Fix slice::ChunksMut aliasing

Author: saethlin

library/core/src/slice/iter.rs

@@ -1629,14 +1629,15 @@ unsafe impl<'a, T> TrustedRandomAccessNoCoerce for Chunks<'a, T> {
 #[stable(feature = "rust1", since = "1.0.0")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct ChunksMut<'a, T: 'a> {
-    v: &'a mut [T],
+    v: *mut [T],
     chunk_size: usize,
+    _marker: PhantomData<&'a mut T>,
 }
 
 impl<'a, T: 'a> ChunksMut<'a, T> {
     #[inline]
     pub(super) fn new(slice: &'a mut [T], size: usize) -> Self {
-        Self { v: slice, chunk_size: size }
+        Self { v: slice, chunk_size: size, _marker: PhantomData }
     }
 }
 
@@ -1650,10 +1651,11 @@ impl<'a, T> Iterator for ChunksMut<'a, T> {
             None
         } else {
             let sz = cmp::min(self.v.len(), self.chunk_size);
-            let tmp = mem::replace(&mut self.v, &mut []);
-            let (head, tail) = tmp.split_at_mut(sz);
+            // SAFETY: sz cannot exceed the slice length based on the calculation above
+            let (head, tail) = unsafe { self.v.split_at_mut(sz) };
             self.v = tail;
-            Some(head)
+            // SAFETY: Nothing points to or will point to the contents of this slice
+            Some(unsafe { &mut *head })
         }
     }
 
@@ -1685,11 +1687,13 @@ impl<'a, T> Iterator for ChunksMut<'a, T> {
                 Some(sum) => cmp::min(self.v.len(), sum),
                 None => self.v.len(),
             };
-            let tmp = mem::replace(&mut self.v, &mut []);
-            let (head, tail) = tmp.split_at_mut(end);
-            let (_, nth) = head.split_at_mut(start);
+            // SAFETY: end is inbounds because we compared above against self.v.len()
+            let (head, tail) = unsafe { self.v.split_at_mut(end) };
+            // SAFETY: start is inbounds because
+            let (_, nth) = unsafe { head.split_at_mut(start) };
             self.v = tail;
-            Some(nth)
+            // SAFETY: Nothing points to or will point to the contents of this slice
+            Some(unsafe { &mut *nth })
         }
     }
 
@@ -1699,7 +1703,8 @@ impl<'a, T> Iterator for ChunksMut<'a, T> {
             None
         } else {
             let start = (self.v.len() - 1) / self.chunk_size * self.chunk_size;
-            Some(&mut self.v[start..])
+            // SAFETY: Nothing points to or will point to the contents of this slice
+            Some(unsafe { &mut *self.v.get_unchecked_mut(start..) })
         }
     }
 
@@ -1727,12 +1732,12 @@ impl<'a, T> DoubleEndedIterator for ChunksMut<'a, T> {
         } else {
             let remainder = self.v.len() % self.chunk_size;
             let sz = if remainder != 0 { remainder } else { self.chunk_size };
-            let tmp = mem::replace(&mut self.v, &mut []);
-            let tmp_len = tmp.len();
+            let len = self.v.len();
             // SAFETY: Similar to `Chunks::next_back`
-            let (head, tail) = unsafe { tmp.split_at_mut_unchecked(tmp_len - sz) };
+            let (head, tail) = unsafe { self.v.split_at_mut_unchecked(len - sz) };
             self.v = head;
-            Some(tail)
+            // SAFETY: Nothing points to or will point to the contents of this slice
+            Some(unsafe { &mut *tail })
         }
     }
 
@@ -1748,10 +1753,12 @@ impl<'a, T> DoubleEndedIterator for ChunksMut<'a, T> {
                 Some(res) => cmp::min(self.v.len(), res),
                 None => self.v.len(),
             };
-            let (temp, _tail) = mem::replace(&mut self.v, &mut []).split_at_mut(end);
-            let (head, nth_back) = temp.split_at_mut(start);
+            // SAFETY: end is inbounds because we compared above against self.v.len()
+            let (temp, _tail) = unsafe { self.v.split_at_mut(end) };
+            let (head, nth_back) = unsafe { temp.split_at_mut(start) };
             self.v = head;
-            Some(nth_back)
+            // SAFETY: Nothing points to or will point to the contents of this slice
+            Some(unsafe { &mut *nth_back })
         }
     }
 }
@@ -1957,9 +1964,10 @@ unsafe impl<'a, T> TrustedRandomAccessNoCoerce for ChunksExact<'a, T> {
 #[stable(feature = "chunks_exact", since = "1.31.0")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct ChunksExactMut<'a, T: 'a> {
-    v: &'a mut [T],
-    rem: &'a mut [T],
+    v: *mut [T],
+    rem: &'a mut [T], // The iterator never yields from here, so this can be unique
     chunk_size: usize,
+    _marker: PhantomData<&'a mut T>,
 }
 
