io: improve panic message of ReadBuf::put_slice() (#6629)

Author: inventednight

tokio/src/io/read_buf.rs

@@ -248,7 +248,9 @@ impl<'a> ReadBuf<'a> {
     pub fn put_slice(&mut self, buf: &[u8]) {
         assert!(
             self.remaining() >= buf.len(),
-            "buf.len() must fit in remaining()"
+            "buf.len() must fit in remaining(); buf.len() = {}, remaining() = {}",
+            buf.len(),
+            self.remaining()
         );
 
         let amt = buf.len();