[pigeon]fix "as Any" workaround due to nested optional (#3658)

## The problem

This PR fixes a weird casting behavior discussed [here](#3545 (comment)): 
```
private func nilOrValue<T>(_ value: Any?) -> T? {
  if value is NSNull { return nil }
  return (value as Any) as! T?  // <- HERE
}
```
Without this intermediate `as Any` cast, [these 3 tests](https://github.com/flutter/packages/blob/5662a7e5799c723f76e9589a75c9d9310e2ba8c1/packages/pigeon/platform_tests/test_plugin/example/ios/RunnerTests/RunnerTests.swift#L10-L29) would crash with `SIGABRT: Could not cast value of type 'Swift.Optional<Any>' (0x7ff865e84b38) to 'Swift.String' (0x7ff865e7de08).`

## Investigation

The crash happens because `value` here is actually of type `Any??` (nested optional!). When it crashes, the debugger simply shows `value` is `nil`. But if we print in `lldb`, the `value` here is actually an inner `Optional.none` case nested by an outer `Optional.some` case. 

### Why does `Any??` crash

Since outer case is `some`, it fails to force cast to `T?` (e.g. `String?`) due to type mismatch. 

### How did we end up with `Any??`

It's related to the signature of these 3 functions: 

- `func toList() -> [Any?]`
- `func fromList(args: [Any])`
- `func nilOrValue<T>(_ value: Any?) -> T?`

Firstly `toList` returns `nil` (of type `Any?`) as the first element of array. Then the type gets coerced as an `Any` type in `fromList`. Then because `nilOrValue` takes `Any?`, this `nil` value gets wrapped by an `Optional.some`. Hence the nested optional. 

## Workarounds

### Workaround 1: `as Any`

This is the current code [in this PR](https://github.com/flutter/packages/pull/3545/files#r1155061282). When casting `Optional.some(nil) as Any`, it erases the outer Optional, so no problem casting to `T?`. 
 
### Workaround 2: 

Handle with nested optional directly: 

```
private func nilOrValue<T>(_ value: Any?) -> T? {
  if value is NSNull { return nil }

  // `if let` deals with "outer some" case and then erase the outer Optional
  if let val = value { 
    // here returns "outer some + inner some" or "outer some + inner none"
    return val as! T?
  }
  // here returns "outer none"
  return nil 
}
```

A similar version of this was also [attempted in this PR](https://github.com/flutter/packages/pull/3545/files/241f0e31e32917f5501dab11f81ab0fbf064687f#diff-bfdb6a91beb03a906435e77e0168117f3f3977ee4d6f8bcaa1724156ae4dc27cR647-R650). It just that we did not know why that worked previously, and now we know! 

### Workaround 3

Casting value to nested optional (`T??`), then stripe the outer optional

```
private func nilOrValue<T>(_ value: Any?) -> T? {
  if value is NSNull { return nil }
  return (value as! T??) ?? nil
}
```

## Solutions

These above workarounds handle nested optionals. However, **a real solution should prevent nested optionals from happening in the first place**, since they are so tricky. 

### Solution 1 (This PR)

The nested optional happens when we do cast from `Any?` to `Any` and then wrapped into `Any?`. (Refer to "How did we end up with Any??" section). 

So the easiest way is just to use `func fromList(args: [Any?])` to match the types of `func toList` and `func nilOrValue`. 

### Solution 2

Solution 2 is the opposite - avoid using `Any?` as much as possible. 

Drawbacks compare to Solution 1: 
a. When inter-op with ObjC, `nullable id` is exported as `Any?`. So we can't 100% prevent `Any?` usage. Though this can be addressed by immediately cast it to `Any`. 
b. Losing of semantic meaning of `Any?` that it <s>can</s> must be optional. The hidden/implicit optional **is** the culprit here in the first place. 
c. While this solution fixes the nested optional issue, it does not address all issues related to implicit optional. For example: 

https://github.com/flutter/packages/blob/c53db71f496b436e48629a8f3e4152c48e63cd66/packages/pigeon/platform_tests/test_plugin/ios/Classes/CoreTests.gen.swift#L563-L564

This is supposed to crash if `args[0]` is `nil`. However, the crash is silenced because `as! [Any]` will make `args[0]` an implicit optional!

The correct codegen should instead be: 
```
let args = message as! [Any?]
let anObjectArg = args[0]!
```

### Solution 3

Just remove `as Any` and update the test. 

The nested optional won't happen in production code, because ObjC `NSArray` contains `NSNull` rather than `nil` when exporting to Swift. 

We can simply fix [the tests](https://github.com/flutter/packages/blob/5662a7e5799c723f76e9589a75c9d9310e2ba8c1/packages/pigeon/platform_tests/test_plugin/example/ios/RunnerTests/RunnerTests.swift#L10-L29) by replacing `nil`s with `NSNull`s. 

However, if we were to re-write engine's codec to Swift, it's actually better practice to use `nil` and not `NSNull` in the array. 

## Additional TODO

We would've caught this earlier if this were an error rather than warning in our unit test. 

![Screenshot 2023-04-06 at 4 15 07 PM](https://user-images.githubusercontent.com/41930132/230510477-1505f830-2fc5-4a4d-858f-e658729fa7bf.png)

*List which issues are fixed by this PR. You must list at least one issue.*

#3545 (comment)

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*

Author: hellohuanlin

packages/pigeon/CHANGELOG.md

@@ -1,5 +1,10 @@
-## NEXT
+## 10.0.0
 
+* [swift] Avoids using `Any` to represent `Optional` in Swift.
+* [swift] **Breaking Change** A raw `List` (without generic type argument) in Dart will be 
+  translated into `[Any?]` (rather than `[Any]`) in Swift.
+* [swift] **Breaking Change** A raw `Map` (without generic type argument) in Dart will be 
+  translated into `[AnyHashable:Any?]` (rather than `[AnyHashable:Any]`) in Swift. 
 * Adds an example application that uses Pigeon directly, rather than in a plugin.
 
 ## 9.2.5

packages/pigeon/lib/generator_tools.dart

@@ -11,7 +11,7 @@ import 'ast.dart';
 /// The current version of pigeon.
 ///
 /// This must match the version in pubspec.yaml.
-const String pigeonVersion = '9.2.5';
+const String pigeonVersion = '10.0.0';
 
 /// Read all the content from [stdin] to a String.
 String readStdin() {

packages/pigeon/lib/swift_generator.dart

@@ -169,7 +169,7 @@ import FlutterMacOS
     Set<String> customEnumNames,
   ) {
     final String className = klass.name;
-    indent.write('static func fromList(_ list: [Any]) -> $className? ');
+    indent.write('static func fromList(_ list: [Any?]) -> $className? ');
 
