At the moment, MaskedStorage requires a Component bound on its type parameter, and it's missing two of the methods that UnprotectedStorage offers. I think it would make the type more flexible, with relatively little work, if the following changes were made:

• Remove the T: Component bound, instead adding a second type parameter S for the storage, with a S: UnprotectedStorage<T> bound.
• Add the missing methods .get and .get_mut that forward to the corresponding UnprotectedStorage methods.

With these changes, the MaskedStorage type could be used for any T, anything that needs an ID for lookup would work, not just components. It would make it simply a safe wrapper around UnprotectedStorage. An example is Amethyst, which currently uses UnprotectedStorage for asset storage, but which has had to build its own safety wrappers. If MaskedStorage were changed in this way, users could use that type instead, without safety concerns.