rust-lang/rust · error · syn::Error

CFG_RELEASE env var: {err}

Error message

CFG_RELEASE env var: {err}

What it means

Emitted by the current_version proc macro in rustc_macros. The macro reads the CFG_RELEASE environment variable via proc_macro::tracked::env_var, then parses it as major.minor.patch (ignoring any -suffix like -dev or -nightly). If the env var is missing (tracked::env_var returns Err) or the value cannot be parsed into three numeric components (parse_str returns None), the macro emits a compile-time syn::Error at the call site. This macro is used by rustc_session to embed the compiler version into the binary.

Source

Thrown at compiler/rustc_macros/src/current_version.rs:12

use proc_macro::TokenStream;
use proc_macro2::Span;
use quote::quote;

pub(crate) fn current_version(_input: TokenStream) -> TokenStream {
    let env_var = "CFG_RELEASE";
    TokenStream::from(match RustcVersion::parse_cfg_release(env_var) {
        Ok(RustcVersion { major, minor, patch }) => quote!(
            // The produced literal has type `rustc_session::RustcVersion`.
            Self { major: #major, minor: #minor, patch: #patch }
        ),
        Err(err) => syn::Error::new(Span::call_site(), format!("{env_var} env var: {err}"))
            .into_compile_error(),
    })
}

struct RustcVersion {
    major: u16,
    minor: u16,
    patch: u16,
}

impl RustcVersion {
    fn parse_cfg_release(env_var: &str) -> Result<Self, Box<dyn std::error::Error>> {
        let value = proc_macro::tracked::env_var(env_var)?;

        Self::parse_str(&value)
            .ok_or_else(|| format!("failed to parse rustc version: {:?}", value).into())
    }

View on GitHub (pinned to 7088e4b63a)

Solutions

  1. Always build the compiler through ./x.py build or ./x.py check, which sets CFG_RELEASE and all other required environment variables.
  2. If you must invoke cargo directly, set CFG_RELEASE manually, e.g.: CFG_RELEASE="1.89.0-dev" cargo build -p rustc_session.
  3. Verify the value is parseable: it must contain at least two dot-separated numeric components (e.g., "1.89.0" or "1.89.0-dev").
  4. Check bootstrap.example.toml and the bootstrap code for how CFG_RELEASE is derived from the version field.

Example fix

# before: direct cargo build without bootstrap env
cargo build -p rustc_session
# error: CFG_RELEASE env var: ...
# after: build through bootstrap which sets CFG_RELEASE
./x.py build --stage 1 compiler/rustc_session
# or set the env var manually:
CFG_RELEASE="1.89.0-dev" cargo build -p rustc_session
Defensive patterns

Strategy: validation

Validate before calling

// Before building a rustc crate that uses the current_version macro,
// verify that CFG_RELEASE is set and parseable.
fn validate_cfg_release() -> Result<(), String> {
    let val = std::env::var("CFG_RELEASE").map_err(|_| "CFG_RELEASE env var is not set")?;
    let core = val.split('-').next().unwrap_or("");
    let parts: Vec<&str> = core.split('.').collect();
    if parts.len() < 2 {
        return Err(format!("CFG_RELEASE '{}' is not a valid version (expected major.minor.patch)", val));
    }
    for p in &parts {
        p.parse::<u16>().map_err(|_| format!("CFG_RELEASE '{}' has non-numeric component", val))?;
    }
    Ok(())
}

Try / catch

// This error is a compile-time syn::Error emitted by the proc macro.
// It cannot be caught at runtime. Prevent it by ensuring the build
// environment has CFG_RELEASE set (normally handled by ./x.py bootstrap).
// The error appears as: error: CFG_RELEASE env var: <message>

Prevention

When it happens

Trigger: Building a rustc compiler crate (e.g., rustc_session) that invokes the current_version macro without the CFG_RELEASE environment variable set, or with a value that doesn't contain at least major.minor as numbers. Normal builds go through ./x.py which sets CFG_RELEASE via the bootstrap system.

Common situations: Running cargo build directly inside a rustc sub-crate instead of through ./x.py; CI scripts that build rustc components without invoking bootstrap; a broken or incomplete bootstrap configuration that fails to export CFG_RELEASE; manually setting CFG_RELEASE to a non-semver string.

Related errors

AI-assisted analysis of rust-lang/rust@7088e4b63a (2026-08-10). Data as JSON: /api/errors/21cf8377337721be. Report an issue: GitHub.