rust-lang/rust · critical

wrong command used for building

Error message

wrong command used for building

What it means

Thrown by the build.rs of rustc_macros (build.rs:1-16) when the RUSTC_BOOTSTRAP environment variable is not set. The rustc compiler must be built through bootstrap (./x.py), which sets RUSTC_BOOTSTRAP; a direct `cargo build` of the compiler crates does not, and the build script panics to prevent an unsupported build path. Helpful messages point to the rustc dev guide build instructions.

Source

Thrown at compiler/rustc_macros/build.rs:14

fn main() {
    println!("cargo:rerun-if-changed=build.rs");
    println!("cargo:rerun-if-env-changed=RUSTC_BOOTSTRAP");
    if std::env::var("RUSTC_BOOTSTRAP").is_err() {
        eprintln!(
            "error: you are attempting to build the compiler without going through bootstrap"
        );
        eprintln!(
            "help: see https://rustc-dev-guide.rust-lang.org/building/how-to-build-and-run.html for how to build the compiler"
        );
        eprintln!(
            "help: if you know what you're doing, set the RUSTC_BOOTSTRAP environment variable to any value"
        );
        panic!("wrong command used for building");
    }
}

View on GitHub (pinned to 22057b88b0)

Solutions

1. Build the compiler through bootstrap: `./x.py build` (or `./x.py build compiler/rustc_macros`). See https://rustc-dev-guide.rust-lang.org/building/how-to-build-and-run.html.
2. If you intentionally need a direct cargo invocation (you know what you are doing), set RUSTC_BOOTSTRAP to any value, e.g. `RUSTC_BOOTSTRAP=1 cargo build -p rustc_macros`.

Example fix

# before (panics)
cargo build -p rustc_macros

# after (option A — recommended)
./x.py build compiler/rustc_macros

# after (option B — explicit, if you know why)
RUSTC_BOOTSTRAP=1 cargo build -p rustc_macros

# rustc_macros/build.rs only expects to be driven by the project build driver
[ -n "${CARGO:-}" ] || { echo "invoke via './x.py build' or 'cargo build', not rustc directly"; exit 1; }
# Confirm against the exact command asserted at compiler/rustc_macros/build.rs:14

When it happens

Trigger: Running `cargo build`/`cargo build -p rustc_macros` (or any compiler crate) directly instead of through `./x.py`, without manually exporting RUSTC_BOOTSTRAP. The build.rs checks `env::var("RUSTC_BOOTSTRAP").is_err()` (build.rs:4) and panics at build.rs:14.

Common situations: New contributors attempting `cargo build` on a rustc checkout before learning bootstrap; IDE tooling or scripts that invoke cargo directly on compiler crates; CI that bypasses ./x.py for a sub-build.

Related errors

• REAL_LIBRARY_PATH_VAR
• LLVM_CONFIG was not set
• command did not execute successfully: {:?} expected success,
• TARGET was not set
• HOST was not set

AI-assisted analysis of rust-lang/rust@22057b88b0 (2026-08-03). Data as JSON: /data/errors/64ec70aab1be446e.json. Report an issue: GitHub.