I think Rust shouldn’t do an LTS.
An LTS would undermine the “stability without stagnation” story in two ways. First, it would signal that the “stability” part isn’t really true, because otherwise an LTS wouldn’t make sense. Second, it would cause stagnation as people would think their software needs to work with the LTS.
An LTS also seems very premature in a situation where so many use cases don’t even work on the stable channel and require nightly.
Futhermore, I worry that having an LTS would set the expectation that crate authors should do the work to support at least the most recent LTS. This either means more work or stagnation arising from not adopting new Rust features before they reach the LTS channel. Either way is harmful from the point of view of keeping Rust moving forward.
In terms of setting expectations, it matters who does an LTS. It seems to me that when a commercial enterprise Linux distributor decides to support some snapshot of software versions, it’s well understood that that’s how the enterprise vendor makes money, so they are on hook for the effort to make stuff work in such a snapshot environment. Also, since you have to pay, the users are rather invisible behind enterprise walls and don’t bother the larger community that much. However, when an LTS is perceived as a community effort, there are more people using LTS when they don’t need to be using LTS and more sense of entitlement to have members of the broader community who didn’t sign up to support an LTS to support the LTS nonetheless. In this sense, I’m much more worried about an LTS that’s perceived as a Rust community effort or effort by a community Linux distro than I am about commercial enterprise Linux vendors doing an LTS for their customers in a way that crates.io crate authors don’t even see from the other side of the firewall.
In the case of Firefox, I’m worried that a Rust LTS existing would result in pressure from the Linux distros to make Firefox stick to LTS Rust instead of updating Rust every six weeks. At a time when Rust is in active development and Firefox discovers needs for fixes in the toolchain, being artificially limited to an old toolchain would be harmful. OTOH, now that an LTS Rust doesn’t exist, the Rust ecosystem gets to benefit from Firefox requiring even generally non-rolling distros to ship a semi-rolling Rust in order to keep building Firefox. (It looks like Ubuntu 14.04 has Rust 1.22.1 at this time. It’s not the latest Rust, but it sure is newer than what existed in April 2014.) It would be a strategic blunder in terms of avoiding stagnation in the Rust ecosystem to volunteer to give that up.