Rollup merge of #71568 - hbina:document_unsafety_slice_sort, r=joshtriplett

Document unsafety in slice/sort.rs

Let me know if these documentations are accurate c:

I don't think I am capable enough to document the safety of `partition_blocks`, however.

Related issue #66219

Author: Manishearth

src/libcore/slice/sort.rs

@@ -1,6 +1,6 @@
 //! Slice sorting
 //!
-//! This module contains an sort algorithm based on Orson Peters' pattern-defeating quicksort,
+//! This module contains a sorting algorithm based on Orson Peters' pattern-defeating quicksort,
 //! published at: https://github.com/orlp/pdqsort
 //!
 //! Unstable sorting is compatible with libcore because it doesn't allocate memory, unlike our
@@ -20,6 +20,9 @@ struct CopyOnDrop<T> {
 
 impl<T> Drop for CopyOnDrop<T> {
     fn drop(&mut self) {
+        // SAFETY:  This is a helper class.
+        //          Please refer to its usage for correctness.
+        //          Namely, one must be sure that `src` and `dst` does not overlap as required by `ptr::copy_nonoverlapping`.
         unsafe {
             ptr::copy_nonoverlapping(self.src, self.dest, 1);
         }
@@ -32,6 +35,21 @@ where
     F: FnMut(&T, &T) -> bool,
 {
     let len = v.len();
+    // SAFETY: The unsafe operations below involves indexing without a bound check (`get_unchecked` and `get_unchecked_mut`)
+    // and copying memory (`ptr::copy_nonoverlapping`).
+    //
+    // a. Indexing:
+    //  1. We checked the size of the array to >=2.
+    //  2. All the indexing that we will do is always between {0 <= index < len} at most.
+    //
+    // b. Memory copying
+    //  1. We are obtaining pointers to references which are guaranteed to be valid.
+    //  2. They cannot overlap because we obtain pointers to difference indices of the slice.
+    //     Namely, `i` and `i-1`.
+    //  3. If the slice is properly aligned, the elements are properly aligned.
+    //     It is the caller's responsibility to make sure the slice is properly aligned.
+    //
+    // See comments below for further detail.
     unsafe {
         // If the first two elements are out-of-order...
         if len >= 2 && is_less(v.get_unchecked(1), v.get_unchecked(0)) {
@@ -62,6 +80,21 @@ where
     F: FnMut(&T, &T) -> bool,
 {
     let len = v.len();
+    // SAFETY: The unsafe operations below involves indexing without a bound check (`get_unchecked` and `get_unchecked_mut`)
+    // and copying memory (`ptr::copy_nonoverlapping`).
+    //
+    // a. Indexing:
+    //  1. We checked the size of the array to >= 2.
+    //  2. All the indexing that we will do is always between `0 <= index < len-1` at most.
+    //
+    // b. Memory copying
+    //  1. We are obtaining pointers to references which are guaranteed to be valid.
+    //  2. They cannot overlap because we obtain pointers to difference indices of the slice.
+    //     Namely, `i` and `i+1`.
+    //  3. If the slice is properly aligned, the elements are properly aligned.
+    //     It is the caller's responsibility to make sure the slice is properly aligned.
+    //
+    // See comments below for further detail.
     unsafe {
         // If the last two elements are out-of-order...
         if len >= 2 && is_less(v.get_unchecked(len - 1), v.get_unchecked(len - 2)) {
@@ -103,6 +136,8 @@ where
     let mut i = 1;
 
     for _ in 0..MAX_STEPS {
+        // SAFETY: We already explicitly did the bound checking with `i < len`.
+        // All our subsequent indexing is only in the range `0 <= index < len`
         unsafe {
             // Find the next pair of adjacent out-of-order elements.
             while i < len && !is_less(v.get_unchecked(i), v.get_unchecked(i - 1)) {
@@ -220,6 +255,7 @@ where
     let mut offsets_l = [MaybeUninit::<u8>::uninit(); BLOCK];
 