 impl<'a, T> ChunksExactMut<'a, T> {
@@ -1969,7 +1977,7 @@ impl<'a, T> ChunksExactMut<'a, T> {
         let fst_len = slice.len() - rem;
         // SAFETY: 0 <= fst_len <= slice.len() by construction above
         let (fst, snd) = unsafe { slice.split_at_mut_unchecked(fst_len) };
-        Self { v: fst, rem: snd, chunk_size }
+        Self { v: fst, rem: snd, chunk_size, _marker: PhantomData }
     }
 
     /// Returns the remainder of the original slice that is not going to be
@@ -1991,10 +1999,11 @@ impl<'a, T> Iterator for ChunksExactMut<'a, T> {
         if self.v.len() < self.chunk_size {
             None
         } else {
-            let tmp = mem::replace(&mut self.v, &mut []);
-            let (head, tail) = tmp.split_at_mut(self.chunk_size);
+            // SAFETY: self.chunk_size is inbounds because we compared above against self.v.len()
+            let (head, tail) = unsafe { self.v.split_at_mut(self.chunk_size) };
             self.v = tail;
-            Some(head)
+            // SAFETY: Nothing points to or will point to the contents of this slice
+            Some(unsafe { &mut *head })
         }
     }
 
@@ -2016,8 +2025,7 @@ impl<'a, T> Iterator for ChunksExactMut<'a, T> {
             self.v = &mut [];
             None
         } else {
-            let tmp = mem::replace(&mut self.v, &mut []);
-            let (_, snd) = tmp.split_at_mut(start);
+            let (_, snd) = unsafe { self.v.split_at_mut(start) };
             self.v = snd;
             self.next()
         }
@@ -2042,11 +2050,11 @@ impl<'a, T> DoubleEndedIterator for ChunksExactMut<'a, T> {
         if self.v.len() < self.chunk_size {
             None
         } else {
-            let tmp = mem::replace(&mut self.v, &mut []);
-            let tmp_len = tmp.len();
-            let (head, tail) = tmp.split_at_mut(tmp_len - self.chunk_size);
+            // SAFETY: This subtraction is inbounds because of the check above
+            let (head, tail) = unsafe { self.v.split_at_mut(self.v.len() - self.chunk_size) };
             self.v = head;
-            Some(tail)
+            // SAFETY: Nothing points to or will point to the contents of this slice
+            Some(unsafe { &mut *tail })
         }
     }
 
@@ -2059,10 +2067,11 @@ impl<'a, T> DoubleEndedIterator for ChunksExactMut<'a, T> {
         } else {
             let start = (len - 1 - n) * self.chunk_size;
             let end = start + self.chunk_size;
-            let (temp, _tail) = mem::replace(&mut self.v, &mut []).split_at_mut(end);
-            let (head, nth_back) = temp.split_at_mut(start);
+            let (temp, _tail) = unsafe { mem::replace(&mut self.v, &mut []).split_at_mut(end) };
+            let (head, nth_back) = unsafe { temp.split_at_mut(start) };
             self.v = head;
-            Some(nth_back)
+            // SAFETY: Nothing points to or will point to the contents of this slice
+            Some(unsafe { &mut *nth_back })
         }
     }
 }
@@ -2646,14 +2655,15 @@ unsafe impl<'a, T> TrustedRandomAccessNoCoerce for RChunks<'a, T> {
 #[stable(feature = "rchunks", since = "1.31.0")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct RChunksMut<'a, T: 'a> {
-    v: &'a mut [T],
+    v: *mut [T],
     chunk_size: usize,
+    _marker: PhantomData<&'a mut T>,
 }
 
 impl<'a, T: 'a> RChunksMut<'a, T> {
     #[inline]
     pub(super) fn new(slice: &'a mut [T], size: usize) -> Self {
-        Self { v: slice, chunk_size: size }
+        Self { v: slice, chunk_size: size, _marker: PhantomData }
     }
 }
 
