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Address some issues in chunk iterator safety comments

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Co-authored-by: the8472 <the8472@users.noreply.github.com>
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Thom Chiovoloni 2021-10-29 19:44:29 -07:00
parent 83aa6d4109
commit e81fefaa50
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1 changed files with 9 additions and 25 deletions
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34
library/core/src/slice/iter.rs
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@ -1475,22 +1475,19 @@ impl<'a, T> DoubleEndedIterator for Chunks<'a, T> {
let remainder = self.v.len() % self.chunk_size;
let chunksz = if remainder != 0 { remainder } else { self.chunk_size };
// SAFETY: split_at_unchecked requires the argument be less than or
// equal to the length. This is guaranteed, but subtle: We need the
// expression `self.v.len() - sz` not to overflow, which means we
// need `sz >= tmp_len`.
//
// `sz` will always either be `self.v.len() % self.chunk_size`,
// which will always evaluate to strictly less than `self.v.len()`
// (or panic, in the case that `self.chunk_size` is zero), or it can
// be `self.chunk_size`, in the case that the length is exactly
// equal to the length. This is guaranteed, but subtle: `chunksz`
// will always either be `self.v.len() % self.chunk_size`, which
// will always evaluate to strictly less than `self.v.len()` (or
// panic, in the case that `self.chunk_size` is zero), or it can be
// `self.chunk_size`, in the case that the length is exactly
// divisible by the chunk size.
//
// While it seems like using `self.chunk_size` in this case could
// lead to a value greater than `self.v.len()`, it cannot: if
// `self.chunk_size` were greater than `self.v.len()`, then
// `self.v.len() % self.chunk_size` would have returned non-zero
// (note that in this branch of the `if`, we already know that
// `self.v` is non-empty).
// `self.v.len() % self.chunk_size` would return nonzero (note that
// in this branch of the `if`, we already know that `self.v` is
// non-empty).
let (fst, snd) = unsafe { self.v.split_at_unchecked(self.v.len() - chunksz) };
self.v = fst;
Some(snd)
@ -2524,20 +2521,7 @@ impl<'a, T> DoubleEndedIterator for RChunks<'a, T> {
} else {
let remainder = self.v.len() % self.chunk_size;
let chunksz = if remainder != 0 { remainder } else { self.chunk_size };
// SAFETY: split_at_unchecked requires the argument be less than or
// equal to the length. This is guaranteed, but subtle: `chunksz`
// will always either be `self.v.len() % self.chunk_size`, which
// will always evaluate to strictly less than `self.v.len()` (or
// panic, in the case that `self.chunk_size` is zero), or it can be
// `self.chunk_size`, in the case that the length is exactly
// divisible by the chunk size.
//
// While it seems like using `self.chunk_size` in this case could
// lead to a value greater than `self.v.len()`, it cannot: if
// `self.chunk_size` were greater than `self.v.len()`, then
// `self.v.len() % self.chunk_size` would return nonzero (note that
// in this branch of the `if`, we already know that `self.v` is
// non-empty).
// SAFETY: similar to Chunks::next_back
let (fst, snd) = unsafe { self.v.split_at_unchecked(chunksz) };
self.v = snd;
Some(fst)