chore(serde_v8): V8Slice SAFETY (#14493)

serde_v8/magic/v8slice.rs

@@ -41,12 +41,26 @@ impl V8Slice {
   }
 
   fn as_slice(&self) -> &[u8] {
+    // SAFETY: v8::SharedRef<v8::BackingStore> is similar to Arc<[u8]>,
+    // it points to a fixed continuous slice of bytes on the heap.
+    // We assume it's initialized and thus safe to read (though may not contain meaningful data)
     unsafe { &*(&self.store[self.range.clone()] as *const _ as *const [u8]) }
   }
 
-  #[allow(clippy::cast_ref_to_mut)]
   fn as_slice_mut(&mut self) -> &mut [u8] {
-    unsafe { &mut *(&self.store[self.range.clone()] as *const _ as *mut [u8]) }
+    // SAFETY: v8::SharedRef<v8::BackingStore> is similar to Arc<[u8]>,
+    // it points to a fixed continuous slice of bytes on the heap.
+    // It's safe-ish to mutate concurrently because it can not be
+    // shrunk/grown/moved/reallocated, thus avoiding dangling refs (unlike a Vec).
+    // Concurrent writes can't lead to meaningful structural invalidation
+    // since we treat them as opaque buffers / "bags of bytes",
+    // concurrent mutation is simply an accepted fact of life.
+    // And in practice V8Slices also do not have overallping read/write phases.
+    // TLDR: permissive interior mutability on slices of bytes is "fine"
+    #[allow(clippy::cast_ref_to_mut)]
+    unsafe {
+      &mut *(&self.store[self.range.clone()] as *const _ as *mut [u8])
+    }
   }
 }