Mention #[default] in E0655 code index

Author: estebank

compiler/rustc_error_codes/src/error_codes/E0665.md

@@ -16,18 +16,30 @@ The `Default` cannot be derived on an enum for the simple reason that the
 compiler doesn't know which value to pick by default whereas it can for a
 struct as long as all its fields implement the `Default` trait as well.
 
-If you still want to implement `Default` on your enum, you'll have to do it "by
-hand":
+For the case where the desired default variant has no data, you can annotate
+it with `#[default]` to derive it:
 
 ```
+#[derive(Default)]
 enum Food {
+    #[default]
     Sweet,
     Salty,
 }
+```
+
+In the case where the default variant does have data, you will have to
+implement `Default` on your enum "by hand":
+
+```
+enum Food {
+    Sweet(i32),
+    Salty,
+}
 
 impl Default for Food {
     fn default() -> Food {
-        Food::Sweet
+        Food::Sweet(1)
     }
 }
 ```