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Restore Send and Sync for BorrowedCursor #163063

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Restore Send and Sync for BorrowedCursor #163063

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8 changes: 8 additions & 0 deletions library/core/src/io/borrowed_buf.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -222,6 +222,14 @@ pub struct BorrowedCursor<'a, T> {
borrowed_buf: NonNull<BorrowedBuf<'a, T>>,
}

// SAFETY: A `BorrowedCursor<'a, T>` is a unique borrow of a `BorrowedBuf<'a, T>`, which is a
// `&'a mut [MaybeUninit<T>]` and two `Copy` fields. The `buf` raw pointer is used like
// `&mut [MaybeUninit<T>]` so `T: Send` -> `Send` and is: `T: Sync` -> `Sync`, and the
// `borrowed_buf` only touches two `Copy` fields, without depending of `T`.
unsafe impl<T: Send> Send for BorrowedCursor<'_, T> {}
// SAFETY: See the `Send` impl above.
unsafe impl<T: Sync> Sync for BorrowedCursor<'_, T> {}

impl<T> Debug for BorrowedCursor<'_, T> {
fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
let buf = BorrowedBufDebug {
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