Honestly, the nightly compiler is _really_ stable for the "nightly" moniker it receives. It's basically just the rolling release of the current Rust codebase each night. I've run into issues before where I get an internal compiler error, but those have been rare and rolling back a version (or waiting to roll forward) resolves them.
For the most part, there's very little risk running Rust nightly and in fact, that's my MO. I like being able to receive the latest benefits and opt into compiler gates and whatnot. I couldn't do that on Rust stable.
Overall, I feel very little friction sticking with nightly rust and using libraries that are still nightly-only.
I think it is healthy to have some projects pushing the envelope on what is possible. It provides a goal to work towards and helps test out those new language features so they can be further refined before hitting stable.
It becomes less healthy if the whole community is relying on nightly.
AFAIK Rocket has always been focused on developing an ergonomic type safe API to building web applications. Many things it depends on, codegen, proc macros, that sort of thing, are yet to be stabilized. I'm not affiliated with the project, this is just what I've read about it.
There are other frameworks you can use if you want to use stable, many of them are async already. Granted, none are really 'mature', but this is a new-ish space for Rust anyway.
Most of that stuff is now stable; the major thing it’s waiting on is a fairly technical detail that gives Rocket more informative errors. If Sergio was willing to regress a bit there, it could be on stable, in my understanding. But developer experience is kind of the whole point, so he’s making the call to wait longer.
(EDIT: I just reviewed the flag list and I am 99% sure this is true; the never type could use a library instead, and I am pretty (but not %100) sure proc_macro_hack could fix the other one. It’s quite possible I’m wrong, but in the end, it’s still basically three features left to go.)
For the most part, there's very little risk running Rust nightly and in fact, that's my MO. I like being able to receive the latest benefits and opt into compiler gates and whatnot. I couldn't do that on Rust stable.
Overall, I feel very little friction sticking with nightly rust and using libraries that are still nightly-only.