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 { // 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 { 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 { let chunk_bytes = mem::size_of::(); 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);