From 2935dd642e897172905c843e355542f60138b3f2 Mon Sep 17 00:00:00 2001
From: Sebastiano Vigna <sebastiano.vigna@gmail.com>
Date: Sun, 17 Nov 2024 08:47:52 +0000
Subject: [PATCH] from_mut is unsafe to mark the possibility of UB

---
 src/algo/llp/mod.rs     |  2 +-
 src/utils/sync_slice.rs | 16 +++++++++++++---
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/src/algo/llp/mod.rs b/src/algo/llp/mod.rs
index d7908c3b..b579e3de 100644
--- a/src/algo/llp/mod.rs
+++ b/src/algo/llp/mod.rs
@@ -408,7 +408,7 @@ fn combine(result: &mut [usize], labels: &[usize], temp_perm: &mut [usize]) -> R
 }
 
 pub fn invert_permutation(perm: &[usize], inv_perm: &mut [usize]) {
-    let sync_slice = SyncSlice::from_mut(inv_perm);
+    let sync_slice = unsafe { SyncSlice::from_mut(inv_perm) };
     perm.par_iter().enumerate().for_each(|(i, &x)| {
         sync_slice.set(x, i);
     });
diff --git a/src/utils/sync_slice.rs b/src/utils/sync_slice.rs
index 5660ea29..fde1d7e0 100644
--- a/src/utils/sync_slice.rs
+++ b/src/utils/sync_slice.rs
@@ -42,8 +42,17 @@ unsafe impl<'a, T: Send> Sync for SyncSlice<'a, T> {}
 
 impl<'a, T> SyncSlice<'a, T> {
     /// Creates a new synchronized slice from a mutable reference.
+    ///
+    /// # Safety
+    ///
+    /// It is the caller's responsibility to ensure that no data
+    /// races occur.
+    ///
+    /// # Undefined Behavior
+    ///
+    /// Data races will cause undefined behaviour.
     #[inline(always)]
-    pub fn from_mut(slice: &'a mut [T]) -> Self {
+    pub unsafe fn from_mut(slice: &'a mut [T]) -> Self {
         Self(Cell::from_mut(slice).as_slice_of_cells())
     }
 }
@@ -152,11 +161,12 @@ impl<'a, T> SyncSlice<'a, T> {
 /// Extension trait providing a [synchronized view](SyncSlice) of a slice via
 /// the [`as_sync_slice`](SyncSliceExt::as_sync_slice) method.
 pub trait SyncSliceExt<'a, T: Copy> {
-    fn as_sync_slice(&'a mut self) -> SyncSlice<'a, T>;
+    /// Invokes [`SyncSlice::from_mut`] on the slice.
+    unsafe fn as_sync_slice(&'a mut self) -> SyncSlice<'a, T>;
 }
 
 impl<'a, T: Copy> SyncSliceExt<'a, T> for [T] {
-    fn as_sync_slice(&'a mut self) -> SyncSlice<'a, T> {
+    unsafe fn as_sync_slice(&'a mut self) -> SyncSlice<'a, T> {
         SyncSlice::from_mut(self)
     }
 }