Implement case indent level equals switch (swiftlang#72)

Author: atamez31

Diff for: Sources/Rules/CaseIndentLevelEqualsSwitch.swift

@@ -11,5 +11,46 @@ import SwiftSyntax
 ///
 /// - SeeAlso: https://google.github.io/swift#switch-statements
 public final class CaseIndentLevelEqualsSwitch: SyntaxFormatRule {
+  public override func visit(_ node: SwitchStmtSyntax) -> StmtSyntax {
+    var cases = [Syntax]()
+    guard let switchIndentation = node.leadingTrivia?.numberOfSpaces else { return node }
+    var spacesDif: Int
+    var isInvalid = false
 
+    // Iterates through the switch stamentent to ensure the number of spaces
+    // in the indentation of each case is the same as the switch keyword.
+    for caseExp in node.cases {
+      guard let caseTrivia = caseExp.leadingTrivia else { continue }
+
+      if caseTrivia.numberOfSpaces != switchIndentation {
+        spacesDif = switchIndentation - caseTrivia.numberOfSpaces
+        diagnose(
+          .adjustIndentationSpaces(
+          count: spacesDif,
+          caseText: caseExp.description),
+          on: node
+        )
+
+        let newCase = replaceTrivia(on: caseExp, token: caseExp.firstToken, leadingTrivia:
+                Trivia.newlines(caseTrivia.numberOfNewlines).appending(.spaces(switchIndentation)))
+        isInvalid = true
+        cases.append(newCase)
+      }
+      else {
+        cases.append(caseExp)
+      }
+    }
+    return isInvalid ? node.withCases(SyntaxFactory.makeSwitchCaseList(cases)) : node
+  }
+}
+
+extension Diagnostic.Message {
+  static func adjustIndentationSpaces(count: Int, caseText: String) -> Diagnostic.Message {
+    let adjustSpaces = count < 0 ? "remove" : "add"
+    let ending = abs(count) == 1 ? "" : "s"
+    return Diagnostic.Message(
+      .warning,
+      "\(adjustSpaces) \(abs(count)) space\(ending) as indentation to the case \(caseText)"
+    )
+  }
 }

Diff for: Sources/Rules/Trivia+Convenience.swift

@@ -12,6 +12,17 @@ extension Trivia {
     return count
   }
 
+  /// Returns the number of newlines after this node.
+  var numberOfNewlines: Int {
+    var count = 0
+    for piece in self {
+      if case .newlines(let n) = piece {
+        count += n
+      }
+    }
+    return count
+  }
+
   /// Returns this set of trivia, without any whitespace characters.
   func withoutSpaces() -> Trivia {
     return Trivia(pieces: filter {

Diff for: Tests/SwiftFormatTests/CaseIndentLevelEqualsSwitchTests.swift

@@ -0,0 +1,71 @@
+import Foundation
+import XCTest
+import SwiftSyntax
+
+@testable import Rules
+
+public class CaseIndentLevelEqualsSwitchTests: DiagnosingTestCase {
+  public func testsInvalidCaseIndent() {
+    XCTAssertFormatting(
+      CaseIndentLevelEqualsSwitch.self,
+      input: """
+             switch order {
+              
+             case .ascending:
+               print("Ascending")
+                          case .descending:
+               print("Descending")
+                case .same:
+               print("Same")
+             }
+             """,
+      expected: """
+                switch order {
+                
+                case .ascending:
+                  print("Ascending")
+                case .descending:
+                  print("Descending")
+                case .same:
+                  print("Same")
+                }
+                """)
+  }
+  
+  public func testsInvalidNestedCaseIndent() {
+    XCTAssertFormatting(
+      CaseIndentLevelEqualsSwitch.self,
+      input: """
+             if true {
+               switch order {
+             case .ascending:
+                 print("Ascending")
+                          case .descending:
+                 print("Descending")
+                case .same:
+                 print("Same")
+               }
+             }
+             """,
+      expected: """
+                if true {
+                  switch order {
+                  case .ascending:
+                    print("Ascending")
+                  case .descending:
+                    print("Descending")
+                  case .same:
+                    print("Same")
+                  }
+                }
+                """)
+  }
+  
+  #if !os(macOS)
+  static let allTests = [
+    CaseIndentLevelEqualsSwitchTests.testsInvalidCaseIndent,
+    CaseIndentLevelEqualsSwitchTests.testsInvalidNestedCaseIndent
+    ]
+  #endif
+  
+}