Auto merge of #75066 - poliorcetics:document-unsafety-in-core-slice, r=LukasKalbertodt

Document unsafety in library/core/src/slice/mod.rs

Restart where #73555 left off, helping with #66219.

1 files changed, 190 insertions, 30 deletions

diff --git a/library/core/src/slice/mod.rs b/library/core/src/slice/mod.rs
index 93608a1ce48..f511ca8eb78 100644
--- a/library/core/src/slice/mod.rs
+++ b/library/core/src/slice/mod.rs
@@ -1,5 +1,4 @@
 // ignore-tidy-filelength
-// ignore-tidy-undocumented-unsafe
 
 //! Slice management and manipulation.
 //!
@@ -70,6 +69,8 @@ impl<T> [T] {
     #[allow(unused_attributes)]
     #[allow_internal_unstable(const_fn_union)]
     pub const fn len(&self) -> usize {
+        // SAFETY: this is safe because `&[T]` and `FatPtr<T>` have the same layout.
+        // Only `std` can make this guarantee.
         unsafe { crate::ptr::Repr { rust: self }.raw.len }
     }
 
@@ -443,7 +444,8 @@ impl<T> [T] {
     #[unstable(feature = "slice_ptr_range", issue = "65807")]
     #[inline]
     pub fn as_ptr_range(&self) -> Range<*const T> {
-        // The `add` here is safe, because:
+        let start = self.as_ptr();
+        // SAFETY: The `add` here is safe, because:
         //
         //   - Both pointers are part of the same object, as pointing directly
         //     past the object also counts.
@@ -460,7 +462,6 @@ impl<T> [T] {
         //     the end of the address space.
         //
         // See the documentation of pointer::add.
-        let start = self.as_ptr();
         let end = unsafe { start.add(self.len()) };
         start..end
     }
@@ -484,8 +485,8 @@ impl<T> [T] {
     #[unstable(feature = "slice_ptr_range", issue = "65807")]
     #[inline]
     pub fn as_mut_ptr_range(&mut self) -> Range<*mut T> {
-        // See as_ptr_range() above for why `add` here is safe.
         let start = self.as_mut_ptr();
+        // SAFETY: See as_ptr_range() above for why `add` here is safe.
         let end = unsafe { start.add(self.len()) };
         start..end
     }
@@ -511,11 +512,15 @@ impl<T> [T] {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn swap(&mut self, a: usize, b: usize) {
+        // Can't take two mutable loans from one vector, so instead just cast
+        // them to their raw pointers to do the swap.
+        let pa: *mut T = &mut self[a];
+        let pb: *mut T = &mut self[b];
+        // SAFETY: `pa` and `pb` have been created from safe mutable references and refer
+        // to elements in the slice and therefore are guaranteed to be valid and aligned.
+        // Note that accessing the elements behind `a` and `b` is checked and will
+        // panic when out of bounds.
         unsafe {
-            // Can't take two mutable loans from one vector, so instead just cast
-            // them to their raw pointers to do the swap
-            let pa: *mut T = &mut self[a];
-            let pb: *mut T = &mut self[b];
             ptr::swap(pa, pb);
         }
     }
@@ -554,6 +559,24 @@ impl<T> [T] {
             // Use the llvm.bswap intrinsic to reverse u8s in a usize
             let chunk = mem::size_of::<usize>();
             while i + chunk - 1 < ln / 2 {
+                // SAFETY: There are several things to check here:
+                //
+                // - Note that `chunk` is either 4 or 8 due to the cfg check
+                //   above. So `chunk - 1` is positive.
+                // - Indexing with index `i` is fine as the loop check guarantees
+                //   `i + chunk - 1 < ln / 2`
+                //   <=> `i < ln / 2 - (chunk - 1) < ln / 2 < ln`.
+                // - Indexing with index `ln - i - chunk = ln - (i + chunk)` is fine:
+                //   - `i + chunk > 0` is trivially true.
+                //   - The loop check guarantees:
+                //     `i + chunk - 1 < ln / 2`
+                //     <=> `i + chunk ≤ ln / 2 ≤ ln`, thus subtraction does not underflow.
+                // - The `read_unaligned` and `write_unaligned` calls are fine:
+                //   - `pa` points to index `i` where `i < ln / 2 - (chunk - 1)`
+                //     (see above) and `pb` points to index `ln - i - chunk`, so
+                //     both are at least `chunk`
+                //     many bytes away from the end of `self`.
+                //   - Any initialized memory is valid `usize`.
                 unsafe {
                     let pa: *mut T = self.get_unchecked_mut(i);
                     let pb: *mut T = self.get_unchecked_mut(ln - i - chunk);
@@ -570,6 +593,20 @@ impl<T> [T] {
             // Use rotate-by-16 to reverse u16s in a u32
             let chunk = mem::size_of::<u32>() / 2;
             while i + chunk - 1 < ln / 2 {
+                // SAFETY: An unaligned u32 can be read from `i` if `i + 1 < ln`
+                // (and obviously `i < ln`), because each element is 2 bytes and
+                // we're reading 4.
+                //
+                // `i + chunk - 1 < ln / 2` # while condition
+                // `i + 2 - 1 < ln / 2`
+                // `i + 1 < ln / 2`
+                //
+                // Since it's less than the length divided by 2, then it must be
+                // in bounds.
+                //
+                // This also means that the condition `0 < i + chunk <= ln` is
+                // always respected, ensuring the `pb` pointer can be used
+                // safely.
                 unsafe {
                     let pa: *mut T = self.get_unchecked_mut(i);
                     let pb: *mut T = self.get_unchecked_mut(ln - i - chunk);
@@ -583,8 +620,13 @@ impl<T> [T] {
         }
 
