Display `impl Sized` correctly

It used to display as just `impl`

src/librustc/util/ppaux.rs

@@ -1325,6 +1325,8 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                         }
                         if !is_sized {
                             write!(f, "{}?Sized", if first { " " } else { "+" })?;
+                        } else if first {
+                            write!(f, " Sized")?;
                         }
                         Ok(())
                     })