WriterPanicked: Use debug_struct

Co-authored-by: Ivan Tham <pickfire@riseup.net>

Author: ijackson

library/std/src/io/buffered/bufwriter.rs

@@ -382,7 +382,9 @@ impl fmt::Display for WriterPanicked {
 #[unstable(feature = "bufwriter_into_raw_parts", issue = "none")]
 impl fmt::Debug for WriterPanicked {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "WriterPanicked{{..buf.len={}..}}", self.buf.len())
+        fmt.debug_struct("WriterPanicked")
+            .field("buffer", &format_args!("{}/{}", self.buf.len(), self.buf.capacity()))
+            .finish()
     }
 }