         while i < ln / 2 {
-            // Unsafe swap to avoid the bounds check in safe swap.
+            // SAFETY: `i` is inferior to half the length of the slice so
+            // accessing `i` and `ln - i - 1` is safe (`i` starts at 0 and
+            // will not go further than `ln / 2 - 1`).
+            // The resulting pointers `pa` and `pb` are therefore valid and
+            // aligned, and can be read from and written to.
             unsafe {
+                // Unsafe swap to avoid the bounds check in safe swap.
                 let pa: *mut T = self.get_unchecked_mut(i);
                 let pb: *mut T = self.get_unchecked_mut(ln - i - 1);
                 ptr::swap(pa, pb);
@@ -609,8 +651,24 @@ impl<T> [T] {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn iter(&self) -> Iter<'_, T> {
+        let ptr = self.as_ptr();
+        // SAFETY: There are several things here:
+        //
+        // `ptr` has been obtained by `self.as_ptr()` where `self` is a valid
+        // reference thus it is non-NUL and safe to use and pass to
+        // `NonNull::new_unchecked` .
+        //
+        // Adding `self.len()` to the starting pointer gives a pointer
+        // at the end of `self`. `end` will never be dereferenced, only checked
+        // for direct pointer equality with `ptr` to check if the iterator is
+        // done.
+        //
+        // In the case of a ZST, the end pointer is just the start pointer plus
+        // the length, to also allows for the fast `ptr == end` check.
+        //
+        // See the `next_unchecked!` and `is_empty!` macros as well as the
+        // `post_inc_start` method for more informations.
         unsafe {
-            let ptr = self.as_ptr();
             assume(!ptr.is_null());
 
             let end = if mem::size_of::<T>() == 0 {
@@ -637,8 +695,24 @@ impl<T> [T] {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn iter_mut(&mut self) -> IterMut<'_, T> {
+        let ptr = self.as_mut_ptr();
+        // SAFETY: There are several things here:
+        //
+        // `ptr` has been obtained by `self.as_ptr()` where `self` is a valid
+        // reference thus it is non-NUL and safe to use and pass to
+        // `NonNull::new_unchecked` .
+        //
+        // Adding `self.len()` to the starting pointer gives a pointer
+        // at the end of `self`. `end` will never be dereferenced, only checked
+        // for direct pointer equality with `ptr` to check if the iterator is
+        // done.
+        //
+        // In the case of a ZST, the end pointer is just the start pointer plus
+        // the length, to also allows for the fast `ptr == end` check.
+        //
+        // See the `next_unchecked!` and `is_empty!` macros as well as the
+        // `post_inc_start` method for more informations.
         unsafe {
-            let ptr = self.as_mut_ptr();
             assume(!ptr.is_null());
 
