io: Avoid marking buffer as uninit when copying to `BufWriter`

1 files changed, 4 insertions, 1 deletions

diff --git a/library/std/src/io/copy.rs b/library/std/src/io/copy.rs
index 8d733325b3b..15e962924ac 100644
--- a/library/std/src/io/copy.rs
+++ b/library/std/src/io/copy.rs
@@ -248,8 +248,11 @@ impl<I: Write + ?Sized> BufferedWriterSpec for BufWriter<I> {
                     Err(e) => return Err(e),
                 }
             } else {
+                // All the bytes that were already in the buffer are initialized,
+                // treat them as such when the buffer is flushed.
+                init += buf.len();
+
                 self.flush_buf()?;
-                init = 0;
             }
         }
     }