Set size_hint for identity compession bodies

tower-http/src/compression/body.rs

@@ -269,6 +269,14 @@ where
             },
         }
     }
+
+    fn size_hint(&self) -> http_body::SizeHint {
+        if let BodyInner::Identity { inner } = &self.inner {
+            inner.size_hint()
+        } else {
+            http_body::SizeHint::new()
+        }
+    }
 }
 
 #[cfg(feature = "compression-gzip")]

tower-http/src/compression/mod.rs

@@ -99,6 +99,7 @@ mod tests {
         ACCEPT_ENCODING, ACCEPT_RANGES, CONTENT_ENCODING, CONTENT_RANGE, CONTENT_TYPE, RANGE,
     };
     use http::{HeaderMap, HeaderName, HeaderValue, Request, Response};
+    use http_body::Body as _;
     use http_body_util::BodyExt;
     use std::convert::Infallible;
     use std::io::Read;
@@ -495,4 +496,16 @@ mod tests {
         assert_eq!(headers[CONTENT_ENCODING], "gzip");
         assert_eq!(decompressed, "Hello, World!");
     }
+
+    #[tokio::test]
+    async fn size_hint_identity() {
+        let msg = "Hello, world!";
+        let svc = service_fn(|_| async { Ok::<_, std::io::Error>(Response::new(Body::from(msg))) });
+        let mut svc = Compression::new(svc);
+
+        let req = Request::new(Body::empty());
+        let res = svc.ready().await.unwrap().call(req).await.unwrap();
+        let body = res.into_body();
+        assert_eq!(body.size_hint().exact().unwrap(), msg.len() as u64);
+    }
 }