@@ -2667,16 +2677,16 @@ impl<'a, T> Iterator for RChunksMut<'a, T> {
             None
         } else {
             let sz = cmp::min(self.v.len(), self.chunk_size);
-            let tmp = mem::replace(&mut self.v, &mut []);
-            let tmp_len = tmp.len();
+            let len = self.v.len();
             // SAFETY: split_at_mut_unchecked just requires the argument be less
             // than the length. This could only happen if the expression
-            // `tmp_len - sz` overflows. This could only happen if `sz >
-            // tmp_len`, which is impossible as we initialize it as the `min` of
-            // `self.v.len()` (e.g. `tmp_len`) and `self.chunk_size`.
-            let (head, tail) = unsafe { tmp.split_at_mut_unchecked(tmp_len - sz) };
+            // `len - sz` overflows. This could only happen if `sz >
+            // len`, which is impossible as we initialize it as the `min` of
+            // `self.v.len()` (e.g. `len`) and `self.chunk_size`.
+            let (head, tail) = unsafe { self.v.split_at_mut_unchecked(len - sz) };
             self.v = head;
-            Some(tail)
+            // SAFETY: Nothing points to or will point to the contents of this slice
+            Some(unsafe { &mut *tail })
         }
     }
 
@@ -2710,11 +2720,11 @@ impl<'a, T> Iterator for RChunksMut<'a, T> {
                 Some(sum) => sum,
                 None => 0,
             };
-            let tmp = mem::replace(&mut self.v, &mut []);
-            let (head, tail) = tmp.split_at_mut(start);
-            let (nth, _) = tail.split_at_mut(end - start);
+            let (head, tail) = unsafe { self.v.split_at_mut(start) };
+            let (nth, _) = unsafe { tail.split_at_mut(end - start) };
             self.v = head;
-            Some(nth)
+            // SAFETY: Nothing points to or will point to the contents of this slice
+            Some(unsafe { &mut *nth })
         }
     }
 
@@ -2725,7 +2735,8 @@ impl<'a, T> Iterator for RChunksMut<'a, T> {
         } else {
             let rem = self.v.len() % self.chunk_size;
             let end = if rem == 0 { self.chunk_size } else { rem };
-            Some(&mut self.v[0..end])
+            // SAFETY: Nothing points to or will point to the contents of this slice
+            Some(unsafe { &mut *self.v.get_unchecked_mut(0..end) })
         }
     }
 
@@ -2750,11 +2761,11 @@ impl<'a, T> DoubleEndedIterator for RChunksMut<'a, T> {
         } else {
             let remainder = self.v.len() % self.chunk_size;
             let sz = if remainder != 0 { remainder } else { self.chunk_size };
-            let tmp = mem::replace(&mut self.v, &mut []);
             // SAFETY: Similar to `Chunks::next_back`
-            let (head, tail) = unsafe { tmp.split_at_mut_unchecked(sz) };
+            let (head, tail) = unsafe { self.v.split_at_mut_unchecked(sz) };
             self.v = tail;
-            Some(head)
+            // SAFETY: Nothing points to or will point to the contents of this slice
+            Some(unsafe { &mut *head })
         }
     }
 
@@ -2769,10 +2780,11 @@ impl<'a, T> DoubleEndedIterator for RChunksMut<'a, T> {
             let offset_from_end = (len - 1 - n) * self.chunk_size;
             let end = self.v.len() - offset_from_end;
             let start = end.saturating_sub(self.chunk_size);
-            let (tmp, tail) = mem::replace(&mut self.v, &mut []).split_at_mut(end);
-            let (_, nth_back) = tmp.split_at_mut(start);
+            let (tmp, tail) = unsafe { self.v.split_at_mut(end) };
+            let (_, nth_back) = unsafe { tmp.split_at_mut(start) };
             self.v = tail;
-            Some(nth_back)
+            // SAFETY: Nothing points to or will point to the contents of this slice
+            Some(unsafe { &mut *nth_back })
         }
     }
 }
@@ -2898,8 +2910,7 @@ impl<'a, T> Iterator for RChunksExact<'a, T> {
     unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> Self::Item {
         let end = self.v.len() - idx * self.chunk_size;
         let start = end - self.chunk_size;
-        // SAFETY:
-        // SAFETY: mostmy identical to `Chunks::__iterator_get_unchecked`.
+        // SAFETY: mostly identical to `Chunks::__iterator_get_unchecked`.
         unsafe { from_raw_parts(self.v.as_ptr().add(start), self.chunk_size) }
     }
 }
@@ -2982,7 +2993,7 @@ unsafe impl<'a, T> TrustedRandomAccessNoCoerce for RChunksExact<'a, T> {
 #[stable(feature = "rchunks", since = "1.31.0")]
 #[must_use = "iterators are lazy and do nothing unless consumed"]
 pub struct RChunksExactMut<'a, T: 'a> {
-    v: &'a mut [T],
+    v: *mut [T],
     rem: &'a mut [T],
     chunk_size: usize,
 }
@@ -3015,11 +3026,11 @@ impl<'a, T> Iterator for RChunksExactMut<'a, T> {
         if self.v.len() < self.chunk_size {
             None
         } else {
-            let tmp = mem::replace(&mut self.v, &mut []);
-            let tmp_len = tmp.len();
-            let (head, tail) = tmp.split_at_mut(tmp_len - self.chunk_size);
+            let len = self.v.len();
+            let (head, tail) = unsafe { self.v.split_at_mut(len - self.chunk_size) };
             self.v = head;
-            Some(tail)
+            // SAFETY: Nothing points to or will point to the contents of this slice
+            Some(unsafe { &mut *tail })
         }
     }
 