             let end = if mem::size_of::<T>() == 0 {
@@ -1107,6 +1181,8 @@ impl<T> [T] {
         let len = self.len();
         let ptr = self.as_mut_ptr();
 
+        // SAFETY: `[ptr; mid]` and `[mid; len]` are inside `self`, which
+        // fulfills the requirements of `from_raw_parts_mut`.
         unsafe {
             assert!(mid <= len);
 
@@ -1655,14 +1731,14 @@ impl<T> [T] {
         while size > 1 {
             let half = size / 2;
             let mid = base + half;
-            // mid is always in [0, size), that means mid is >= 0 and < size.
-            // mid >= 0: by definition
-            // mid < size: mid = size / 2 + size / 4 + size / 8 ...
+            // SAFETY: the call is made safe by the following inconstants:
+            // - `mid >= 0`: by definition
+            // - `mid < size`: `mid = size / 2 + size / 4 + size / 8 ...`
             let cmp = f(unsafe { s.get_unchecked(mid) });
             base = if cmp == Greater { base } else { mid };
             size -= half;
         }
-        // base is always in [0, size) because base <= mid.
+        // SAFETY: base is always in [0, size) because base <= mid.
         let cmp = f(unsafe { s.get_unchecked(base) });
         if cmp == Equal { Ok(base) } else { Err(base + (cmp == Less) as usize) }
     }
@@ -2120,6 +2196,21 @@ impl<T> [T] {
         let mut next_read: usize = 1;
         let mut next_write: usize = 1;
 
+        // SAFETY: the `while` condition guarantees `next_read` and `next_write`
+        // are less than `len`, thus are inside `self`. `prev_ptr_write` points to
+        // one element before `ptr_write`, but `next_write` starts at 1, so
+        // `prev_ptr_write` is never less than 0 and is inside the slice.
+        // This fulfils the requirements for dereferencing `ptr_read`, `prev_ptr_write`
+        // and `ptr_write`, and for using `ptr.add(next_read)`, `ptr.add(next_write - 1)`
+        // and `prev_ptr_write.offset(1)`.
+        //
+        // `next_write` is also incremented at most once per loop at most meaning
+        // no element is skipped when it may need to be swapped.
+        //
+        // `ptr_read` and `prev_ptr_write` never point to the same element. This
+        // is required for `&mut *ptr_read`, `&mut *prev_ptr_write` to be safe.
+        // The explanation is simply that `next_read >= next_write` is always true,
+        // thus `next_read > next_write - 1` is too.
         unsafe {
             // Avoid bounds checks by using raw pointers.
             while next_read < len {
@@ -2203,9 +2294,11 @@ impl<T> [T] {
     pub fn rotate_left(&mut self, mid: usize) {
         assert!(mid <= self.len());
         let k = self.len() - mid;
+        let p = self.as_mut_ptr();
 
+        // SAFETY: The range `[p.add(mid) - mid, p.add(mid) + k)` is trivially
+        // valid for reading and writing, as required by `ptr_rotate`.
         unsafe {
-            let p = self.as_mut_ptr();
             rotate::ptr_rotate(mid, p.add(mid), k);
         }
     }
@@ -2244,9 +2337,11 @@ impl<T> [T] {
     pub fn rotate_right(&mut self, k: usize) {
         assert!(k <= self.len());
         let mid = self.len() - k;
+        let p = self.as_mut_ptr();
 
