split this into individual/individual_mut

src/ship/items/inventory.rs

@@ -198,7 +198,14 @@ impl InventoryItem {
         Ok(())
     }
 
-    pub fn individual(&mut self) -> Option<&mut IndividualInventoryItem> {
+    pub fn individual(&self) -> Option<&IndividualInventoryItem> {
+        match self {
+            InventoryItem::Individual(ref individual_inventory_item) => Some(individual_inventory_item),
+            _ => None
+        }
+    }
+
+    pub fn individual_mut(&mut self) -> Option<&mut IndividualInventoryItem> {
         match self {
             InventoryItem::Individual(ref mut individual_inventory_item) => Some(individual_inventory_item),
             _ => None

src/ship/items/manager.rs

@@ -611,7 +611,7 @@ impl ItemManager {
 
         let individual_item = mag_handle.item_mut()
             .ok_or(ItemManagerError::NoSuchItemId(mag_id))?
-            .individual()
+            .individual_mut()
             .ok_or(ItemManagerError::WrongItemType(mag_id))?;
         let mag = individual_item
             .mag_mut()

src/ship/items/use_tool.rs

@@ -52,7 +52,7 @@ async fn mag_cell<EG: EntityGateway>(entity_gateway: &mut EG, used_cell: &Consum
     let mag_item = mag_handle.item_mut()
         .ok_or(UseItemError::InvalidItem)?;
     let actual_mag = mag_item
-        .individual()
+        .individual_mut()
         .ok_or(UseItemError::InvalidItem)?
         .mag_mut()
         .ok_or(UseItemError::InvalidItem)?;