     // The current block on the right side (from `r.sub(block_r)` to `r`).
+    // SAFETY: The documentation for .add() specifically mention that `vec.as_ptr().add(vec.len())` is always safe`
     let mut r = unsafe { l.add(v.len()) };
     let mut block_r = BLOCK;
     let mut start_r = ptr::null_mut();
@@ -268,6 +304,16 @@ where
             let mut elem = l;
 
             for i in 0..block_l {
+                // SAFETY: The unsafety operations below involve the usage of the `offset`.
+                //         According to the conditions required by the function, we satisfy them because:
+                //         1. `offsets_l` is stack-allocated, and thus considered separate allocated object.
+                //         2. The function `is_less` returns a `bool`.
+                //            Casting a `bool` will never overflow `isize`.
+                //         3. We have guaranteed that `block_l` will be `<= BLOCK`.
+                //            Plus, `end_l` was initially set to the begin pointer of `offsets_` which was declared on the stack.
+                //            Thus, we know that even in the worst case (all invocations of `is_less` returns false) we will only be at most 1 byte pass the end.
+                //        Another unsafety operation here is dereferencing `elem`.
+                //        However, `elem` was initially the begin pointer to the slice which is always valid.
                 unsafe {
                     // Branchless comparison.
                     *end_l = i as u8;
@@ -284,6 +330,17 @@ where
             let mut elem = r;
 
             for i in 0..block_r {
+                // SAFETY: The unsafety operations below involve the usage of the `offset`.
+                //         According to the conditions required by the function, we satisfy them because:
+                //         1. `offsets_r` is stack-allocated, and thus considered separate allocated object.
+                //         2. The function `is_less` returns a `bool`.
+                //            Casting a `bool` will never overflow `isize`.
+                //         3. We have guaranteed that `block_r` will be `<= BLOCK`.
+                //            Plus, `end_r` was initially set to the begin pointer of `offsets_` which was declared on the stack.
+                //            Thus, we know that even in the worst case (all invocations of `is_less` returns true) we will only be at most 1 byte pass the end.
+                //        Another unsafety operation here is dereferencing `elem`.
+                //        However, `elem` was initially `1 * sizeof(T)` past the end and we decrement it by `1 * sizeof(T)` before accessing it.
+                //        Plus, `block_r` was asserted to be less than `BLOCK` and `elem` will therefore at most be pointing to the beginning of the slice.
                 unsafe {
                     // Branchless comparison.
                     elem = elem.offset(-1);
@@ -404,8 +461,13 @@ where
         // Find the first pair of out-of-order elements.
         let mut l = 0;
         let mut r = v.len();
+
+        // SAFETY: The unsafety below involves indexing an array.
+        // For the first one: We already do the bounds checking here with `l < r`.
+        // For the second one: We initially have `l == 0` and `r == v.len()` and we checked that `l < r` at every indexing operation.
+        //                     From here we know that `r` must be at least `r == l` which was shown to be valid from the first one.
         unsafe {
-            // Find the first element greater then or equal to the pivot.
+            // Find the first element greater than or equal to the pivot.
             while l < r && is_less(v.get_unchecked(l), pivot) {
                 l += 1;
             }
@@ -444,6 +506,7 @@ where
 
     // Read the pivot into a stack-allocated variable for efficiency. If a following comparison
     // operation panics, the pivot will be automatically written back into the slice.
+    // SAFETY: The pointer here is valid because it is obtained from a reference to a slice.
     let mut tmp = mem::ManuallyDrop::new(unsafe { ptr::read(pivot) });
     let _pivot_guard = CopyOnDrop { src: &mut *tmp, dest: pivot };
     let pivot = &*tmp;
@@ -452,8 +515,12 @@ where
     let mut l = 0;
     let mut r = v.len();
     loop {
+        // SAFETY: The unsafety below involves indexing an array.
+        // For the first one: We already do the bounds checking here with `l < r`.
+        // For the second one: We initially have `l == 0` and `r == v.len()` and we checked that `l < r` at every indexing operation.
+        //                     From here we know that `r` must be at least `r == l` which was shown to be valid from the first one.
         unsafe {
-            // Find the first element greater that the pivot.
+            // Find the first element greater than the pivot.
             while l < r && !is_less(pivot, v.get_unchecked(l)) {
                 l += 1;
             }