+        // SAFETY: The range `[p.add(mid) - mid, p.add(mid) + k)` is trivially
+        // valid for reading and writing, as required by `ptr_rotate`.
         unsafe {
-            let p = self.as_mut_ptr();
             rotate::ptr_rotate(mid, p.add(mid), k);
         }
     }
@@ -2407,6 +2502,9 @@ impl<T> [T] {
         T: Copy,
     {
         assert_eq!(self.len(), src.len(), "destination and source slices have different lengths");
+        // SAFETY: `self` is valid for `self.len()` elements by definition, and `src` was
+        // checked to have the same length. The slices cannot overlap because
+        // mutable references are exclusive.
         unsafe {
             ptr::copy_nonoverlapping(src.as_ptr(), self.as_mut_ptr(), self.len());
         }
@@ -2460,6 +2558,8 @@ impl<T> [T] {
         assert!(src_end <= self.len(), "src is out of bounds");
         let count = src_end - src_start;
         assert!(dest <= self.len() - count, "dest is out of bounds");
+        // SAFETY: the conditions for `ptr::copy` have all been checked above,
+        // as have those for `ptr::add`.
         unsafe {
             ptr::copy(self.as_ptr().add(src_start), self.as_mut_ptr().add(dest), count);
         }
@@ -2515,6 +2615,9 @@ impl<T> [T] {
     #[stable(feature = "swap_with_slice", since = "1.27.0")]
     pub fn swap_with_slice(&mut self, other: &mut [T]) {
         assert!(self.len() == other.len(), "destination and source slices have different lengths");
+        // SAFETY: `self` is valid for `self.len()` elements by definition, and `src` was
+        // checked to have the same length. The slices cannot overlap because
+        // mutable references are exclusive.
         unsafe {
             ptr::swap_nonoverlapping(self.as_mut_ptr(), other.as_mut_ptr(), self.len());
         }
@@ -2546,6 +2649,8 @@ impl<T> [T] {
             // iterative stein’s algorithm
             // We should still make this `const fn` (and revert to recursive algorithm if we do)
             // because relying on llvm to consteval all this is… well, it makes me uncomfortable.
+
+            // SAFETY: `a` and `b` are checked to be non-zero values.
             let (ctz_a, mut ctz_b) = unsafe {
                 if a == 0 {
                     return b;
@@ -2565,6 +2670,7 @@ impl<T> [T] {
                     mem::swap(&mut a, &mut b);
                 }
                 b = b - a;
+                // SAFETY: `b` is checked to be non-zero.
                 unsafe {
                     if b == 0 {
                         break;
@@ -2626,6 +2732,7 @@ impl<T> [T] {
         // First, find at what point do we split between the first and 2nd slice. Easy with
         // ptr.align_offset.
         let ptr = self.as_ptr();
+        // SAFETY: See the `align_to_mut` method for the detailed safety comment.
         let offset = unsafe { crate::ptr::align_offset(ptr, mem::align_of::<U>()) };
         if offset > self.len() {
             (self, &[], &[])
@@ -2685,6 +2792,13 @@ impl<T> [T] {
         // First, find at what point do we split between the first and 2nd slice. Easy with
         // ptr.align_offset.
         let ptr = self.as_ptr();
+        // SAFETY: Here we are ensuring we will use aligned pointers for U for the
+        // rest of the method. This is done by passing a pointer to &[T] with an
+        // alignment targeted for U.
+        // `crate::ptr::align_offset` is called with a correctly aligned and
+        // valid pointer `ptr` (it comes from a reference to `self`) and with
+        // a size that is a power of two (since it comes from the alignement for U),
+        // satisfying its safety constraints.
         let offset = unsafe { crate::ptr::align_offset(ptr, mem::align_of::<U>()) };
         if offset > self.len() {
             (self, &mut [], &mut [])
@@ -2810,15 +2924,13 @@ impl<T> [T] {
 
