Ensure that the engine converts std::filesystem::path objects to UTF-8 strings on Windows (flutter#179528)

Fixes flutter#178896

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: jason-simmons

engine/src/flutter/fml/string_conversion.cc

@@ -44,4 +44,9 @@ std::u16string Utf8ToUtf16(const std::string_view string) {
   return converter.from_bytes(string.data());
 }
 
+std::string PathToUtf8(const std::filesystem::path& path) {
+  const std::u8string path_u8 = path.u8string();
+  return std::string(path_u8.begin(), path_u8.end());
+}
+
 }  // namespace fml

engine/src/flutter/fml/string_conversion.h

@@ -5,6 +5,7 @@
 #ifndef FLUTTER_FML_STRING_CONVERSION_H_
 #define FLUTTER_FML_STRING_CONVERSION_H_
 
+#include <filesystem>
 #include <string>
 #include <vector>
 
@@ -19,6 +20,9 @@ std::string Utf16ToUtf8(const std::u16string_view string);
 // Returns a UTF-16 encoded equivalent of a UTF-8 encoded input string.
 std::u16string Utf8ToUtf16(const std::string_view string);
 
+// Returns the pathname encoded in UTF-8.
+std::string PathToUtf8(const std::filesystem::path& path);
+
 }  // namespace fml
 
 #endif  // FLUTTER_FML_STRING_CONVERSION_H_

engine/src/flutter/fml/string_conversion_unittests.cc

@@ -33,5 +33,10 @@ TEST(StringConversion, Utf16ToUtf8Unicode) {
   EXPECT_EQ(Utf16ToUtf8(u"\x2603"), "\xe2\x98\x83");
 }
 
+TEST(StringConversion, PathToUtf8) {
+  EXPECT_EQ(PathToUtf8(std::filesystem::path("abc")), "abc");
+  EXPECT_EQ(PathToUtf8(std::filesystem::path(u"\x2603")), "\xe2\x98\x83");
+}
+
 }  // namespace testing
 }  // namespace fml

engine/src/flutter/shell/platform/windows/flutter_project_bundle.cc

@@ -7,6 +7,7 @@
 #include <filesystem>
 
 #include "flutter/fml/logging.h"
+#include "flutter/fml/string_conversion.h"
 #include "flutter/shell/platform/common/engine_switches.h"  // nogncheck
 #include "flutter/shell/platform/common/path_utils.h"
 
@@ -73,7 +74,7 @@ UniqueAotDataPtr FlutterProjectBundle::LoadAotData(
                    << "; no such file.";
     return UniqueAotDataPtr(nullptr, nullptr);
   }
-  std::string path_string = aot_library_path_.string();
+  std::string path_string = fml::PathToUtf8(aot_library_path_);
   FlutterEngineAOTDataSource source = {};
   source.type = kFlutterEngineAOTDataSourceTypeElfPath;
   source.elf_path = path_string.c_str();

engine/src/flutter/shell/platform/windows/flutter_windows_engine.cc

@@ -274,8 +274,8 @@ bool FlutterWindowsEngine::Run(std::string_view entrypoint) {
     FML_LOG(ERROR) << "Missing or unresolvable paths to assets.";
     return false;
   }
-  std::string assets_path_string = project_->assets_path().string();
-  std::string icu_path_string = project_->icu_path().string();
+  std::string assets_path_string = fml::PathToUtf8(project_->assets_path());
+  std::string icu_path_string = fml::PathToUtf8(project_->icu_path());
   if (embedder_api_.RunsAOTCompiledDartCode()) {
     aot_data_ = project_->LoadAotData(embedder_api_);
     if (!aot_data_) {

engine/src/flutter/tools/templater/templater_main.cc

@@ -12,6 +12,7 @@
 #include "flutter/fml/file.h"
 #include "flutter/fml/logging.h"
 #include "flutter/fml/mapping.h"
+#include "flutter/fml/string_conversion.h"
 #include "inja/inja.hpp"
 
 namespace flutter {
@@ -50,9 +51,9 @@ bool TemplaterMain(const fml::CommandLine& command_line) {
       reinterpret_cast<const uint8_t*>(rendered_template.data()),
       rendered_template.size()};
 
-  auto current_dir =
-      fml::OpenDirectory(std::filesystem::current_path().string().c_str(),
-                         false, fml::FilePermission::kReadWrite);
+  auto current_dir = fml::OpenDirectory(
+      fml::PathToUtf8(std::filesystem::current_path()).c_str(), false,
+      fml::FilePermission::kReadWrite);
   if (!current_dir.is_valid()) {
     FML_LOG(ERROR) << "Could not open current directory.";
     return false;