Fix @differentiable attribute formatting.

- Don't insert a space before `where` if it's the first thing
  in the attribute.
- Apply formatting to comma-delimited parameter "tuples"
  following the `wrt:` clause.

Fixes SR-12414.

Author: allevato

Sources/SwiftFormatPrettyPrint/TokenStreamCreator.swift

@@ -2020,16 +2020,27 @@ fileprivate final class TokenStreamCreator: SyntaxVisitor {
 
   override func visit(_ node: DifferentiableAttributeArgumentsSyntax) -> SyntaxVisitorContinueKind {
     // This node encapsulates the entire list of arguments in a `@differentiable(...)` attribute.
+    after(node.diffParamsComma, tokens: .break(.same))
+
+    var needsBreakBeforeWhereClause = false
+
     if let vjp = node.maybeVJP {
-      before(vjp.firstToken, tokens: .break(.same), .open)
+      before(vjp.firstToken, tokens: .open)
       after(vjp.lastToken, tokens: .close)
+      after(vjp.trailingComma, tokens: .break(.same))
+      needsBreakBeforeWhereClause = true
     }
     if let jvp = node.maybeJVP {
-      before(jvp.firstToken, tokens: .break(.same), .open)
+      before(jvp.firstToken, tokens: .open)
       after(jvp.lastToken, tokens: .close)
+      after(jvp.trailingComma, tokens: .break(.same))
+      needsBreakBeforeWhereClause = true
     }
     if let whereClause = node.whereClause {
-      before(whereClause.firstToken, tokens: .break(.same), .open)
+      if needsBreakBeforeWhereClause {
+        before(whereClause.firstToken, tokens: .break(.same))
+      }
+      before(whereClause.firstToken, tokens: .open)
       after(whereClause.lastToken, tokens: .close)
     }
     return .visitChildren
@@ -2044,6 +2055,17 @@ fileprivate final class TokenStreamCreator: SyntaxVisitor {
     return .visitChildren
   }
 
+  override func visit(_ node: DifferentiationParamsSyntax) -> SyntaxVisitorContinueKind {
+    after(node.leftParen, tokens: .break(.open, size: 0), .open)
+    before(node.rightParen, tokens: .break(.close, size: 0), .close)
+    return .visitChildren
+  }
+
+  override func visit(_ node: DifferentiationParamSyntax) -> SyntaxVisitorContinueKind {
+    after(node.trailingComma, tokens: .break(.same))
+    return .visitChildren
+  }
+
   // `DerivativeRegistrationAttributeArguments` was added after the Swift 5.2 release was cut.
   #if HAS_DERIVATIVE_REGISTRATION_ATTRIBUTE
     override func visit(_ node: DerivativeRegistrationAttributeArgumentsSyntax)

Tests/SwiftFormatPrettyPrintTests/DifferentiationAttributeTests.swift

@@ -43,6 +43,68 @@ final class DifferentiationAttributeTests: PrettyPrintTestCase {
     assertPrettyPrintEqual(input: input, expected: expected, linelength: 43)
   }
 
+  func testDifferentiableWithOnlyWhereClause() {
+    let input =
+      """
+      @differentiable(where T: D)
+      func foo<T>(_ x: T) -> T {}
+
+      @differentiable(where T: Differentiable)
+      func foo<T>(_ x: T) -> T {}
+      """
+
+    let expected =
+      """
+      @differentiable(where T: D)
+      func foo<T>(_ x: T) -> T {}
+
+      @differentiable(
+        where T: Differentiable
+      )
+      func foo<T>(_ x: T) -> T {}
+
+      """
+
+    assertPrettyPrintEqual(input: input, expected: expected, linelength: 28)
+  }
+
+  func testDifferentiableWithMultipleParameters() {
+    let input =
+      """
+      @differentiable(wrt: (x, y))
+      func foo<T>(_ x: T) -> T {}
+
+      @differentiable(wrt: (self, x, y))
+      func foo<T>(_ x: T) -> T {}
+
+      @differentiable(wrt: (theVariableNamedSelf, theVariableNamedX, theVariableNamedY))
+      func foo<T>(_ x: T) -> T {}
+      """
+
+    let expected =
+      """
+      @differentiable(wrt: (x, y))
+      func foo<T>(_ x: T) -> T {}
+
+      @differentiable(
+        wrt: (self, x, y)
+      )
+      func foo<T>(_ x: T) -> T {}
+
+      @differentiable(
+        wrt: (
+          theVariableNamedSelf,
+          theVariableNamedX,
+          theVariableNamedY
+        )
+      )
+      func foo<T>(_ x: T) -> T {}
+
+      """
+
+    assertPrettyPrintEqual(input: input, expected: expected, linelength: 28)
+  }
+
   func testDerivative() {
     #if HAS_DERIVATIVE_REGISTRATION_ATTRIBUTE
       let input =

Tests/SwiftFormatPrettyPrintTests/XCTestManifests.swift

@@ -209,6 +209,8 @@ extension DifferentiationAttributeTests {
     static let __allTests__DifferentiationAttributeTests = [
         ("testDerivative", testDerivative),
         ("testDifferentiable", testDifferentiable),
+        ("testDifferentiableWithMultipleParameters", testDifferentiableWithMultipleParameters),
+        ("testDifferentiableWithOnlyWhereClause", testDifferentiableWithOnlyWhereClause),
         ("testTranspose", testTranspose),
     ]
 }