From a5064618cb788b458034c2604348c67c50343004 Mon Sep 17 00:00:00 2001
From: Nanami <12036195+yokodake@users.noreply.github.com>
Date: Mon, 14 Sep 2020 01:11:02 +0200
Subject: [PATCH] Document unsafety in core::slice::memchr
Contributes to #66219
---
library/core/src/slice/memchr.rs | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)
diff --git a/library/core/src/slice/memchr.rs b/library/core/src/slice/memchr.rs
index 0c0f1750264eb..08077c700da69 100644
--- a/library/core/src/slice/memchr.rs
+++ b/library/core/src/slice/memchr.rs
@@ -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);