Show if-case syntax for enums

Author: amartini51

TSPL.docc/LanguageGuide/ControlFlow.md

@@ -880,6 +880,8 @@ on the right-hand side of an assignment,
 as shown in the examples above,
 you can also use them as the value that a function or closure returns.
 
+<!-- XXX forward reference to if-case syntax? -->
+
 ### Switch
 
 A `switch` statement considers a value
@@ -1296,6 +1298,8 @@ Because `anotherPoint` is always a tuple of two values,
 this case matches all possible remaining values,
 and a `default` case isn't needed to make the `switch` statement exhaustive.
 
+<!-- XXX cross reference if-case syntax from here -->
+
 #### Where
 
 A `switch` case can use a `where` clause to check for additional conditions.

TSPL.docc/LanguageGuide/Enumerations.md

@@ -479,6 +479,31 @@ case let .qrCode(productCode):
   ```
 -->
 
+When you're matching just one case of an enumeration,
+you can use the shorter `if case` syntax
+instead of writing a full switch statement.
+For example:
+
+```swift
+if case .qrCode(let productCode) = productBarcode {
+    print("QR code: \(productCode).")
+}
+```
+
+In this code,
+the condition for the `if` statement starts with `case`,
+indicating that the condition is a pattern instead of a Boolean value.
+If the pattern matches,
+the condition for the `if` is considered to be true;
+otherwise it's false.
+Just like in the switch statement earlier,
+the `productBarcode` variable is matched against
+the pattern `.qrCode(let productCode)` here.
+And like in the switch case,
+writing `let` extracts the associated value as a constant.
+
+<!-- XXX mention guards, and maybe for loops -->
+
 ## Raw Values
 
 The barcode example in <doc:Enumerations#Associated-Values>
@@ -520,6 +545,8 @@ Raw values can be
 strings, characters, or any of the integer or floating-point number types.
 Each raw value must be unique within its enumeration declaration.
 
+<!-- XXX fix this note box abuse; this is important, not optional -->
+
 > Note: Raw values are *not* the same as associated values.
 > Raw values are set to prepopulated values
 > when you first define the enumeration in your code,