         while left != right {
             let mid = left + (right - left) / 2;
-            // SAFETY:
-            // When left < right, left <= mid < right.
-            // Therefore left always increases and right always decreases,
-            // and eigher of them is selected.
-            // In both cases left <= right is satisfied.
-            // Therefore if left < right in a step,
-            // left <= right is satisfied in the next step.
-            // Therefore as long as left != right, 0 <= left < right <= len is satisfied
-            // and if this case 0 <= mid < len is satisfied too.
+            // SAFETY: When `left < right`, `left <= mid < right`.
+            // Therefore `left` always increases and `right` always decreases,
+            // and either of them is selected. In both cases `left <= right` is
+            // satisfied. Therefore if `left < right` in a step, `left <= right`
+            // is satisfied in the next step. Therefore as long as `left != right`,
+            // `0 <= left < right <= len` is satisfied and if this case
+            // `0 <= mid < len` is satisfied too.
             let value = unsafe { self.get_unchecked(mid) };
             if pred(value) {
                 left = mid + 1;
@@ -2938,7 +3050,8 @@ fn is_ascii(s: &[u8]) -> bool {
     // above.
     debug_assert!(offset_to_aligned <= len);
 
-    // word_ptr is the (properly aligned) usize ptr we use to read the middle chunk of the slice.
+    // SAFETY: word_ptr is the (properly aligned) usize ptr we use to read the
+    // middle chunk of the slice.
     let mut word_ptr = unsafe { start.add(offset_to_aligned) as *const usize };
 
     // `byte_pos` is the byte index of `word_ptr`, used for loop end checks.
@@ -3126,11 +3239,13 @@ unsafe impl<T> SliceIndex<[T]> for usize {
 
     #[inline]
     fn get(self, slice: &[T]) -> Option<&T> {
+        // SAFETY: `self` is checked to be in bounds.
         if self < slice.len() { unsafe { Some(&*self.get_unchecked(slice)) } } else { None }
     }
 
     #[inline]
     fn get_mut(self, slice: &mut [T]) -> Option<&mut T> {
+        // SAFETY: `self` is checked to be in bounds.
         if self < slice.len() { unsafe { Some(&mut *self.get_unchecked_mut(slice)) } } else { None }
     }
 
@@ -3171,6 +3286,7 @@ unsafe impl<T> SliceIndex<[T]> for ops::Range<usize> {
         if self.start > self.end || self.end > slice.len() {
             None
         } else {
+            // SAFETY: `self` is checked to be valid and in bounds above.
             unsafe { Some(&*self.get_unchecked(slice)) }
         }
     }
@@ -3180,6 +3296,7 @@ unsafe impl<T> SliceIndex<[T]> for ops::Range<usize> {
         if self.start > self.end || self.end > slice.len() {
             None
         } else {
+            // SAFETY: `self` is checked to be valid and in bounds above.
             unsafe { Some(&mut *self.get_unchecked_mut(slice)) }
         }
     }
@@ -3208,6 +3325,7 @@ unsafe impl<T> SliceIndex<[T]> for ops::Range<usize> {
         } else if self.end > slice.len() {
             slice_end_index_len_fail(self.end, slice.len());
         }
+        // SAFETY: `self` is checked to be valid and in bounds above.
         unsafe { &*self.get_unchecked(slice) }
     }
 
@@ -3218,6 +3336,7 @@ unsafe impl<T> SliceIndex<[T]> for ops::Range<usize> {
         } else if self.end > slice.len() {
             slice_end_index_len_fail(self.end, slice.len());
         }
+        // SAFETY: `self` is checked to be valid and in bounds above.
         unsafe { &mut *self.get_unchecked_mut(slice) }
     }
 }
@@ -3290,6 +3409,7 @@ unsafe impl<T> SliceIndex<[T]> for ops::RangeFrom<usize> {
         if self.start > slice.len() {
             slice_start_index_len_fail(self.start, slice.len());
         }
+        // SAFETY: `self` is checked to be valid and in bounds above.
         unsafe { &*self.get_unchecked(slice) }
     }
 
