Rust and ARM is just fine. For example, Cloudflare famously keeps their entire stack cross-compilable to ARM, and even ships Rust on iPhones.

There's two areas where I believe you could call it second-class at the moment, though:

1. There is no ARM targets in Rust's "Tier 1" platform support.

2. std::arch doesn't have ARM intrinsics in stable.

For 1, ARM targets are in a weird space; they aren't Tier 1, but they're closer than most of the other Tier 2 targets. Several ARM targets are Tier 1 for Firefox, for example, so they get a bunch of work done there.

For 2, well, there hasn't been as much demand before. I expect that to change because of this.

... we'll see what the future holds :)