         } else {
+///             Some(*count.lock().await)
+///         }
+///     }
+/// });
+///
+/// pin_utils::pin_mut!(s);
+/// assert_eq!(s.next().await, Some(1));
+/// assert_eq!(s.next().await, Some(2));
+/// assert_eq!(s.next().await, Some(3));
+/// assert_eq!(s.next().await, None);
+/// #
+/// # }) }
+///
+/// ```
+pub fn from_fn<T, F, Fut>(f: F) -> FromFn<F, Fut, T>
+where
+    F: FnMut() -> Fut,
+    Fut: Future<Output = Option<T>>,
+{
+    FromFn {
+        f,
+        future: None,
+        __t: PhantomData,
+    }
+}
+
+impl<F, Fut, T> FromFn<F, Fut, T> {
+    pin_utils::unsafe_unpinned!(f: F);
+    pin_utils::unsafe_pinned!(future: Option<Fut>);
+}
+
+impl<F, Fut, T> Stream for FromFn<F, Fut, T>
+where
+    F: FnMut() -> Fut,
+    Fut: Future<Output = Option<T>>,
+{
+    type Item = T;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        loop {
+            match &self.future {
+                Some(_) => {
+                    let next =
+                        futures_core::ready!(self.as_mut().future().as_pin_mut().unwrap().poll(cx));
+                    self.as_mut().future().set(None);
+
+                    return Poll::Ready(next);
+                }
+                None => {
+                    let fut = (self.as_mut().f())();
+                    self.as_mut().future().set(Some(fut));
+                }
+            }
+        }
+    }
+}

src/stream/mod.rs

@@ -24,6 +24,7 @@
 use cfg_if::cfg_if;
 
 pub use empty::{empty, Empty};
+pub use from_fn::{from_fn, FromFn};
 pub use once::{once, Once};
 pub use repeat::{repeat, Repeat};
 pub use stream::{
@@ -33,6 +34,7 @@ pub use stream::{
 pub(crate) mod stream;
 
 mod empty;
+mod from_fn;
 mod once;
 mod repeat;