Add rule to prefer for loops over functional forEach(…) method (#234)

Author: calda

README.md

@@ -1239,6 +1239,40 @@ _You can enable the following settings in Xcode by running [this script](resourc
 
   </details>
 
+* <a id='prefer-for-loop-over-forEach'></a>(<a href='#prefer-for-loop-over-forEach'>link</a>) **Prefer using `for` loops over the functional `forEach(…)` method**, unless using `forEach(…)` as the last element in a functional chain. [![SwiftFormat: forLoop](https://img.shields.io/badge/SwiftFormat-forLoop-7B0051.svg)](https://github.com/nicklockwood/SwiftFormat/blob/master/Rules.md#forLoop)
+
+  <details>
+
+  #### Why?
+  For loops are more idiomatic than the `forEach(…)` method, and are typically familiar to all developers who have experience with C-family languages. 
+
+  For loops are also more expressive than the `forEach(…)` method. For loops support the `return`, `continue`, and `break` control flow keywords, while `forEach(…)` only supports `return` (which has the same behavior as `continue` in a for loop).
+  
+  ```swift
+  // WRONG
+  planets.forEach { planet in
+    planet.terraform()
+  }
+
+  // WRONG
+  planets.forEach {
+    $0.terraform()
+  }
+
+  // RIGHT
+  for planet in planets {
+    planet.terraform()
+  }
+
+  // ALSO FINE, since forEach is useful when paired with other functional methods in a chain.
+  planets
+    .filter { !$0.isGasGiant }
+    .map { PlanetTerraformer(planet: $0) }
+    .forEach { $0.terraform() }
+  ```
+    
+  </details>
+
 * <a id='omit-internal-keyword'></a>(<a href='#omit-internal-keyword'>link</a>) **Omit the `internal` keyword** when defining types, properties, or functions with an internal access control level. [![SwiftFormat: redundantInternal](https://img.shields.io/badge/SwiftFormat-redundantInternal-7B0051.svg)](https://github.com/nicklockwood/SwiftFormat/blob/master/Rules.md#redundantInternal)
 
   <details>
@@ -1564,6 +1598,8 @@ _You can enable the following settings in Xcode by running [this script](resourc
   }
   ```
 
+  </details>
+
 * <a id='anonymous-trailing-closures'></a>(<a href='#anonymous-trailing-closures'>link</a>) **Prefer trailing closure syntax for closure arguments with no parameter name.** [![SwiftFormat: trailingClosures](https://img.shields.io/badge/SwiftFormat-trailingClosures-7B0051.svg)](https://github.com/nicklockwood/SwiftFormat/blob/master/Rules.md#trailingClosures)
 
   <details>
@@ -1584,6 +1620,8 @@ _You can enable the following settings in Xcode by running [this script](resourc
   planets.first { $0.isGasGiant }
   ```
 
+  </details>
+
 ### Operators
 
 * <a id='infix-operator-spacing'></a>(<a href='#infix-operator-spacing'>link</a>) **Infix operators should have a single space on either side.** Prefer parenthesis to visually group statements with many operators rather than varying widths of whitespace. This rule does not apply to range operators (e.g. `1...3`) and postfix or prefix operators (e.g. `guest?` or `-1`). [![SwiftLint: operator_usage_whitespace](https://img.shields.io/badge/SwiftLint-operator__usage__whitespace-007A87.svg)](https://realm.github.io/SwiftLint/operator_usage_whitespace)

Sources/AirbnbSwiftFormatTool/airbnb.swiftformat

@@ -32,6 +32,7 @@
 --someAny disabled # opaqueGenericParameters
 --elseposition same-line #elseOnSameLine
 --guardelse next-line #elseOnSameLine
+--oneLineForEach wrap #forLoop
 
 # We recommend a max width of 100 but _strictly enforce_ a max width of 130
 --maxwidth 130 # wrap
@@ -89,3 +90,4 @@
 --rules trailingClosures
 --rules elseOnSameLine
 --rules sortTypealiases
+--rules forLoop