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Add borrow capability constant to Reader #243

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Merged
cpubot merged 1 commit into from
Mar 19, 2026
Merged

Add borrow capability constant to Reader #243

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2 changes: 2 additions & 0 deletions wincode/src/io/cursor.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -141,6 +141,8 @@ impl<'a, T> Reader<'a> for Cursor<T>
where
T: AsRef<[u8]>,
{
const BORROW_KINDS: u8 = BorrowKind::CallSite.mask();

#[inline]
fn copy_into_slice(&mut self, dst: &mut [MaybeUninit<u8>]) -> ReadResult<()> {
let src = self.advance_slice_checked(dst.len())?;
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41 changes: 40 additions & 1 deletion wincode/src/io/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -8,7 +8,8 @@ use {
thiserror::Error,
};

#[derive(Debug)]
#[derive(Debug, Clone, Copy)]
#[repr(u8)]
pub enum BorrowKind {
/// Borrowed from the call site, not extending past it.
CallSite,
Expand All @@ -18,6 +19,13 @@ pub enum BorrowKind {
BackingMut,
}

impl BorrowKind {
#[inline]
pub const fn mask(self) -> u8 {
1u8 << (self as u8)
}
}

impl core::fmt::Display for BorrowKind {
fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
match self {
Expand Down Expand Up @@ -62,6 +70,35 @@ pub(super) const fn transpose<const N: usize, T>(
/// compatible with readers that don't support borrowing, if possible.
/// - Returns [`ReadError::UnsupportedBorrow`] for readers that do not support borrowing.
pub trait Reader<'a> {
/// Borrow capabilities of this reader.
///
/// A bitmask of [`BorrowKind`] values indicating which kinds of borrows are supported.
///
/// Users of [`Reader`] can call [`Reader::supports_borrow`] to check if a borrow kind
/// is supported.
const BORROW_KINDS: u8 = 0;

/// Checks if this reader supports the given borrow kind.
///
/// # Examples
/// ```
/// # use wincode::io::{Reader, BorrowKind, Cursor};
/// #
/// let reader = [1, 2, 3, 4, 5];
/// assert!(reader.as_slice().supports_borrow(BorrowKind::Backing));
///
/// let mut reader = [1, 2, 3, 4, 5];
/// assert!(reader.as_mut_slice().supports_borrow(BorrowKind::BackingMut));
///
/// let reader = Cursor::new([1, 2, 3, 4, 5]);
/// assert!(reader.supports_borrow(BorrowKind::CallSite));
/// assert!(!reader.supports_borrow(BorrowKind::Backing));
/// ```
#[inline]
fn supports_borrow(&self, kind: BorrowKind) -> bool {
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@kskalski kskalski Mar 17, 2026

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can we make it const?

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@cpubot cpubot Mar 18, 2026

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Unfortunately not -- const functions in traits isn't yet supported. But, I looked at the assembly in a few examples, and the compiler reliably constant-folds conditions out since BORROW_CAP and kind.mask is const

Self::BORROW_KINDS & kind.mask() != 0
}

/// Return exactly `N` bytes as `&[u8; N]` without advancing.
///
/// Errors if fewer than `N` bytes are available.
Expand Down Expand Up @@ -270,6 +307,8 @@ pub trait Reader<'a> {
}

impl<'a, R: Reader<'a> + ?Sized> Reader<'a> for &mut R {
const BORROW_KINDS: u8 = R::BORROW_KINDS;

#[inline(always)]
fn by_ref(&mut self) -> impl Reader<'a> {
&mut **self
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12 changes: 12 additions & 0 deletions wincode/src/io/slice.rs
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Expand Up @@ -71,6 +71,8 @@ impl<'a, T> SliceUnchecked<'a, T> {
}

impl<'a> Reader<'a> for SliceUnchecked<'a, u8> {
const BORROW_KINDS: u8 = BorrowKind::Backing.mask() | BorrowKind::CallSite.mask();

#[inline]
fn copy_into_slice(&mut self, buf: &mut [MaybeUninit<u8>]) -> ReadResult<()> {
// SAFETY: by constructing `SliceUnchecked`, caller guarantees
Expand Down Expand Up @@ -171,6 +173,9 @@ impl<'a, T> SliceMutUnchecked<'a, T> {
}

impl<'a> Reader<'a> for SliceMutUnchecked<'a, u8> {
const BORROW_KINDS: u8 =
BorrowKind::Backing.mask() | BorrowKind::BackingMut.mask() | BorrowKind::CallSite.mask();

#[inline]
fn copy_into_slice(&mut self, buf: &mut [MaybeUninit<u8>]) -> ReadResult<()> {
// SAFETY: by constructing `SliceMutUnchecked`, caller guarantees
Expand Down Expand Up @@ -260,6 +265,8 @@ impl<'b, T> SliceScopedUnchecked<'_, 'b, T> {
}

impl<'a> Reader<'a> for SliceScopedUnchecked<'a, '_, u8> {
const BORROW_KINDS: u8 = BorrowKind::CallSite.mask();

#[inline(always)]
fn copy_into_slice(&mut self, buf: &mut [MaybeUninit<u8>]) -> ReadResult<()> {
self.inner.copy_into_slice(buf)
Expand Down Expand Up @@ -292,6 +299,8 @@ impl<'a> Reader<'a> for SliceScopedUnchecked<'a, '_, u8> {
}

impl<'a> Reader<'a> for &'a [u8] {
const BORROW_KINDS: u8 = BorrowKind::Backing.mask() | BorrowKind::CallSite.mask();

#[inline]
fn take_borrowed(&mut self, len: usize) -> ReadResult<&'a [u8]> {
let Some(src) = advance_slice_checked(self, len) else {
Expand Down Expand Up @@ -357,6 +366,9 @@ impl<'a> Reader<'a> for &'a [u8] {
}

impl<'a> Reader<'a> for &'a mut [u8] {
const BORROW_KINDS: u8 =
BorrowKind::Backing.mask() | BorrowKind::BackingMut.mask() | BorrowKind::CallSite.mask();

#[inline(always)]
unsafe fn as_trusted_for(&mut self, n_bytes: usize) -> ReadResult<impl Reader<'a>> {
let Some(window) = advance_slice_mut_checked(self, n_bytes) else {
Expand Down