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use core::marker::PhantomData;
use core::mem::{align_of, size_of, transmute};
use core::ptr;
use crate::traits::ZeroCopy;
/// A struct padder as returned from [`BufMut::store_struct`].
///
/// This knows how to find and initialize padding regions in `repr(C)` types,
/// and provides a builder-like API to doing so.
///
/// [`BufMut::store_struct`]: crate::buf::BufMut::store_struct
#[must_use = "For the writer to have an effect on `AlignedBuf` you must call `Padder::finish`"]
pub struct Padder<'a, T> {
ptr: *mut u8,
offset: usize,
_marker: PhantomData<&'a mut T>,
}
impl<'a, T> Padder<'a, T>
where
T: ZeroCopy,
{
#[inline]
pub(crate) fn new(ptr: *mut u8) -> Self {
Self {
ptr,
offset: 0,
_marker: PhantomData,
}
}
/// Indicate that this validate is transparent over `U`.
//
/// # Safety
///
/// This is only allowed if `T` is `#[repr(transparent)]` over `U`.
#[inline]
pub unsafe fn transparent<U>(&mut self) -> &mut Padder<'a, U> {
transmute(self)
}
/// Pad around the given field with zeros.
///
/// Note that this is necessary to do correctly in order to satisfy the
/// safety requirements by [`end()`].
///
/// This is typically not called directly, but rather is implemented by the
/// [`ZeroCopy`] derive.
///
/// [`end()`]: Self::end
/// [`ZeroCopy`]: derive@crate::ZeroCopy
///
/// # Safety
///
/// The caller must ensure that the field type `F` is an actual field in
/// order in the struct being padded.
///
/// # Examples
///
/// ```
/// use musli_zerocopy::{AlignedBuf, ZeroCopy};
/// use musli_zerocopy::buf::BufMut;
///
/// #[derive(Debug, PartialEq, Eq, ZeroCopy)]
/// #[repr(C)]
/// struct ZeroPadded(u8, u16);
///
/// let padded = ZeroPadded(0x01u8.to_be(), 0x0203u16.to_be());
///
/// let mut buf = AlignedBuf::new();
///
/// // SAFETY: We do not pad beyond known fields and are
/// // making sure to initialize all of the buffer.
/// unsafe {
/// let mut w = buf.store_struct(&padded);
/// w.pad::<u8>(&padded.0);
/// w.pad::<u16>(&padded.1);
/// w.end();
/// }
///
/// // Note: The bytes are explicitly convert to big-endian encoding above.
/// assert_eq!(buf.as_slice(), &[1, 0, 2, 3]);
/// # Ok::<_, musli_zerocopy::Error>(())
/// ```
#[inline]
pub unsafe fn pad<F>(&mut self, field: *const F)
where
F: ZeroCopy,
{
self.pad_with(field, align_of::<F>());
}
/// Pad around the given field with zeros using a custom alignment `align`.
///
/// Note that this is necessary to do correctly in order to satisfy the
/// safety requirements by [`end()`].
///
/// This is typically not called directly, but rather is implemented by the
/// [`ZeroCopy`] derive.
///
/// [`end()`]: Self::end
/// [`ZeroCopy`]: derive@crate::ZeroCopy
///
/// # Safety
///
/// The caller must ensure that the field type `F` is an actual field in
/// order in the struct being padded and that `align` matches the argument
/// provided to `#[repr(packed)]` (note that empty means 1).
///
/// # Examples
///
/// ```
/// use core::ptr;
/// use musli_zerocopy::{AlignedBuf, ZeroCopy};
/// use musli_zerocopy::buf::BufMut;
///
/// #[derive(Debug, PartialEq, Eq, ZeroCopy)]
/// #[repr(C, packed(2))]
/// struct ZeroPadded(u8, u32);
///
/// let padded = ZeroPadded(0x01u8.to_be(), 0x02030405u32.to_be());
///
/// let mut buf = AlignedBuf::new();
///
/// // SAFETY: We do not pad beyond known fields and are
/// // making sure to initialize all of the buffer.
