protobuf/rust/internal.rs

// Protocol Buffers - Google's data interchange format
// Copyright 2023 Google Inc.  All rights reserved.
// https://developers.google.com/protocol-buffers/
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
//     * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
//     * Redistributions in binary form must reproduce the above
// copyright notice, this list of conditions and the following disclaimer
// in the documentation and/or other materials provided with the
// distribution.
//     * Neither the name of Google Inc. nor the names of its
// contributors may be used to endorse or promote products derived from
// this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

//! Kernel-agnostic logic for the Rust Protobuf runtime that should not be
//! exposed to through the `protobuf` path but must be public for use by
//! generated code.

use std::slice;

/// Represents an ABI-stable version of `NonNull<[u8]>`/`string_view` (a
/// borrowed slice of bytes) for FFI use only.
///
/// Has semantics similar to `std::string_view` in C++ and `&[u8]` in Rust,
/// but is not ABI-compatible with either.
///
/// If `len` is 0, then `ptr` can be null or dangling. C++ considers a dangling
/// 0-len `std::string_view` to be invalid, and Rust considers a `&[u8]` with a
/// null data pointer to be invalid.
#[repr(C)]
#[derive(Copy, Clone)]
pub struct PtrAndLen {
    /// Pointer to the first byte.
    /// Borrows the memory.
    pub ptr: *const u8,

    /// Length of the `[u8]` pointed to by `ptr`.
    pub len: usize,
}

impl PtrAndLen {
    /// Unsafely dereference this slice.
    ///
    /// # Safety
    /// - `ptr` must be valid for `len` bytes. It can be null or dangling if
    ///   `self.len == 0`.
    pub unsafe fn as_ref<'a>(self) -> &'a [u8] {
        if self.ptr.is_null() {
            assert_eq!(self.len, 0, "Non-empty slice with null data pointer");
            &[]
        } else {
            slice::from_raw_parts(self.ptr, self.len)
        }
    }
}