Add empty structs

This should be quick, but introduces the syntax and the concept of a
ZST.

Author: djmitche

src/SUMMARY.md

@@ -64,6 +64,7 @@
 - [User-Defined Types](user-defined-types.md)
   - [Named Structs](user-defined-types/named-structs.md)
   - [Tuple Structs](user-defined-types/tuple-structs.md)
+  - [Empty Structs](user-defined-types/empty-structs.md)
   - [Enums](user-defined-types/enums.md)
   - [Type Aliases](user-defined-types/aliases.md)
   - [Const](user-defined-types/const.md)

src/user-defined-types/empty-structs.md

@@ -0,0 +1,27 @@
+---
+minutes: 2
+---
+
+# Empty Structs
+
+Sometimes we just need a type, without any data. The only value of such a type
+has the same name as the type. This is a zero-sized type (ZST).
+
+```rust,editable
+struct NullMessageDestination;
+
+fn main() {
+    let _message_destination = NullMessageDestination;
+    // ...
+}
+```
+
+ZSTs will be useful when we talk about traits and generics on day 2.
+
+<details>
+
+- This is a shorthand for a tuple struct with zero tuple elements.
+- There isn't much to see here, yet, but students might imagine a trivial
+  implementation of an interface may have no data to store.
+
+</details>

src/user-defined-types/tuple-structs.md

@@ -2,10 +2,6 @@
 minutes: 10
 ---
 
-<!-- NOTES:
-Tuple structs, newtype wrappers, unit-like structs, including initialization syntax
--->
-
 # Tuple Structs
 
 If the field names are unimportant, you can use a tuple struct: