Make C++ struct generator type-safe (#35656)

Summary:
Pull Request resolved: #35656

Changelog: [Internal]

## Change:

#35265 added a struct generator - but it is not type safe :-(

E.g. you can write a TM Spec type as:

```
export type CustomType = {
   key: string;
   enabled: boolean;
   time?: number;
 }
```
And a C++ type as:
```
using CustomType = NativeSampleModuleBaseCustomType<float, bool, std::optional<int32_t>>;
template <>
struct Bridging<CustomType>
    : NativeSampleModuleBaseCustomTypeBridging<float, bool, std::optional<int32_t>> {};
```
and it will still compile :-( - which should not.

The reason is that the generated structs don't validate the members type :-(
```
template <typename P0, typename P1, typename P2>
struct NativeSampleModuleBaseCustomType {
  P0 key;
  P1 enabled;
  P2 time;
  bool operator==(const NativeSampleModuleBaseCustomType &other) const {
    return key == other.key && enabled == other.enabled && time == other.time;
  }
};

template <typename P0, typename P1, typename P2>
struct NativeSampleModuleBaseCustomTypeBridging {
  static NativeSampleModuleBaseCustomType<P0, P1, P2> fromJs(
      jsi::Runtime &rt,
      const jsi::Object &value,
      const std::shared_ptr<CallInvoker> &jsInvoker) {
    NativeSampleModuleBaseCustomType<P0, P1, P2> result{
      bridging::fromJs<P0>(rt, value.getProperty(rt, "key"), jsInvoker),
      bridging::fromJs<P1>(rt, value.getProperty(rt, "enabled"), jsInvoker),
      bridging::fromJs<P2>(rt, value.getProperty(rt, "time"), jsInvoker)};
    return result;
  }

  static jsi::Object toJs(
      jsi::Runtime &rt,
      const NativeSampleModuleBaseCustomType<P0, P1, P2> &value) {
    auto result = facebook::jsi::Object(rt);
    result.setProperty(rt, "key", bridging::toJs(rt, value.key));
    result.setProperty(rt, "enabled", bridging::toJs(rt, value.enabled));
    if (value.time) {
      result.setProperty(rt, "time", bridging::toJs(rt, value.time.value()));
    }
    keyToJs(rt, value.key);
    return result;
  }
};
```

This fixes that, by simply emitting conversion functions for each member such as
```
#ifdef DEBUG
  static bool keyToJs(jsi::Runtime &rt, P0 value) {
    return bridging::toJs(rt, value);
  }
  static double enabledToJs(jsi::Runtime &rt, P1 value) {
    return bridging::toJs(rt, value);
  }
  static jsi::String timeToJs(jsi::Runtime &rt, P2 value) {
    return bridging::toJs(rt, value);
  }
#endif
```

Reviewed By: cipolleschi

Differential Revision: D42082423

fbshipit-source-id: 5133f14e2aa8351e9bbbf614117a3d5894b17fa6

packages/react-native-codegen/src/generators/modules/GenerateModuleH.js

@@ -210,6 +210,20 @@ function createStructs(
       const templateParameter = value.properties
         .map((v, i) => 'P' + i)
         .join(', ');
+      const paramemterConversion = value.properties
+        .map((v, i) => {
+          const translatedParam = translatePrimitiveJSTypeToCpp(
+            v.typeAnnotation,
+            false,
+            typeName =>
+              `Unsupported type for param "${v.name}". Found: ${typeName}`,
+            resolveAlias,
+          );
+          return `  static ${translatedParam} ${v.name}ToJs(jsi::Runtime &rt, P${i} value) {
+    return bridging::toJs(rt, value);
+  }`;
+        })
+        .join('\n');
       return `#pragma mark - ${structName}
 
 template <${templateParameterWithTypename}>
@@ -238,25 +252,29 @@ ${value.properties
     return result;
   }
 
+#ifdef DEBUG
+${paramemterConversion}
+#endif
+
   static jsi::Object toJs(
-      jsi::Runtime &rt,
-      const ${structName}<${templateParameter}> &value,
-      const std::shared_ptr<CallInvoker> &jsInvoker) {
-    auto result = facebook::jsi::Object(rt);
-${value.properties
-  .map((v, i) => {
-    if (v.optional) {
-      return `    if (value.${v.name}) {
-      result.setProperty(rt, "${v.name}", bridging::toJs(rt, value.${v.name}.value(), jsInvoker));
-    }`;
-    } else {
-      return `    result.setProperty(rt, "${v.name}", bridging::toJs(rt, value.${v.name}, jsInvoker));`;
-    }
-  })
-  .join('\n')}
-    return result;
-  }
-};
+    jsi::Runtime &rt,
+    const ${structName}<${templateParameter}> &value,
+    const std::shared_ptr<CallInvoker> &jsInvoker) {
+      auto result = facebook::jsi::Object(rt);
+      ${value.properties
+        .map((v, i) => {
+          if (v.optional) {
+            return `    if (value.${v.name}) {
+            result.setProperty(rt, "${v.name}", bridging::toJs(rt, value.${v.name}.value(), jsInvoker));
+          }`;
+          } else {
+            return `    result.setProperty(rt, "${v.name}", bridging::toJs(rt, value.${v.name}, jsInvoker));`;
+          }
+        })
+        .join('\n')}
+          return result;
+        }
+      };
 
 `;
     })