Fix integer overflow in BlobVec::push for ZST (#10799)

`reserve_exact` is no-op for ZST because `self.item_layout.size() > 0`
is always `false`.


https://github.com/bevyengine/bevy/blob/daa8bf20dfb0d5e3e2c8dbd032d532bde133f221/crates/bevy_ecs/src/storage/blob_vec.rs#L112-L120

Then in `push` we just increase `.len` ignoring integer overflow.


https://github.com/bevyengine/bevy/blob/daa8bf20dfb0d5e3e2c8dbd032d532bde133f221/crates/bevy_ecs/src/storage/blob_vec.rs#L232-L237

crates/bevy_ecs/src/storage/blob_vec.rs

@@ -61,6 +61,8 @@ impl BlobVec {
         if item_layout.size() == 0 {
             BlobVec {
                 data,
+                // ZST `BlobVec` max size is `usize::MAX`, and `reserve_exact` for ZST assumes
+                // the capacity is always `usize::MAX` and panics if it overflows.
                 capacity: usize::MAX,
                 len: 0,
                 item_layout,
@@ -109,20 +111,29 @@ impl BlobVec {
     ///
     /// Note that the allocator may give the collection more space than it requests. Therefore, capacity can not be relied upon
     /// to be precisely minimal.
+    ///
+    /// # Panics
+    ///
+    /// Panics if new capacity overflows `usize`.
     pub fn reserve_exact(&mut self, additional: usize) {
         let available_space = self.capacity - self.len;
-        if available_space < additional && self.item_layout.size() > 0 {
+        if available_space < additional {
             // SAFETY: `available_space < additional`, so `additional - available_space > 0`
             let increment = unsafe { NonZeroUsize::new_unchecked(additional - available_space) };
-            // SAFETY: not called for ZSTs
-            unsafe { self.grow_exact(increment) };
+            self.grow_exact(increment);
         }
     }
 
-    // SAFETY: must not be called for a ZST item layout
-    #[warn(unsafe_op_in_unsafe_fn)] // to allow unsafe blocks in unsafe fn
-    unsafe fn grow_exact(&mut self, increment: NonZeroUsize) {
-        debug_assert!(self.item_layout.size() != 0);
+    /// Grows the capacity by `increment` elements.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the new capacity overflows `usize`.
+    /// For ZST it panics unconditionally because ZST `BlobVec` capacity
+    /// is initialized to `usize::MAX` and always stays that way.
+    fn grow_exact(&mut self, increment: NonZeroUsize) {
+        // If we got here with ZST, we requested total capacity > usize::MAX.
+        assert!(self.item_layout.size() != 0, "ZST capacity overflow");
 
         let new_capacity = self.capacity + increment.get();
         let new_layout =
@@ -609,6 +620,30 @@ mod tests {
         let _ = unsafe { BlobVec::new(item_layout, Some(drop), 0) };
     }
 
+    #[test]
+    #[should_panic(expected = "ZST capacity overflow")]
+    fn blob_vec_zst_size_overflow() {
+        // SAFETY: no drop is correct drop for `()`.
+        let mut blob_vec = unsafe { BlobVec::new(Layout::new::<()>(), None, 0) };
+
+        assert_eq!(usize::MAX, blob_vec.capacity(), "Self-check");
+
+        // SAFETY: Because `()` is a ZST trivial drop type, and because `BlobVec` capacity
+        //   is always `usize::MAX` for ZSTs, we can arbitrarily set the length
+        //   and still be sound.
+        unsafe {
+            blob_vec.set_len(usize::MAX);
+        }
+
+        // SAFETY: `BlobVec` was initialized for `()`, so it is safe to push `()` to it.
+        unsafe {
+            OwningPtr::make((), |ptr| {
+                // This should panic because len is usize::MAX, remaining capacity is 0.
+                blob_vec.push(ptr);
+            });
+        }
+    }
+
     #[test]
     fn aligned_zst() {
         // NOTE: This test is explicitly for uncovering potential UB with miri.