Add a default flag for enum documentation

fixes #115438

Author: omid

src/librustdoc/html/render/print_item.rs

@@ -1356,6 +1356,9 @@ fn item_enum(w: &mut Buffer, cx: &mut Context<'_>, it: &clean::Item, e: &clean::
                 toggle_open(&mut w, format_args!("{count_variants} variants"));
             }
             for v in e.variants() {
+                if v.is_default() {
+                    w.write_str("    <span class=\"code-attribute\">#[default]</span>");
+                }
                 w.write_str("    ");
                 let name = v.name.unwrap();
                 match *v.kind {
@@ -1419,6 +1422,9 @@ fn item_enum(w: &mut Buffer, cx: &mut Context<'_>, it: &clean::Item, e: &clean::
             if let clean::VariantKind::Tuple(ref s) = variant_data.kind {
                 write!(w, "({})", print_tuple_struct_fields(cx, s));
             }
+            if variant.is_default() {
+                w.write_str(" <span class=\"code-attribute\">(Default)</span>");
+            }
             w.write_str("</h3></section>");
 
             let heading_and_fields = match &variant_data.kind {