What causes "missing environment variable {var}" in rust-lang/rust?

Identify the required var from the error message and set it: export = .

rust-lang/rust · error · anyhow::Error

missing environment variable {var}

Error message

missing environment variable {var}

What it means

Bailed by env_string when std::env::var returns VarError::NotPresent — the required environment variable is not set in the process environment at all. generate-copyright delegates some required inputs through env vars and treats them as mandatory.

Source

Thrown at src/tools/generate-copyright/src/main.rs:217

        }
    }
}

/// A License has an SPDX license name and a list of copyright holders.
#[derive(serde::Deserialize, Clone, Debug, PartialEq, Eq)]
struct License {
    spdx: String,
    copyright: Vec<String>,
}

/// Grab an environment variable as string, or fail nicely.
fn env_string(var: &str) -> Result<String, Error> {
    match std::env::var(var) {
        Ok(var) => Ok(var),
        Err(std::env::VarError::NotUnicode(_)) => {
            anyhow::bail!("environment variable {var} is not utf-8")
        }
        Err(std::env::VarError::NotPresent) => anyhow::bail!("missing environment variable {var}"),
    }
}

/// Grab an environment variable as a PathBuf, or fail nicely.
fn env_path(var: &str) -> Result<PathBuf, Error> {
    if let Some(var) = std::env::var_os(var) {
        Ok(var.into())
    } else {
        anyhow::bail!("missing environment variable {var}")
    }
}

View on GitHub (pinned to 7088e4b63a)

Solutions

Identify the required var from the error message and set it: export <VAR>=<value>.
Run the tool through its intended wrapper (e.g. the rustdoc/xtask entrypoint) rather than invoking the binary directly.
Check the tool's README/help for the list of required env vars.

Example fix

// before: hard requirement, no guidance
Err(std::env::VarError::NotPresent) =>
    anyhow::bail!("missing environment variable {var}"),

// after: include a hint on how to set it
Err(std::env::VarError::NotPresent) => anyhow::bail!(
    "missing environment variable {var}; set it via `{var}=<value>` \
     or run through `./x.py run generate-copyright`"
)

Defensive patterns

Strategy: validation

Validate before calling

// Pre-flight: assert required env vars exist before running generate-copyright:
const REQUIRED: &[&str] = &["LICENSES_DIR", "SRC_DIR"];
for v in REQUIRED {
    if std::env::var_os(v).is_none() {
        eprintln!("missing required env var {v}"); std::process::exit(1);
    }
}

Type guard

null

Try / catch

match env_string(var) {
    Ok(s) => Ok(s),
    Err(e) => {
        eprintln!("{e}. Set it with: export {var}=<value>");
        Err(e)
    }
}

Prevention

Run the tool through its xtask wrapper which sets every required var.
Document the required env vars in one place (README) and keep it in sync.
Add a CI check that asserts all required vars are present.

When it happens

Trigger: Running generate-copyright without one of its required environment variables set (whatever var name is passed to env_string at the failing call site).

Common situations: Running the tool manually instead of through its usual xtask/wrapper that sets the vars; a wrapper script that skips setting a var on a certain platform; CI environment that doesn't forward the var.

Related errors

AI-assisted analysis of rust-lang/rust@7088e4b63a (2026-08-10). Data as JSON: /api/errors/3c59e47ce8b21eda. Report an issue: GitHub.