trpl: move tuple-structs.md into structs.md

Author: geofft

src/doc/trpl/SUMMARY.md

@@ -43,7 +43,6 @@
     * [Universal Function Call Syntax](ufcs.md)
     * [Crates and Modules](crates-and-modules.md)
     * [`const` and `static`](const-and-static.md)
-    * [Tuple Structs](tuple-structs.md)
     * [Attributes](attributes.md)
     * [`type` aliases](type-aliases.md)
     * [Casting between types](casting-between-types.md)

src/doc/trpl/structs.md

@@ -117,3 +117,63 @@ ones, and it will copy the values you don’t specify:
 let origin = Point3d { x: 0, y: 0, z: 0 };
 let point = Point3d { z: 1, x: 2, .. origin };
 ```
+
+# 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
+their fields don’t:
+
+```rust
+struct Color(i32, i32, i32);
+struct Point(i32, i32, i32);
+```
+
+[tuple]: primitive-types.html#tuples
+
+These two will not be equal, even if they have the same values:
+
+```rust
+# struct Color(i32, i32, i32);
+# struct Point(i32, i32, i32);
+let black = Color(0, 0, 0);
+let origin = Point(0, 0, 0);
+```
+
+It is almost always better to use a struct than a tuple struct. We would write
+`Color` and `Point` like this instead:
+
+```rust
+struct Color {
+    red: i32,
+    blue: i32,
+    green: i32,
+}
+
+struct Point {
+    x: i32,
+    y: i32,
+    z: i32,
+}
+```
+
+Now, we have actual names, rather than positions. Good names are important,
+and with a struct, we have actual names.
+
+There _is_ one case when a tuple struct is very useful, though, and that’s a
+tuple struct with only one element. We call this the ‘newtype’ pattern, because
+it allows you to create a new type, distinct from that of its contained value
+and expressing its own semantic meaning:
+
+```rust
+struct Inches(i32);
+
+let length = Inches(10);
+
+let Inches(integer_length) = length;
+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`.