protobuf/rust/internal.rs

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//! 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;

/// Used to protect internal-only items from being used accidentally.
pub struct Private;

/// 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
    /// - `self.ptr` must be dereferencable and immutable for `self.len` bytes
    ///   for the lifetime `'a`. 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 {
            // SAFETY:
            // - `ptr` is non-null
            // - `ptr` is valid for `len` bytes as promised by the caller.
            unsafe { slice::from_raw_parts(self.ptr, self.len) }
        }
    }
}