base/containers/span_rust.h

// Copyright 2021 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_CONTAINERS_SPAN_RUST_H_
#define BASE_CONTAINERS_SPAN_RUST_H_

#include <stdint.h>

#include "base/containers/span.h"
#include "build/build_config.h"
#include "third_party/rust/cxx/v1/cxx.h"

namespace base {

// Creates a Rust slice from a span.
inline rust::Slice<const uint8_t> SpanToRustSlice(span<const uint8_t> span) {
  return rust::Slice<const uint8_t>(span.data(), span.size());
}

// Note to future editors: if you add code to convert from a rust::Slice to a
// span, you should be aware that Rust slices (and cxx) return a fabricated
// non-null pointer for zero-length slices, and that this will likely need
// converting to NULL before constructing a span. cxx handles the conversion
// from NULL to an artificial pointer (specifically it uses alignof<T>) in the
// C++ -> Rust direction, but does nothing cunning in the other direction,
// presumably on the assumption that C++ won't access the data pointer if the
// length is 0. But we do not think (reinterpret_cast<T*>(alignof(T)), 0) can
// safely be stored in span today. The pointer arithmetic rules have special
// cases for NULL, but otherwise it is only defined if there is actually an
// object at the address, and there is no object at alignof(T).
// https://eel.is/c++draft/expr.add#4
// https://eel.is/c++draft/expr.add#5

}  // namespace base

#endif  // BASE_CONTAINERS_SPAN_RUST_H_