@@ -3041,9 +3052,8 @@ impl<'a, T> Iterator for RChunksExactMut<'a, T> {
             self.v = &mut [];
             None
         } else {
-            let tmp = mem::replace(&mut self.v, &mut []);
-            let tmp_len = tmp.len();
-            let (fst, _) = tmp.split_at_mut(tmp_len - end);
+            let len = self.v.len();
+            let (fst, _) = unsafe { self.v.split_at_mut(len - end) };
             self.v = fst;
             self.next()
         }
@@ -3069,10 +3079,10 @@ impl<'a, T> DoubleEndedIterator for RChunksExactMut<'a, T> {
         if self.v.len() < self.chunk_size {
             None
         } else {
-            let tmp = mem::replace(&mut self.v, &mut []);
-            let (head, tail) = tmp.split_at_mut(self.chunk_size);
+            let (head, tail) = unsafe { self.v.split_at_mut(self.chunk_size) };
             self.v = tail;
-            Some(head)
+            // SAFETY: Nothing points to or will point to the contents of this slice
+            Some(unsafe { &mut *head })
         }
     }
 
@@ -3088,10 +3098,11 @@ impl<'a, T> DoubleEndedIterator for RChunksExactMut<'a, T> {
             let offset = (len - n) * self.chunk_size;
             let start = self.v.len() - offset;
             let end = start + self.chunk_size;
-            let (tmp, tail) = mem::replace(&mut self.v, &mut []).split_at_mut(end);
-            let (_, nth_back) = tmp.split_at_mut(start);
+            let (tmp, tail) = unsafe { self.v.split_at_mut(end) };
+            let (_, nth_back) = unsafe { tmp.split_at_mut(start) };
             self.v = tail;
-            Some(nth_back)
+            // SAFETY: Nothing points to or will point to the contents of this slice
+            Some(unsafe { &mut *nth_back })
         }
     }
 }
@@ -3174,7 +3185,11 @@ where
             let mut len = 1;
             let mut iter = self.slice.windows(2);
             while let Some([l, r]) = iter.next() {
-                if (self.predicate)(l, r) { len += 1 } else { break }
+                if (self.predicate)(l, r) {
+                    len += 1
+                } else {
+                    break;
+                }
             }
             let (head, tail) = self.slice.split_at(len);
             self.slice = tail;
@@ -3206,7 +3221,11 @@ where
             let mut len = 1;
             let mut iter = self.slice.windows(2);
             while let Some([l, r]) = iter.next_back() {
-                if (self.predicate)(l, r) { len += 1 } else { break }
+                if (self.predicate)(l, r) {
+                    len += 1
+                } else {
+                    break;
+                }
             }
             let (head, tail) = self.slice.split_at(self.slice.len() - len);
             self.slice = head;
@@ -3261,7 +3280,11 @@ where
             let mut len = 1;
             let mut iter = self.slice.windows(2);
             while let Some([l, r]) = iter.next() {
-                if (self.predicate)(l, r) { len += 1 } else { break }
+                if (self.predicate)(l, r) {
+                    len += 1
+                } else {
+                    break;
+                }
             }
             let slice = mem::take(&mut self.slice);
             let (head, tail) = slice.split_at_mut(len);
@@ -3294,7 +3317,11 @@ where
             let mut len = 1;
             let mut iter = self.slice.windows(2);
             while let Some([l, r]) = iter.next_back() {
-                if (self.predicate)(l, r) { len += 1 } else { break }
+                if (self.predicate)(l, r) {
+                    len += 1
+                } else {
+                    break;
+                }
             }
             let slice = mem::take(&mut self.slice);
             let (head, tail) = slice.split_at_mut(slice.len() - len);