trpl/structs: Document unit-like structs

Author: geofft

src/doc/trpl/structs.md

@@ -118,7 +118,7 @@ let origin = Point3d { x: 0, y: 0, z: 0 };
 let point = Point3d { z: 1, x: 2, .. origin };
 ```
 
-# Tuple Structs
+# Tuple structs
 
 Rust has another data type that's like a hybrid between a [tuple][tuple] and a
 struct, called a ‘tuple struct’. Tuple structs have a name, but
@@ -177,3 +177,22 @@ println!("length is {} inches", integer_length);
 As you can see here, you can extract the inner integer type through a
 destructuring `let`, just as with regular tuples. In this case, the
 `let Inches(integer_length)` assigns `10` to `integer_length`.
+
+# Unit-like structs
+
+You can define a struct with no members at all:
+
+```rust
+struct Electron;
+```
+
+Such a struct is called ‘unit-like’ because it resembles the empty
+tuple, `()`, sometimes called ‘unit’. Like a tuple struct, it defines a
+new type.
+
+This is rarely useful on its own (although sometimes it can serve as a
+marker type), but in combination with other features, it can become
+useful. For instance, a library may ask you to create a structure that
+implements a certain [trait][trait] to handle events. If you don’t have
+any data you need to store in the structure, you can just create a
+unit-like struct.