Implement return void instead of empty tuple (swiftlang#60)

Author: LaurenWhite

tools/swift-format/Sources/Rules/ReturnVoidInsteadOfEmptyTuple.swift

@@ -13,5 +13,19 @@ import SwiftSyntax
 ///
 /// - SeeAlso: https://google.github.io/swift#types-with-shorthand-names
 public final class ReturnVoidInsteadOfEmptyTuple: SyntaxFormatRule {
+  public override func visit(_ node: FunctionTypeSyntax) -> TypeSyntax {
+    guard let returnType = node.returnType as? TupleTypeSyntax,
+      returnType.elements.count == 0 else { return node }
+    diagnose(.returnVoid, on: node.returnType)
+    let voidKeyword =  SyntaxFactory.makeSimpleTypeIdentifier(
+      name: SyntaxFactory.makeIdentifier(
+        "Void",
+        trailingTrivia: returnType.rightParen.trailingTrivia),
+      genericArgumentClause: nil)
+    return node.withReturnType(voidKeyword)
+  }
+}
 
+extension Diagnostic.Message {
+  static let returnVoid = Diagnostic.Message(.warning, "replace '()' with 'Void'")
 }

tools/swift-format/Tests/SwiftFormatTests/ReturnVoidInsteadOfEmptyTupleTests.swift

@@ -0,0 +1,30 @@
+import Foundation
+import XCTest
+import SwiftSyntax
+
+@testable import Rules
+
+public class ReturnVoidInsteadOfEmptyTupleTests: DiagnosingTestCase {
+  public func testEmptyTupleReturns() {
+    XCTAssertFormatting(
+      ReturnVoidInsteadOfEmptyTuple.self,
+      input: """
+             let callback: () -> ()
+             typealias x = Int -> ()
+             func y() -> Int -> () { return }
+             func z(d: Bool -> ()) {}
+             """,
+      expected: """
+                let callback: () -> Void
+                typealias x = Int -> Void
+                func y() -> Int -> Void { return }
+                func z(d: Bool -> Void) {}
+                """)
+  }
+
+  #if !os(macOS)
+  static let allTests = [
+    ReturnVoidInsteadOfEmptyTupleTests.testEmptyTupleReturns,
+    ]
+  #endif
+}