/// unsafe {
/// let mut w = buf.store_struct(&padded);
/// w.pad_with::<u8>(ptr::addr_of!(padded.0), 2);
/// w.pad_with::<u32>(ptr::addr_of!(padded.1), 2);
/// w.end();
/// }
///
/// // Note: The bytes are explicitly convert to big-endian encoding above.
/// assert_eq!(buf.as_slice(), &[1, 0, 2, 3, 4, 5]);
/// # Ok::<_, musli_zerocopy::Error>(())
/// ```
#[inline]
pub unsafe fn pad_with<F>(&mut self, field: *const F, align: usize)
where
F: ZeroCopy,
{
let count = crate::buf::padding_to(self.offset, align);
// zero out padding.
ptr::write_bytes(self.ptr.add(self.offset), 0, count);
self.offset = self.offset.wrapping_add(count);
if F::PADDED {
let mut padder = Padder::new(self.ptr.wrapping_add(self.offset));
F::pad(field, &mut padder);
padder.end();
}
self.offset = self.offset.wrapping_add(size_of::<F>());
}
/// Only pad a field where the value of the field doesn't matter.
///
/// # Safety
///
/// The caller must ensure that the field type `F` is an actual field in
/// order in the struct being padded and that `F` is a primitive that does
/// not contain any interior padding.
#[inline]
pub unsafe fn pad_primitive<F>(&mut self)
where
F: ZeroCopy,
{
let count = crate::buf::padding_to(self.offset, align_of::<F>());
// zero out padding.
ptr::write_bytes(self.ptr.add(self.offset), 0, count);
self.offset = self.offset.wrapping_add(count.wrapping_add(size_of::<F>()));
}
/// Finish writing the current buffer.
///
/// This is typically not called directly, but rather is implemented by the
/// [`ZeroCopy`] derive.
///
/// [`ZeroCopy`]: derive@crate::ZeroCopy
///
/// # Safety
///
/// Before calling `end()`, the caller must ensure that they've called
/// [`pad::<F>()`] *in order* for every field in a struct being serialized
/// where `F` is the type of the field. Otherwise we might not have written
/// the necessary padding to ensure that all bytes related to the struct are
/// initialized. Failure to do so would result in undefined behavior.
///
/// Fields which are [`ZeroSized`] can be skipped.
///
/// [`pad::<F>()`]: Self::pad
/// [`ZeroSized`]: crate::traits::ZeroSized
///
/// # Examples
///
/// ```
/// use musli_zerocopy::{AlignedBuf, ZeroCopy};
/// use musli_zerocopy::buf::BufMut;
/// use musli_zerocopy::pointer::Ref;
///
/// #[derive(Debug, PartialEq, Eq, ZeroCopy)]
/// #[repr(C)]
/// struct ZeroPadded(u8, u16);
///
/// let padded = ZeroPadded(0x01u8.to_be(), 0x0203u16.to_be());
///
/// let mut buf = AlignedBuf::new();
///
/// let reference = Ref::<ZeroPadded>::new(buf.next_offset::<ZeroPadded>());
///
/// // SAFETY: We do not pad beyond known fields and are
/// // making sure to initialize all of the buffer.
/// unsafe {
/// let mut w = buf.store_struct(&padded);
/// w.pad(&padded.0);
/// w.pad(&padded.1);
/// w.end();
/// }
///
/// // Note: The bytes are explicitly convert to big-endian encoding above.
/// assert_eq!(buf.as_slice(), &[1, 0, 2, 3]);
///
/// let buf = buf.as_aligned();
///
/// assert_eq!(buf.load(reference)?, &padded);
/// # Ok::<_, musli_zerocopy::Error>(())
/// ```
#[inline]
pub unsafe fn end(self) {
let count = size_of::<T>() - self.offset;
ptr::write_bytes(self.ptr.wrapping_add(self.offset), 0, count);
}
}