io: add SyncIOBridge::into_inner (#5971)

tokio-util/tests/io_sync_bridge.rs

@@ -43,6 +43,18 @@ async fn test_async_write_to_sync() -> Result<(), Box<dyn Error>> {
     Ok(())
 }
 
+#[tokio::test]
+async fn test_into_inner() -> Result<(), Box<dyn Error>> {
+    let mut buf = Vec::new();
+    SyncIoBridge::new(tokio::io::empty())
+        .into_inner()
+        .read_to_end(&mut buf)
+        .await
+        .unwrap();
+    assert_eq!(buf.len(), 0);
+    Ok(())
+}
+
 #[tokio::test]
 async fn test_shutdown() -> Result<(), Box<dyn Error>> {
     let (s1, mut s2) = tokio::io::duplex(1024);