With Rust, I've only heard of some people making Slacks. I don't use Slack, but at least one person mentioned on IRC that the Rust slack they were in was pretty much dead, so I'm not aware of this happening to Rust yet. We'll see as time goes on. There are something like 50 #rust- channels on Mozilla's IRC and ##rust on Freenode, though.

I've also heard of quite a few very active JavaScript Freenode channels, though Slack does have a lot of inroads there depending on which particular segment of the community you're into.