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Auto merge of #76688 - yokodake:patch-2, r=kodrAus

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Document unsafety in core::slice::memchr

Contributes to #66219

Note sure if that's good enough, especially for the `align_to` call.
The docs only mention transmuting and I don't think that everything related to reference lifetimes and state validity mentioned in the [nomicon](https://doc.rust-lang.org/nomicon/transmutes.html) are relevant here.
This commit is contained in:
bors 2020-11-25 02:49:28 +00:00
commit 3f7ccb4cf5
1 changed files with 6 additions and 2 deletions
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8
library/core/src/slice/memchr.rs
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@ -1,8 +1,6 @@
// Original implementation taken from rust-memchr.
// Copyright 2015 Andrew Gallant, bluss and Nicolas Koch
// ignore-tidy-undocumented-unsafe
use crate::cmp;
use crate::mem;
@ -72,6 +70,8 @@ fn memchr_general_case(x: u8, text: &[u8]) -> Option<usize> {
// search the body of the text
let repeated_x = repeat_byte(x);
while offset <= len - 2 * USIZE_BYTES {
// SAFETY: the while's predicate guarantees a distance of at least 2 * usize_bytes
// between the offset and the end of the slice.
unsafe {
let u = *(ptr.add(offset) as *const usize);
let v = *(ptr.add(offset + USIZE_BYTES) as *const usize);
@ -105,6 +105,8 @@ pub fn memrchr(x: u8, text: &[u8]) -> Option<usize> {
let (min_aligned_offset, max_aligned_offset) = {
// We call this just to obtain the length of the prefix and suffix.
// In the middle we always process two chunks at once.
// SAFETY: transmuting `[u8]` to `[usize]` is safe except for size differences
// which are handled by `align_to`.
let (prefix, _, suffix) = unsafe { text.align_to::<(Chunk, Chunk)>() };
(prefix.len(), len - suffix.len())
};
@ -121,6 +123,8 @@ pub fn memrchr(x: u8, text: &[u8]) -> Option<usize> {
let chunk_bytes = mem::size_of::<Chunk>();
while offset > min_aligned_offset {
// SAFETY: offset starts at len - suffix.len(), as long as it is greater than
// min_aligned_offset (prefix.len()) the remaining distance is at least 2 * chunk_bytes.
unsafe {
let u = *(ptr.offset(offset as isize - 2 * chunk_bytes as isize) as *const Chunk);
let v = *(ptr.offset(offset as isize - chunk_bytes as isize) as *const Chunk);