chore: satisfy warnings about async fn in traits

src/operator_ext.rs

@@ -3,18 +3,24 @@ use opendal::{Buffer, Error, ErrorKind, FuturesAsyncReader, Operator};
 
 #[extension(pub trait OperatorExt)]
 impl Operator {
-    async fn read_if_exists(&self, path: &str) -> Result<Option<Buffer>, Error> {
+    fn read_if_exists(
+        &self,
+        path: &str,
+    ) -> impl Future<Output = Result<Option<Buffer>, Error>> + Send {
+        async {
             match self.read(path).await {
                 Ok(buffer) => Ok(Some(buffer)),
                 Err(error) if matches!(error.kind(), ErrorKind::NotFound) => Ok(None),
                 Err(error) => Err(error),
             }
         }
+    }
 
-    async fn async_reader_if_exists(
+    fn async_reader_if_exists(
         &self,
         path: &str,
-    ) -> Result<Option<FuturesAsyncReader>, Error> {
+    ) -> impl Future<Output = Result<Option<FuturesAsyncReader>, Error>> + Send {
+        async {
             let reader = self.reader(path).await?;
             match reader.into_futures_async_read(..).await {
                 Ok(reader) => Ok(Some(reader)),
@@ -22,4 +28,5 @@ impl Operator {
                 Err(error) => Err(error),
             }
         }
+    }
 }