@@ -3298,6 +3418,7 @@ unsafe impl<T> SliceIndex<[T]> for ops::RangeFrom<usize> {
         if self.start > slice.len() {
             slice_start_index_len_fail(self.start, slice.len());
         }
+        // SAFETY: `self` is checked to be valid and in bounds above.
         unsafe { &mut *self.get_unchecked_mut(slice) }
     }
 }
@@ -3543,6 +3664,9 @@ macro_rules! iterator {
             // Helper function for creating a slice from the iterator.
             #[inline(always)]
             fn make_slice(&self) -> &'a [T] {
+                // SAFETY: the iterator was created from a slice with pointer
+                // `self.ptr` and length `len!(self)`. This guarantees that all
+                // the prerequisites for `from_raw_parts` are fulfilled.
                 unsafe { from_raw_parts(self.ptr.as_ptr(), len!(self)) }
             }
 
@@ -3601,6 +3725,11 @@ macro_rules! iterator {
             #[inline]
             fn next(&mut self) -> Option<$elem> {
                 // could be implemented with slices, but this avoids bounds checks
+
+                // SAFETY: `assume` calls are safe since a slice's start pointer
+                // must be non-null, and slices over non-ZSTs must also have a
+                // non-null end pointer. The call to `next_unchecked!` is safe
+                // since we check if the iterator is empty first.
                 unsafe {
                     assume(!self.ptr.as_ptr().is_null());
                     if mem::size_of::<T>() != 0 {
@@ -3634,14 +3763,14 @@ macro_rules! iterator {
                         // could be (due to wrapping).
                         self.end = self.ptr.as_ptr();
                     } else {
+                        // SAFETY: end can't be 0 if T isn't ZST because ptr isn't 0 and end >= ptr
                         unsafe {
-                            // End can't be 0 if T isn't ZST because ptr isn't 0 and end >= ptr
                             self.ptr = NonNull::new_unchecked(self.end as *mut T);
                         }
                     }
                     return None;
                 }
-                // We are in bounds. `post_inc_start` does the right thing even for ZSTs.
+                // SAFETY: We are in bounds. `post_inc_start` does the right thing even for ZSTs.
                 unsafe {
                     self.post_inc_start(n as isize);
                     Some(next_unchecked!(self))
@@ -3748,6 +3877,8 @@ macro_rules! iterator {
                 let mut i = 0;
                 while let Some(x) = self.next() {
                     if predicate(x) {
+                        // SAFETY: we are guaranteed to be in bounds by the loop invariant:
+                        // when `i >= n`, `self.next()` returns `None` and the loop breaks.
                         unsafe { assume(i < n) };
                         return Some(i);
                     }
@@ -3769,6 +3900,8 @@ macro_rules! iterator {
                 while let Some(x) = self.next_back() {
                     i -= 1;
                     if predicate(x) {
+                        // SAFETY: `i` must be lower than `n` since it starts at `n`
+                        // and is only decreasing.
                         unsafe { assume(i < n) };
                         return Some(i);
                     }
@@ -3784,6 +3917,11 @@ macro_rules! iterator {
             #[inline]
             fn next_back(&mut self) -> Option<$elem> {
                 // could be implemented with slices, but this avoids bounds checks
+
+                // SAFETY: `assume` calls are safe since a slice's start pointer must be non-null,
+                // and slices over non-ZSTs must also have a non-null end pointer.
+                // The call to `next_back_unchecked!` is safe since we check if the iterator is
+                // empty first.
                 unsafe {
                     assume(!self.ptr.as_ptr().is_null());
                     if mem::size_of::<T>() != 0 {
@@ -3804,7 +3942,7 @@ macro_rules! iterator {
                     self.end = self.ptr.as_ptr();
                     return None;
                 }
-                // We are in bounds. `pre_dec_end` does the right thing even for ZSTs.
+                // SAFETY: We are in bounds. `pre_dec_end` does the right thing even for ZSTs.
                 unsafe {
                     self.pre_dec_end(n as isize);
                     Some(next_back_unchecked!(self))
@@ -3999,6 +4137,9 @@ impl<'a, T> IterMut<'a, T> {
     /// ```
     #[stable(feature = "iter_to_slice", since = "1.4.0")]
     pub fn into_slice(self) -> &'a mut [T] {
+        // SAFETY: the iterator was created from a mutable slice with pointer
+        // `self.ptr` and length `len!(self)`. This guarantees that all the prerequisites
+        // for `from_raw_parts_mut` are fulfilled.
         unsafe { from_raw_parts_mut(self.ptr.as_ptr(), len!(self)) }
     }
 
