FileSystem: Backport 3-arg Path::Combine()

src/common/file_system.cpp

@@ -891,6 +891,28 @@ std::string Path::Combine(std::string_view base, std::string_view next)
   return ret;
 }
 
+std::string Path::Combine(std::string_view base, std::string_view subdir, std::string_view next)
+{
+  std::string ret;
+  ret.reserve(base.length() + subdir.length() + next.length() + 2);
+
+  PathAppendString(ret, base);
+  while (!ret.empty() && ret.back() == FS_OSPATH_SEPARATOR_CHARACTER)
+    ret.pop_back();
+
+  ret += FS_OSPATH_SEPARATOR_CHARACTER;
+  PathAppendString(ret, subdir);
+  while (!ret.empty() && ret.back() == FS_OSPATH_SEPARATOR_CHARACTER)
+    ret.pop_back();
+
+  ret += FS_OSPATH_SEPARATOR_CHARACTER;
+  PathAppendString(ret, next);
+  while (!ret.empty() && ret.back() == FS_OSPATH_SEPARATOR_CHARACTER)
+    ret.pop_back();
+
+  return ret;
+}
+
 std::FILE* FileSystem::OpenCFile(const char* path, const char* mode, Error* error)
 {
 #ifdef _WIN32

src/common/path.h

@@ -27,6 +27,7 @@ std::string BuildRelativePath(std::string_view filename, std::string_view new_fi
 
 /// Joins path components together, producing a new path.
 std::string Combine(std::string_view base, std::string_view next);
+std::string Combine(std::string_view base, std::string_view subdir, std::string_view next);
 
 /// Removes all .. and . components from a path.
 std::string Canonicalize(std::string_view path);