rust/cpp_kernel/strings.h

// Protocol Buffers - Google's data interchange format
// Copyright 2024 Google LLC.  All rights reserved.
//
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file or at
// https://developers.google.com/open-source/licenses/bsd

#ifndef GOOGLE_PROTOBUF_RUST_CPP_KERNEL_STRINGS_H__
#define GOOGLE_PROTOBUF_RUST_CPP_KERNEL_STRINGS_H__

#include <cstddef>
#include <cstring>
#include <string>
#include <type_traits>

#include "absl/strings/string_view.h"

namespace google {
namespace protobuf {
namespace rust {

// Represents an ABI-stable version of &[u8]/string_view (borrowed slice of
// bytes) for FFI use only.
//
// Note that the intent is for either Rust or C++ to construct one of these with
// the pointer that they would have from a Rust slice or C++ string_view.
// Notably this means that if len==0, ptr may be any value, including nullptr or
// an invalid pointer, which may be a value incompatible for use with either a
// Rust slice or C++ string_view.
//
// It may be constructed trivially, but use the provided conversion methods
// when converting from this type into any other type to avoid obscure undefined
// behavior.
struct PtrAndLen {
  /// Borrows the memory.
  const char* ptr;
  size_t len;

  std::string CopyToString() const;
  absl::string_view AsStringView() const;
  void PlacementNewString(void* location);
};

// Represents an owned string for FFI purposes.
//
// This must only be used to transfer a string from C++ to Rust. The
// below invariants must hold:
//   * Rust and C++ versions of this struct are ABI compatible.
//   * The data were allocated using the Rust allocator and are 1 byte aligned.
//   * The data is valid UTF-8.
//
// This type is returned by value from `extern "C"` functions, so it must not
// declare any constructors: the Microsoft C++ ABI returns a class with a
// user-provided constructor through a hidden pointer, whereas Rust follows the
// C rules for the equivalent `#[repr(C)]` struct. On 32-bit x86 and ARM64
// Windows these two conventions differ.
struct RustStringRawParts {
  // Owns the memory.
  const char* data;
  size_t len;

  // Copies src into a buffer allocated with the Rust allocator.
  static RustStringRawParts CopyFrom(absl::string_view src);
};

// These types cross the FFI boundary by value, so C++ must pass and return them
// the same way C would. The checks below reject constructors, non-public data
// members and virtual functions. They cannot reject a public base class, which
// the Microsoft C++ ABI also returns through a hidden pointer, so do not add
// one.
static_assert(std::is_aggregate_v<PtrAndLen> &&
              std::is_trivially_copyable_v<PtrAndLen> &&
              std::is_standard_layout_v<PtrAndLen>);
static_assert(std::is_aggregate_v<RustStringRawParts> &&
              std::is_trivially_copyable_v<RustStringRawParts> &&
              std::is_standard_layout_v<RustStringRawParts>);

}  // namespace rust
}  // namespace protobuf
}  // namespace google

extern "C" {

// Allocates a new std::string on the C++ heap and returns a pointer to it.
std::string* proto2_rust_cpp_new_string(google::protobuf::rust::PtrAndLen src);

// Deletes a std::string object from the C++ heap.
void proto2_rust_cpp_delete_string(std::string* str);

// Obtain a PtrAndLen, the FFI-safe view type, from a std::string.
google::protobuf::rust::PtrAndLen proto2_rust_cpp_string_to_view(std::string* str);
}

#endif  // GOOGLE_PROTOBUF_RUST_CPP_KERNEL_STRINGS_H__