for RefCell borrows the cell, this means that any mutable borrow of a DOM object's member will trigger a panic if a GC occurs while the borrow is still active. We often refer to this in Servo as a borrow proposing a static analysis. Verifying borrow hazards To verify that a particular mutable borrow can trigger be triggered when a GC occurs, we need 1) deterministic garbage collection, 2) a way to run the suspicious occurs, and is guaranteed to trigger any latent borrow hazards. It is also very slow, so minimizing your testcase will save you time. If you are unsure of exactly how to trigger the suspicious code, add a