Rust "no override and no default toolchain set" in CI

rustup is installed but has no toolchain selected - no per-directory override and no global default. With nothing to dispatch to, cargo and rustc shims refuse to run.

What this error means

Any cargo or rustc invocation fails with error: no override and no default toolchain set. It happens after a minimal rustup install (e.g. --no-default-toolchain) where no toolchain was ever made the default.

rustup output

error: rustup could not choose a version of cargo to run, because one wasn't
specified explicitly, and no default is configured.

help: run 'rustup default stable' to download the latest stable release of Rust
and set it as your default toolchain.

Common causes

rustup installed without a default toolchain

Installing with --no-default-toolchain (or a stripped image) leaves rustup with the shims but no toolchain selected, so it can’t choose what to run.

No directory override either

Without a rust-toolchain.toml, a rustup override, or a global default, rustup has no signal for which toolchain a command should use.

How to fix it

Set a default toolchain

Terminal

rustup default stable
rustc --version   # confirms a toolchain is now selected

Or pin one via the setup action / toolchain file

A setup action or a committed rust-toolchain.toml selects the toolchain without a manual default.

rust-toolchain.toml

# rust-toolchain.toml at repo root
[toolchain]
channel = "1.85.0"

How to prevent it

Run rustup default stable (or commit a rust-toolchain.toml) early in CI.
Use a setup action that installs and selects a toolchain.
Avoid --no-default-toolchain unless you immediately pin one.