@@ -5568,6 +5709,8 @@ impl<T, const N: usize> FusedIterator for ArrayChunks<'_, T, N> {}
 #[unstable(feature = "array_chunks", issue = "74985")]
 unsafe impl<'a, T, const N: usize> TrustedRandomAccess for ArrayChunks<'a, T, N> {
     unsafe fn get_unchecked(&mut self, i: usize) -> &'a [T; N] {
+        // SAFETY: The safety guarantees of `get_unchecked` are transferred to
+        // the caller.
         unsafe { self.iter.get_unchecked(i) }
     }
     fn may_have_side_effect() -> bool {
@@ -6288,12 +6431,20 @@ pub unsafe fn from_raw_parts_mut<'a, T>(data: *mut T, len: usize) -> &'a mut [T]
 /// Converts a reference to T into a slice of length 1 (without copying).
 #[stable(feature = "from_ref", since = "1.28.0")]
 pub fn from_ref<T>(s: &T) -> &[T] {
+    // SAFETY: a reference is guaranteed to be valid for reads. The returned
+    // reference cannot be mutated as it is an immutable reference.
+    // `mem::size_of::<T>()` cannot be larger than `isize::MAX`.
+    // Thus the call to `from_raw_parts` is safe.
     unsafe { from_raw_parts(s, 1) }
 }
 
 /// Converts a reference to T into a slice of length 1 (without copying).
 #[stable(feature = "from_ref", since = "1.28.0")]
 pub fn from_mut<T>(s: &mut T) -> &mut [T] {
+    // SAFETY: a mutable reference is guaranteed to be valid for writes.
+    // The reference cannot be accessed by another pointer as it is an mutable reference.
+    // `mem::size_of::<T>()` cannot be larger than `isize::MAX`.
+    // Thus the call to `from_raw_parts_mut` is safe.
     unsafe { from_raw_parts_mut(s, 1) }
 }
 
@@ -6414,6 +6565,8 @@ where
         if self.as_ptr().guaranteed_eq(other.as_ptr()) {
             return true;
         }
+        // SAFETY: `self` and `other` are references and are thus guaranteed to be valid.
+        // The two slices have been checked to have the same size above.
         unsafe {
             let size = mem::size_of_val(self);
             memcmp(self.as_ptr() as *const u8, other.as_ptr() as *const u8, size) == 0
@@ -6516,6 +6669,9 @@ impl SliceOrd for u8 {
     #[inline]
     fn compare(left: &[Self], right: &[Self]) -> Ordering {
         let order =
+            // SAFETY: `left` and `right` are references and are thus guaranteed to be valid.
+            // We use the minimum of both lengths which guarantees that both regions are
+            // valid for reads in that interval.
             unsafe { memcmp(left.as_ptr(), right.as_ptr(), cmp::min(left.len(), right.len())) };
         if order == 0 {
             left.len().cmp(&right.len())
@@ -6590,6 +6746,10 @@ impl SliceContains for u8 {
 impl SliceContains for i8 {
     fn slice_contains(&self, x: &[Self]) -> bool {
         let byte = *self as u8;
+        // SAFETY: `i8` and `u8` have the same memory layout, thus casting `x.as_ptr()`
+        // as `*const u8` is safe. The `x.as_ptr()` comes from a reference and is thus guaranteed
+        // to be valid for reads for the length of the slice `x.len()`, which cannot be larger
+        // than `isize::MAX`. The returned slice is never mutated.
         let bytes: &[u8] = unsafe { from_raw_parts(x.as_ptr() as *const u8, x.len()) };
         memchr::memchr(byte, bytes).is_some()
     }