Implement only one trailing closure argument (swiftlang#85)

Author: LaurenWhite

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

@@ -10,4 +10,15 @@ import SwiftSyntax
 /// - SeeAlso: https://google.github.io/swift#trailing-closures
 public final class OnlyOneTrailingClosureArgument: SyntaxLintRule {
 
+  public override func visit(_ node: FunctionCallExprSyntax) {
+    guard (node.argumentList.contains { $0.expression is ClosureExprSyntax }) else { return }
+    guard node.trailingClosure != nil else { return }
+    diagnose(.removeTrailingClosure, on: node)
+  }
+}
+
+extension Diagnostic.Message {
+  static let removeTrailingClosure =
+    Diagnostic.Message(.warning,
+                      "function call shouldn't have both closure arguments and a trailing closure")
 }

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

@@ -0,0 +1,30 @@
+import Foundation
+import SwiftSyntax
+import XCTest
+
+@testable import Rules
+
+public class OnlyOneTrailingClosureArgumentTests: DiagnosingTestCase {
+  public func testInvalidTrailingClosureCall() {
+    let input =
+      """
+      callWithBoth(someClosure: {}) {
+        // ...
+      }
+      callWithClosure(someClosure: {})
+      callWithTrailingClosure {
+        // ...
+      }
+      """
+    performLint(OnlyOneTrailingClosureArgument.self, input: input)
+    XCTAssertDiagnosed(.removeTrailingClosure)
+    XCTAssertNotDiagnosed(.removeTrailingClosure)
+    XCTAssertNotDiagnosed(.removeTrailingClosure)
+  }
+
+  #if !os(macOS)
+  static let allTests = [
+    OnlyOneTrailingClosureArgumentTests.testInvalidTrailingClosureCall,
+    ]
+  #endif
+}