Implement size_hint for BufReader

library/std/src/io/mod.rs

@@ -2462,6 +2462,34 @@ impl<R: Read> Iterator for Bytes<R> {
             };
         }
     }
+
+    default fn size_hint(&self) -> (usize, Option<usize>) {
+        (&self.inner as &dyn SizeHint).size_hint()
+    }
+}
+
+trait SizeHint {
+    fn lower_bound(&self) -> usize;
+
+    fn upper_bound(&self) -> Option<usize> {
+        None
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (self.lower_bound(), self.upper_bound())
+    }
+}
+
+impl<T> SizeHint for T {
+    default fn lower_bound(&self) -> usize {
+        0
+    }
+}
+
+impl<T> SizeHint for BufReader<T> {
+    fn lower_bound(&self) -> usize {
+        self.buffer().len()
+    }
 }
 
 /// An iterator over the contents of an instance of `BufRead` split on a