`enqueue` is a mutable behavior but with a immutable function type

[yanganto]

The enqueue function of the public trait MessageQueue is immutable. In general, we do mutate some data from the Queue, when we put things into it.

[yanganto]

It may be an intentional design to keep the mutability inside. The thing I should use is TestMessageQueue, but the test queue is private, and the trait is public.

[tnull]

It may be an intentional design to keep the mutability inside. The thing I should use is TestMessageQueue, but the test queue is private, and the trait is public.

It is in fact intentional. As noted in #73, the trait was introduced essentially as an intermediate step when moving the PeerManager dependency from all of the message handlers to just one place: the message queue. However, users probably wouldn't benefit from using a custom MessageQueue implementation, which is why we're considering to just remove it again and just use DefaultMessageQueue everywhere.

[yanganto]

Got it