Fix #1253: Document enum type aliases

Author: adamchalmers

src/custom_types/enum.md

@@ -53,6 +53,46 @@ fn main() {
 
 ```
 
+## Type aliases
+
+If you use a type alias, you can refer to each enum variant via its alias. 
+This might be useful if the enum's name is too long or too generic, and you
+want to rename it.
+
+```rust,editable
+enum VeryVerboseEnumOfThingsToDoWithNumbers {
+    Add,
+    Subtract,
+}
+
+// Creates a type alias
+type Operations = VeryVerboseEnumOfThingsToDoWithNumbers;
+
+fn main() {
+    // We can refer to each variant via its alias, not its long and inconvenient
+    // name.
+    let x = Operations::Add;
+}
+```
+
+The most common place you'll see this is in `impl` blocks using the `Self` alias.
+
+```rust,editable
+enum VeryVerboseEnumOfThingsToDoWithNumbers {
+    Add,
+    Subtract,
+}
+
+impl VeryVerboseEnumOfThingsToDoWithNumbers {
+    fn run(&self, x: i32, y: i32) -> i32 {
+        match self {
+            Self::Add => x + y,
+            Self::Subtract => x - y,
+        }
+    }
+}
+```
+
 ### See also:
 
 [`match`][match], [`fn`][fn], and [`String`][str]