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io: improve safety comment on FillBuf (#4476)

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Alice Ryhl 2022-02-07 10:07:58 +01:00 committed by GitHub
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commit d6143c9566
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6
tokio/src/io/util/fill_buf.rs
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@ -40,6 +40,12 @@ impl<'a, R: AsyncBufRead + ?Sized + Unpin> Future for FillBuf<'a, R> {
// Safety: This is necessary only due to a limitation in the
// borrow checker. Once Rust starts using the polonius borrow
// checker, this can be simplified.
//
// The safety of this transmute relies on the fact that the
// value of `reader` is `None` when we return in this branch.
// Otherwise the caller could poll us again after
// completion, and access the mutable reference while the
// returned immutable reference still exists.
let slice = std::mem::transmute::<&[u8], &'a [u8]>(slice);
Poll::Ready(Ok(slice))
},