     indent.addScoped('{', '}', () {
       enumerate(getFieldsInSerializationOrder(klass),
@@ -431,7 +431,7 @@ import FlutterMacOS
             indent.addScoped('{ $messageVarName, reply in', '}', () {
               final List<String> methodArgument = <String>[];
               if (components.arguments.isNotEmpty) {
-                indent.writeln('let args = message as! [Any]');
+                indent.writeln('let args = message as! [Any?]');
                 enumerate(components.arguments,
                     (int index, _SwiftFunctionArgument arg) {
                   final String argName =
@@ -524,7 +524,7 @@ import FlutterMacOS
               indent.writeln('case ${customClass.enumeration}:');
               indent.nest(1, () {
                 indent.writeln(
-                    'return ${customClass.name}.fromList(self.readValue() as! [Any])');
+                    'return ${customClass.name}.fromList(self.readValue() as! [Any?])');
               });
             }
             indent.writeln('default:');
@@ -605,8 +605,7 @@ import FlutterMacOS
             'nullable enums require special code that this helper does not supply');
         return '${_swiftTypeForDartType(type)}(rawValue: $value as! Int)!';
       } else if (type.baseName == 'Object') {
-        // Special-cased to avoid warnings about using 'as' with Any.
-        return value;
+        return value + (type.isNullable ? '' : '!');
       } else if (type.baseName == 'int') {
         if (type.isNullable) {
           // Nullable ints need to check for NSNull, and Int32 before casting can be done safely.
@@ -628,7 +627,7 @@ import FlutterMacOS
       if (listEncodedClassNames != null &&
           listEncodedClassNames.contains(type.baseName)) {
         indent.writeln('var $variableName: $fieldType? = nil');
-        indent.write('if let ${variableName}List = $value as! [Any]? ');
+        indent.write('if let ${variableName}List = $value as! [Any?]? ');
         indent.addScoped('{', '}', () {
           indent.writeln(
               '$variableName = $fieldType.fromList(${variableName}List)');
@@ -652,7 +651,7 @@ import FlutterMacOS
       if (listEncodedClassNames != null &&
           listEncodedClassNames.contains(type.baseName)) {
         indent.writeln(
-            'let $variableName = $fieldType.fromList($value as! [Any])!');
+            'let $variableName = $fieldType.fromList($value as! [Any?])!');
       } else {
         indent.writeln(
             'let $variableName = ${castForceUnwrap(value, type, root)}');
@@ -695,7 +694,7 @@ import FlutterMacOS
 
 private func nilOrValue<T>(_ value: Any?) -> T? {
   if value is NSNull { return nil }
-  return (value as Any) as! T?
+  return value as! T?
 }''');
   }
 
@@ -739,9 +738,9 @@ String _flattenTypeArguments(List<TypeDeclaration> args) {
 String _swiftTypeForBuiltinGenericDartType(TypeDeclaration type) {
   if (type.typeArguments.isEmpty) {
     if (type.baseName == 'List') {
-      return '[Any]';
+      return '[Any?]';
     } else if (type.baseName == 'Map') {
-      return '[AnyHashable: Any]';
+      return '[AnyHashable: Any?]';
     } else {
       return 'Any';
     }

packages/pigeon/platform_tests/test_plugin/example/ios/RunnerTests/EchoBinaryMessenger.swift

@@ -28,13 +28,14 @@ class EchoBinaryMessenger: NSObject, FlutterBinaryMessenger {
 
     guard
       let args = self.codec.decode(message) as? [Any?],
-      let firstArg: Any? = nilOrValue(args.first)
+      let firstArg = args.first,
+      let castedFirstArg: Any? = nilOrValue(firstArg)
     else {
       callback(self.defaultReturn.flatMap { self.codec.encode($0) })
       return
     }
 
-    callback(self.codec.encode(firstArg))
+    callback(self.codec.encode(castedFirstArg))
   }
 
   func setMessageHandlerOnChannel(

packages/pigeon/platform_tests/test_plugin/example/ios/RunnerTests/PrimitiveTests.swift

@@ -11,8 +11,8 @@ class MockPrimitiveHostApi: PrimitiveHostApi {
   func aBool(value: Bool) -> Bool { value }
   func aString(value: String) -> String { value  }
   func aDouble(value: Double) -> Double { value }
-  func aMap(value: [AnyHashable: Any]) -> [AnyHashable: Any] { value }
-  func aList(value: [Any]) -> [Any] { value }
+  func aMap(value: [AnyHashable: Any?]) -> [AnyHashable: Any?] { value }
+  func aList(value: [Any?]) -> [Any?] { value }
   func anInt32List(value: FlutterStandardTypedData) -> FlutterStandardTypedData { value }
   func aBoolList(value: [Bool?]) -> [Bool?] { value }
   func aStringIntMap(value: [String?: Int64?]) -> [String?: Int64?] { value }