I could write a novel on this topic, but I’ll try to keep it brief…
Since you’re talking about this being “unofficial” there’s nothing to stop you from creating a Slack team for the Rust community. Those who aren’t interested can simply not use it, and the Mozilla IRC channels aren’t going away.
That said, I really don’t like the trend of open communities using Slack. It’s a system owned and controlled by a business that does not value openness, interoperability, or the privacy of its users. Despite the hype it currently gets (which is largely deserved) it’s still just a business, and there’s no guarantee it’ll still be around in a year or two. It’s fine for a company to use Slack for their employees to chat (which is its primary use case) but it’s a very poor fit for something like this.
While the features Slack provides a team are great and it’s a good thing to want to bring them to communities using less powerful technology like IRC, the closed nature of Slack makes it less than an ideal for us. Many people are already getting annoyed with having to set up yet another Slack account just to discuss or get support for an otherwise open project. (For me, a good example of this was Kubernetes moving from IRC to Slack.)
My obviously biased opinion is that the best system to use for an open source project’s community chat is Matrix, because it is an open protocol, anyone can build a client or server for it, it’s federated, so no single company owns or controls the data, and it’s on par feature-wise with Slack (give or take, since all the Matrix clients are still beta-ish at best.)
I’m the author of Ruma, a fledgling Matrix server written in Rust. If you’re not already familiar with Matrix, we prepared a good Introduction to Matrix that explains it at a high level. Even the information in those documents should be enough to make it a compelling alternative to Slack for the Rust community’s purposes. More info about Matrix can be found at the official website. The premiere web-based Matrix client right now is Riot. You can either create an account to chat, or log in as a guest without providing any information about yourself, just like IRC.
It’s also important to note that Matrix bridges into other networks, including IRC. The Matrix server provided by Matrix.org bridges into both Freenode and Mozilla IRC, so users of Matrix and users of IRC can talk to each other in the same channels transparently, without even knowing that some other users are connected through the other system. This means that using Matrix as the community chat for Rust would make it completely transparently compatible with our existing IRC channels. In fact, this is already the case. I’ve been using Matrix through Riot to chat in the Mozilla IRC channels for some time now. This means that it’s not even necessary to “choose” Matrix. It’s simply there and working already. As such, any effort by an organized group who would’ve otherwise “chosen” Slack could simply put their effort into publicizing the fact that users can use either Matrix or IRC to participate in community chat.
I’m happy to answer more questions about Matrix, Riot, Matrix/IRC integration, etc.