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Reference Chunks::next_back in more of the chunk iterators safety comments

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Thom Chiovoloni 2021-10-29 20:08:27 -07:00
parent e81fefaa50
commit b54381640d
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1 changed files with 2 additions and 34 deletions
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36
library/core/src/slice/iter.rs
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@ -1653,25 +1653,7 @@ impl<'a, T> DoubleEndedIterator for ChunksMut<'a, T> {
let sz = if remainder != 0 { remainder } else { self.chunk_size };
let tmp = mem::replace(&mut self.v, &mut []);
let tmp_len = tmp.len();
// SAFETY: split_at_unchecked requires the argument be less than or
// equal to the length. This is guaranteed, but subtle: We need the
// expression `tmp_len - sz` not to overflow, which means we need
// `sz >= tmp_len`.
//
// `sz` will always either be `tmp_or_v.len() % self.chunk_size`
// (where `tmp_or_v` is the slice that at the time was `self.v` but
// now is `tmp`, and thus `tmp_len` and `tmp_or_v.len()` are the
// same), which will always evaluate to strictly less than
// `tmp_or_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 `tmp_len`, it cannot: if
// `self.chunk_size` were greater than `tmp_len`, then
// `tmp_or_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).
// SAFETY: Similar to `Chunks::next_back`
let (head, tail) = unsafe { tmp.split_at_mut_unchecked(tmp_len - sz) };
self.v = head;
Some(tail)
@ -2691,21 +2673,7 @@ impl<'a, T> DoubleEndedIterator for RChunksMut<'a, T> {
let remainder = self.v.len() % self.chunk_size;
let sz = if remainder != 0 { remainder } else { self.chunk_size };
let tmp = mem::replace(&mut self.v, &mut []);
// SAFETY: split_at_mut_unchecked requires the argument be less than
// or equal to the length. This is guaranteed, but subtle: `chunksz`
// will always either be `tmp_or_v.len() % self.chunk_size` (where
// `tmp_or_v` is the slice that at the time was `self.v` but now is
// `tmp`), which will always evaluate to strictly less than
// `tmp_or_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 `tmp_or_v.len()`, it cannot: if
// `self.chunk_size` were greater than `tmp_or_v.len()`, then
// `tmp_or_v.len() % self.chunk_size` would return nonzero (note
// that in this branch of the `if`, we already know that `tmp_or_v`
// is non-empty).
// SAFETY: Similar to `Chunks::next_back`
let (head, tail) = unsafe { tmp.split_at_mut_unchecked(sz) };
self.v = tail;
Some(head)