Rollup merge of #87127 - poliorcetics:ptr-rotate-safety, r=scottmcm

Add safety comments in private core::slice::rotate::ptr_rotate function

Helps with #66219.

```@rustbot``` label C-cleanup T-compiler T-libs

Author: JohnTitor

library/core/src/slice/rotate.rs

@@ -1,5 +1,3 @@
-// ignore-tidy-undocumented-unsafe
-
 use crate::cmp;
 use crate::mem::{self, MaybeUninit};
 use crate::ptr;
@@ -79,8 +77,10 @@ pub unsafe fn ptr_rotate<T>(mut left: usize, mut mid: *mut T, mut right: usize)
             // the way until about `left + right == 32`, but the worst case performance breaks even
             // around 16. 24 was chosen as middle ground. If the size of `T` is larger than 4
             // `usize`s, this algorithm also outperforms other algorithms.
+            // SAFETY: callers must ensure `mid - left` is valid for reading and writing.
             let x = unsafe { mid.sub(left) };
             // beginning of first round
+            // SAFETY: see previous comment.
             let mut tmp: T = unsafe { x.read() };
             let mut i = right;
             // `gcd` can be found before hand by calculating `gcd(left + right, right)`,
@@ -92,6 +92,21 @@ pub unsafe fn ptr_rotate<T>(mut left: usize, mut mid: *mut T, mut right: usize)
             // the very end. This is possibly due to the fact that swapping or replacing temporaries
             // uses only one memory address in the loop instead of needing to manage two.
             loop {
+                // [long-safety-expl]
+                // SAFETY: callers must ensure `[left, left+mid+right)` are all valid for reading and
+                // writing.
+                //
+                // - `i` start with `right` so `mid-left <= x+i = x+right = mid-left+right < mid+right`
+                // - `i <= left+right-1` is always true
+                //   - if `i < left`, `right` is added so `i < left+right` and on the next
+                //     iteration `left` is removed from `i` so it doesn't go further
+                //   - if `i >= left`, `left` is removed immediately and so it doesn't go further.
+                // - overflows cannot happen for `i` since the function's safety contract ask for
+                //   `mid+right-1 = x+left+right` to be valid for writing
+                // - underflows cannot happen because `i` must be bigger or equal to `left` for
+                //   a substraction of `left` to happen.
+                //
+                // So `x+i` is valid for reading and writing if the caller respected the contract
                 tmp = unsafe { x.add(i).replace(tmp) };
                 // instead of incrementing `i` and then checking if it is outside the bounds, we
                 // check if `i` will go outside the bounds on the next increment. This prevents
@@ -100,6 +115,8 @@ pub unsafe fn ptr_rotate<T>(mut left: usize, mut mid: *mut T, mut right: usize)
                     i -= left;
                     if i == 0 {
                         // end of first round
+                        // SAFETY: tmp has been read from a valid source and x is valid for writing
+                        // according to the caller.
                         unsafe { x.write(tmp) };
                         break;
                     }
@@ -113,13 +130,24 @@ pub unsafe fn ptr_rotate<T>(mut left: usize, mut mid: *mut T, mut right: usize)
             }
             // finish the chunk with more rounds
             for start in 1..gcd {
+                // SAFETY: `gcd` is at most equal to `right` so all values in `1..gcd` are valid for
+                // reading and writing as per the function's safety contract, see [long-safety-expl]
+                // above
                 tmp = unsafe { x.add(start).read() };
+                // [safety-expl-addition]
+                //
+                // Here `start < gcd` so `start < right` so `i < right+right`: `right` being the
+                // greatest common divisor of `(left+right, right)` means that `left = right` so
+                // `i < left+right` so `x+i = mid-left+i` is always valid for reading and writing
+                // according to the function's safety contract.
                 i = start + right;
                 loop {
+                    // SAFETY: see [long-safety-expl] and [safety-expl-addition]
                     tmp = unsafe { x.add(i).replace(tmp) };
                     if i >= left {
                         i -= left;
                         if i == start {
+                            // SAFETY: see [long-safety-expl] and [safety-expl-addition]
                             unsafe { x.add(start).write(tmp) };
                             break;
                         }
@@ -135,14 +163,30 @@ pub unsafe fn ptr_rotate<T>(mut left: usize, mut mid: *mut T, mut right: usize)
             // The `[T; 0]` here is to ensure this is appropriately aligned for T
             let mut rawarray = MaybeUninit::<(BufType, [T; 0])>::uninit();
             let buf = rawarray.as_mut_ptr() as *mut T;
+            // SAFETY: `mid-left <= mid-left+right < mid+right`
             let dim = unsafe { mid.sub(left).add(right) };
             if left <= right {
+                // SAFETY:
+                //
+                // 1) The `else if` condition about the sizes ensures `[mid-left; left]` will fit in
+                //    `buf` without overflow and `buf` was created just above and so cannot be
+                //    overlapped with any value of `[mid-left; left]`
+                // 2) [mid-left, mid+right) are all valid for reading and writing and we don't care
+                //    about overlaps here.
+                // 3) The `if` condition about `left <= right` ensures writing `left` elements to
+                //    `dim = mid-left+right` is valid because:
+                //    - `buf` is valid and `left` elements were written in it in 1)
+                //    - `dim+left = mid-left+right+left = mid+right` and we write `[dim, dim+left)`
                 unsafe {
+                    // 1)
                     ptr::copy_nonoverlapping(mid.sub(left), buf, left);
+                    // 2)
                     ptr::copy(mid, mid.sub(left), right);
+                    // 3)
                     ptr::copy_nonoverlapping(buf, dim, left);
                 }
             } else {
+                // SAFETY: same reasoning as above but with `left` and `right` reversed
                 unsafe {
                     ptr::copy_nonoverlapping(mid, buf, right);
                     ptr::copy(mid.sub(left), dim, left);
@@ -156,6 +200,10 @@ pub unsafe fn ptr_rotate<T>(mut left: usize, mut mid: *mut T, mut right: usize)
             // of this algorithm would be, and swapping using that last chunk instead of swapping
             // adjacent chunks like this algorithm is doing, but this way is still faster.
             loop {
+                // SAFETY:
+                // `left >= right` so `[mid-right, mid+right)` is valid for reading and writing
+                // Substracting `right` from `mid` each turn is counterbalanced by the addition and
+                // check after it.
                 unsafe {
                     ptr::swap_nonoverlapping(mid.sub(right), mid, right);
                     mid = mid.sub(right);
@@ -168,6 +216,10 @@ pub unsafe fn ptr_rotate<T>(mut left: usize, mut mid: *mut T, mut right: usize)
         } else {
             // Algorithm 3, `left < right`
             loop {
+                // SAFETY: `[mid-left, mid+left)` is valid for reading and writing because
+                // `left < right` so `mid+left < mid+right`.
+                // Adding `left` to `mid` each turn is counterbalanced by the substraction and check
+                // after it.
                 unsafe {
                     ptr::swap_nonoverlapping(mid.sub(left), mid, left);
                     mid = mid.add(left);