Rollup merge of rust-lang#40144 - MajorBreakfast:patch-7, r=frewsxcv

Unit-like structs doc: Improve code sample

r? @steveklabnik

BTW it seems that
```Rust
let p = Proton {};
```
compiles without an error. That's why I didn't add it to the example. It's about consistency anyway.

src/doc/book/src/structs.md

@@ -255,13 +255,14 @@ rather than positions.
 
 You can define a `struct` with no members at all:
 
-```rust
+```rust,compile_fail,E0423
 struct Electron {} // Use empty braces...
 struct Proton;     // ...or just a semicolon.
 
-// Whether you declared the struct with braces or not, do the same when creating one.
+// Use the same notation when creating an instance.
 let x = Electron {};
 let y = Proton;
+let z = Electron; // Error
 ```
 
 Such a `struct` is called ‘unit-like’ because it resembles the empty