Rust "the option Z is only accepted on the nightly" in CI

You used a nightly-only compiler flag (-Z ...) or an unstable #![feature(...)] while building on stable Rust. Those are gated to the nightly channel and stable rustc rejects them outright.

What this error means

The build fails with the option Z is only accepted on the nightly compiler or #![feature] may not be used on the stable release channel. The same code compiles on nightly but not on the stable CI toolchain.

cargo output

error: the option `Z` is only accepted on the nightly compiler

help: consider switching to a nightly toolchain: `rustup default nightly`
note: selecting a toolchain with `+toolchain` arguments ...

Common causes

Nightly-only flag on stable

A -Z flag (often in RUSTFLAGS, .cargo/config.toml, or a build script) is unstable and only works on nightly. Stable rustc refuses it.

Unstable feature gate on stable

A #![feature(...)] attribute enables an unstable language/library feature. Those are nightly-only and error on the stable channel.

How to fix it

Use nightly where the feature requires it

If the unstable feature is essential, build that job on nightly.

Terminal

rustup toolchain install nightly
cargo +nightly build

Or remove the unstable dependency

Find the -Z flag (check RUSTFLAGS and .cargo/config.toml) and drop it if not needed.
Replace #![feature(...)] with a stable alternative where one exists.
Pin the job to nightly only when there is genuinely no stable path.

How to prevent it

Keep CI on stable and isolate any nightly-only job explicitly.
Avoid -Z flags and #![feature] gates unless you commit to nightly.
Document which jobs require nightly and why.