chore: checkpoint before Python removal

vendor/futures/tests/recurse.rs

@@ -0,0 +1,25 @@
+use futures::executor::block_on;
+use futures::future::{self, BoxFuture, FutureExt};
+use std::sync::mpsc;
+use std::thread;
+
+#[test]
+fn lots() {
+    #[cfg(not(futures_sanitizer))]
+    const N: i32 = 1_000;
+    #[cfg(futures_sanitizer)] // If N is many, asan reports stack-overflow: https://gist.github.com/taiki-e/099446d21cbec69d4acbacf7a9646136
+    const N: i32 = 100;
+
+    fn do_it(input: (i32, i32)) -> BoxFuture<'static, i32> {
+        let (n, x) = input;
+        if n == 0 {
+            future::ready(x).boxed()
+        } else {
+            future::ready((n - 1, x + n)).then(do_it).boxed()
+        }
+    }
+
+    let (tx, rx) = mpsc::channel();
+    thread::spawn(|| block_on(do_it((N, 0)).map(move |x| tx.send(x).unwrap())));
+    assert_eq!((0..=N).sum::<i32>(), rx.recv().unwrap());
+}