Implement typed routes for ROUTER

[simonwuelker]

Implements ROUTER::add_typed_route, which (unlike ROUTER::add_route) knows about the type expected by the callback and makes it impossible to mismatch types between the receiver and the callback.

This method can be used pretty much exactly like ROUTER::add_route, I added a small test case that demonstrates this.

Closes #242

[jdm]

This is a great usability improvement! Thanks.

[mrobinson]

This is a great usability improvement! Thanks.

Agreed! Unfortunately, add_route is a bit easy to use because of #239. Does it make sense to mark it as unsafe?

[simonwuelker]

Agreed! Unfortunately, add_route is a bit easy to use because of #239. Does it make sense to mark it as unsafe?

unsafe is usually reserved for functions that can cause undefined behavior if used incorrectly, and I don't think add_route can quite do that.

Perhaps a
#[deprecated(since="0.18.4", note="please use 'add_typed_route' instead")]
annotation would be more appropriate?

[simonwuelker]

Hmm, perhaps I should have implemented the callback to receive a Result<T, Error> instead.
Functions like servo's route_ipc_receiver_to_new_crossbeam_receiver_preserving_errors need to know about any errors that occured, because constellation is not allowed to crash.

I'll make a follow-up PR once we have decided whether to mark add_route as unsafe/deprecated.

[jdm]

Agreed that deprecated feels more appropriate here than unsafe. If there isn't a way to trigger memory unsafety, unsafe would be overkill.

[mrobinson]

I guess since serializable types cannot contain pointers, it is not possible to create memory unsafety with this issue. 🤔