feat: Add FlattenCompat struct

src/lib.rs

@@ -47,6 +47,7 @@
 #![doc(test(attr(allow(unused_extern_crates, unused_variables))))]
 #![doc(html_logo_url = "https://async.rs/images/logo--hero.svg")]
 #![recursion_limit = "1024"]
+#![feature(associated_type_bounds)]
 
 use cfg_if::cfg_if;
 

src/stream/stream/flatten.rs

@@ -0,0 +1,24 @@
+use std::pin::Pin;
+
+use crate::stream::{IntoStream, Stream};
+use crate::task::{Context, Poll};
+
+/// Real logic of both `Flatten` and `FlatMap` which simply delegate to
+/// this type.
+#[derive(Clone, Debug)]
+struct FlattenCompat<S, U> {
+    stream: S,
+    frontiter: Option<U>,
+}
+impl<S, U> FlattenCompat<S, U> {
+    pin_utils::unsafe_unpinned!(stream: S);
+    pin_utils::unsafe_unpinned!(frontiter: Option<U>);
+
+    /// Adapts an iterator by flattening it, for use in `flatten()` and `flat_map()`.
+    pub fn new(stream: S) -> FlattenCompat<S, U> {
+        FlattenCompat {
+            stream,
+            frontiter: None,
+        }
+    }
+}

src/stream/stream/mod.rs

@@ -106,6 +106,7 @@ cfg_if! {
 cfg_if! {
     if #[cfg(any(feature = "unstable", feature = "docs"))] {
         mod merge;
+        mod flatten;
 
         use std::pin::Pin;