Add a real example of #[diagnostic::on_unimplemented]

Author: cuviper

posts/2024-05-02-Rust-1.78.0.md

@@ -70,6 +70,16 @@ error[E0277]: My Message for `ImportantTrait<i32>` is not implemented for `Strin
    = note: Note 2
 ```
 
+For trait authors, this kind of diagnostic is more useful if you can provide a better hint than just talking about the missing implementation itself. For example, this is an abridged sample from the standard library:
+
+```rust
+#[diagnostic::on_unimplemented(
+    message = "the size for values of type `{Self}` cannot be known at compilation time",
+    label = "doesn't have a size known at compile-time"
+)]
+pub trait Sized {}
+```
+
 For more information, see the reference section on [the `diagnostic` tool attribute namespace](https://doc.rust-lang.org/stable/reference/attributes/diagnostics.html#the-diagnostic-tool-attribute-namespace).
 
 ### Asserting `unsafe` preconditions