SwiftLint: Add custom rule for placing each let on its own line

Summary:
This rule will flag when there are 2 let conditions on the same line. For example:

```
if let lhs = lhs, let rhs = rhs {
```

However one limitation is that it is unable to flag boolean conditions since this is difficult to detect via a regex

```
if let lhs = lhs, a == b {
if let lhs = lhs, isBooleanCondition {
```

Reviewed By: samodom

Differential Revision: D33844926

fbshipit-source-id: 4f629131ed22d6d7446c1500bcefb787ec4b7861

.swiftlint.yml

@@ -67,6 +67,9 @@ custom_rules:
   swiftlint_disable_without_this_or_next:
     regex: "// swiftlint:disable (?!implicitly_unwrapped_optional|force_unwrapping)"
     message: "Prefer using swiftlint:disable:this or swiftlint:disable:next over swiftlint:disable"
+  one_let_per_line:
+    regex: " let [^\n]*?, let "
+    message: "Place each let statement and condition on its own line"
 
 only_rules:
   - array_init

FBSDKCoreKit/FBSDKCoreKitTests/Internal/ServerConfiguration/ServerConfigurationTests.swift

@@ -2082,7 +2082,8 @@ class ServerConfigurationTests: XCTestCase {
     file: StaticString = #filePath,
     line: UInt = #line
   ) {
-    if let lhs = lhs, let rhs = rhs {
+    if let lhs = lhs,
+       let rhs = rhs {
       let dict1 = NSDictionary(dictionary: lhs)
       let dict2 = NSDictionary(dictionary: rhs)
       XCTAssertEqual(dict1, dict2, message(), file: file, line: line)

FBSDKGamingServicesKit/FacebookGamingServices/Internal/GamingServiceController.swift

@@ -61,7 +61,8 @@ class GamingServiceController: NSObject {
 
   func isValidCallbackURL(_ url: URL, forService service: String) -> Bool {
     // verify the URL is intended as a callback for the SDK's friend finder
-    guard let appID = settings.appID, let scheme = url.scheme else { return false }
+    guard let appID = settings.appID,
+          let scheme = url.scheme else { return false }
 
     return scheme.hasPrefix("fb\(appID)") && url.host == service
   }

FBSDKGamingServicesKit/FacebookGamingServices/Internal/ShareTournamentDialog.swift

@@ -138,7 +138,9 @@ public class ShareTournamentDialog: NSObject, URLOpening {
   }
 
   func isTournamentURL(url: URL) -> Bool {
-    if let scheme = url.scheme, let host = url.host, let appID = Settings.shared.appID {
+    if let scheme = url.scheme,
+       let host = url.host,
+       let appID = Settings.shared.appID {
       return scheme.hasPrefix("fb\(appID)") && host.elementsEqual("instant_tournament")
     }
     return false

FBSDKShareKit/FBSDKShareKitTests/Models/ShareMediaContentTests.swift

@@ -16,9 +16,11 @@ class ShareMediaContentTests: XCTestCase {
     let media = ShareModelTestUtility.media
 
     for (item1, item2) in zip(media, mediaContentMedia) {
-      if let photo1 = item1 as? SharePhoto, let photo2 = item2 as? SharePhoto {
+      if let photo1 = item1 as? SharePhoto,
+         let photo2 = item2 as? SharePhoto {
         XCTAssertEqual(photo1, photo2)
-      } else if let video1 = item1 as? ShareVideo, let video2 = item2 as? ShareVideo {
+      } else if let video1 = item1 as? ShareVideo,
+                let video2 = item2 as? ShareVideo {
         XCTAssertEqual(video1, video2)
       } else {
         XCTFail("Unexpected type implementing the ShareMedia protocol. Item1: \(item1), Item2: \(item2)")