Merge pull request swiftlang#188 from dylansturg/derivative_attr_crash

Handle derivative attr without params.

Author: allevato

Sources/SwiftFormatPrettyPrint/TokenStreamCreator.swift

@@ -2140,7 +2140,8 @@ fileprivate final class TokenStreamCreator: SyntaxVisitor {
  // `@transpose(...)` attribute.
  before(node.ofLabel, tokens: .open)
  after(node.colon, tokens: .break(.continue, newlines: .elective(ignoresDiscretionary: true)))
- after(node.comma, tokens: .close)
+ // The comma after originalDeclName is optional and is only present if there are diffParams.
+ after(node.comma ?? node.originalDeclName.lastToken, tokens: .close)
 
  if let diffParams = node.diffParams {
  before(diffParams.firstToken, tokens: .break(.same), .open)

Tests/SwiftFormatPrettyPrintTests/DifferentiationAttributeTests.swift

@@ -99,6 +99,9 @@ final class DifferentiationAttributeTests: PrettyPrintTestCase {
  #if HAS_DERIVATIVE_REGISTRATION_ATTRIBUTE
  let input =
  """
+ @derivative(of: foo)
+ func deriv<T>() {}
+
  @derivative(of: foo, wrt: x)
  func deriv<T>(_ x: T) {}
 
@@ -111,6 +114,9 @@ final class DifferentiationAttributeTests: PrettyPrintTestCase {
 
  let expected =
  """
+ @derivative(of: foo)
+ func deriv<T>() {}
+
  @derivative(of: foo, wrt: x)
  func deriv<T>(_ x: T) {}