From the reply to my sibling post and yours, I stand corrected. It seems IRC is big with older languages like C and related (as in, a similar domain) ones like Rust[1]. JavaScript and new, web related languages like Elm are more likely to use Slack.

[1] I'm curious as to whether that IRC channel is the only "blessed" Rust chat out there. The problem with other languages is that the community makes a Slack for them even though there's an IRC channel for them already and most new users start only going to the Slack.

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.

There are a bunch of substantially less trafficked, though definitely regularly active, channels for Rust: #rust-offtopic, #rust-internals, #rust-beginners (this one is recommended in the official documentation and is well-traficked), #rust-osdev, several others too.