I agree regarding the use of nightly Rust but fortunately it shouldn't be too long until a stable-compatible Rocket is released: https://github.com/SergioBenitez/Rocket/issues/19

Wasn't Rust "stabilized" with its 1.0 release in 2015?

Yes. But meta programming facilities aren't stabilized for obvious reasons (it ossifies the current implementation).

Most meta programming facilities are stabilized, most importantly procedural macros.

I'm not up to date with Rust tbh. Last time I checked Rocket used proc macros which were nightly only. But maybe there is some other issue.

Most procedural macros were stabilised in 2018, but some smaller features related to them were stabilised later. Rocket also used a ton of other nightly features beyond metaprogramming, which have either been stabilised by the compiler or replaced with stable solutions by Rocket.

Technically speaking, stable Rocket requires nightly Rust, but nightly (master) Rocket runs on stable Rust. The compiler has finally stabilised all the necessary features for Rocket 1.0, it's just up to Rocket to finish their next release.

Do you have any idea why that is? I am pretty sure procedural macros in an expression context landed in stable 1 or 2 releases ago.

I believe they are waiting for releases of dependencies to not declare feature requirements. There's an issue linked in this subthread already for tracking it.

That is already done, and master builds on stable. They haven't cut a release yet because there's more they want in the release than just "whatever is on HEAD when we compile on stable."

Rocket's development process is quite slow, and they haven't but out a release since then.