avoid writing to readonly refcounts

crates/roc_std/src/roc_list.rs

@@ -174,9 +174,12 @@ where
     /// should be considered for marking read-only.
     pub unsafe fn set_readonly(&mut self) {
         if let Some((_, storage)) = self.elements_and_storage() {
+            // Only safe to write to the pointer if it is not constant (0)
+            if !matches!(storage.get(), Storage::Readonly) {
                 storage.set(Storage::Readonly);
             }
         }
+    }
 
     /// Note that there is no way to convert directly to a Vec.
     ///
@@ -676,7 +679,10 @@ where
         let ptr = self.ptr_to_refcount();
         unsafe {
             let value = std::ptr::read(ptr);
-            std::ptr::write(ptr, Ord::max(0, ((value as isize) + 1) as usize));
+            // Only safe to write to the pointer if it is not constant (0)
+            if value != 0 {
+                std::ptr::write(ptr, (value as isize + 1) as usize);
+            }
         }
     }
 

crates/roc_std/src/roc_str.rs

@@ -943,14 +943,20 @@ impl BigString {
         assert_ne!(self.capacity(), 0);
 
         let ptr = self.ptr_to_refcount();
+        // Only safe to write to the pointer if it is not constant (0)
+        if unsafe { std::ptr::read(ptr) } != 0 {
             unsafe { std::ptr::write(ptr, 0) }
         }
+    }
 
     fn inc(&mut self) {
         let ptr = self.ptr_to_refcount();
         unsafe {
             let value = std::ptr::read(ptr);
-            std::ptr::write(ptr, Ord::max(0, ((value as isize) + 1) as usize));
+            // Only safe to write to the pointer if it is not constant (0)
+            if value != 0 {
+                std::ptr::write(ptr, (value as isize + 1) as usize);
+            }
         }
     }