rust-lang/rust · error

Failed to wait for `as`.

Error message

Failed to wait for `as`.

What it means

This fires in Cranelift codegen's `compile_global_asm` after spawning the system assembler (`as`) with `CG_CLIF_FORCE_GNU_AS` set. The code calls `child.wait().expect("Failed to wait for `as`.")`. This panics if `wait()` fails — which happens when the child process was already reaped (e.g., by a SIGCHLD handler), killed by the OS (OOM killer), or if the system call itself errors (e.g., EINTR without retry, or the PID is invalid).

Source

Thrown at compiler/rustc_codegen_cranelift/src/global_asm.rs:235

    // Remove all LLVM style comments
    let mut global_asm = global_asm
        .lines()
        .map(|line| if let Some(index) = line.find("//") { &line[0..index] } else { line })
        .collect::<Vec<_>>()
        .join("\n");
    global_asm.push('\n');

    // Assemble `global_asm`
    if option_env!("CG_CLIF_FORCE_GNU_AS").is_some() {
        let mut child = Command::new(&config.assembler)
            .arg("-o")
            .arg(&global_asm_object_file)
            .stdin(Stdio::piped())
            .spawn()
            .expect("Failed to spawn `as`.");
        child.stdin.take().unwrap().write_all(global_asm.as_bytes()).unwrap();
        let status = child.wait().expect("Failed to wait for `as`.");
        if !status.success() {
            return Err(format!("Failed to assemble `{}`", global_asm));
        }
    } else {
        // Escape { and }
        let global_asm = global_asm.replace('{', "{{").replace('}', "}}");

        let mut child = Command::new(std::env::current_exe().unwrap())
            // Avoid a warning about the jobserver fd not being passed
            .env_remove("CARGO_MAKEFLAGS")
            .arg("--target")
            .arg(&config.target)
            .arg("--crate-type")
            .arg("staticlib")
            .arg("--emit")
            .arg("obj")
            .arg("-o")
            .arg(&global_asm_object_file)

View on GitHub (pinned to 7088e4b63a)

Solutions

1. Check system memory — `as` may have been OOM-killed. Increase container/host memory limits.
2. Retry the build — transient signal/OOM issues are often non-reproducible.
3. Remove `CG_CLIF_FORCE_GNU_AS` to use the LLVM-based asm path which doesn't spawn a subprocess.
4. If running under a process supervisor that reaps children, disable it or configure it to not reap cg_clif's children.
5. Check dmesg/syslog for OOM killer activity: `dmesg | grep -i 'killed process'`.

Example fix

# before (large global_asm + memory pressure → as killed → wait fails)
CG_CLIF_FORCE_GNU_AS=1 cargo build

# after (remove the flag to avoid spawning `as`)
unset CG_CLIF_FORCE_GNU_AS
cargo build

// Check available memory before building large global_asm blocks:
// In a CI script:
// free -m | awk '/Mem:/ { if ($4 < 512) { echo "Low memory; as may be OOM-killed"; exit 1; } }'
// Or simply avoid CG_CLIF_FORCE_GNU_AS to use the in-process LLVM path.

// In a build script, retry on assembler wait failure:
fn assemble_with_retry(asm: &str, n: u32) -> bool {
    for _ in 0..n {
        let mut child = match Command::new("as").stdin(Stdio::piped()).spawn() {
            Ok(c) => c,
            Err(_) => return false,
        };
        if child.wait().is_ok() { return true; }
    }
    false
}

When it happens

Trigger: The assembler (`as`) was spawned successfully but `wait()` on the child handle fails. This can happen if another part of the process reaps the child (signal handler, threaded runtime), if the OOM killer terminates `as`, or if the process receives a signal that interrupts the wait.

Common situations: Systems under memory pressure where `as` gets OOM-killed during assembly of large `global_asm!` blocks. Process managers or threaded runtimes that install SIGCHLD handlers that race with `wait()`. Rare on normal systems; more common in CI under resource constraints or in containers with strict memory limits.

Related errors

• Failed to spawn `as`.
• cannot allocate registers
• TOOLCHAIN_NAME
• TOOLCHAIN_NAME
• TOOLCHAIN_NAME

AI-assisted analysis of rust-lang/rust@7088e4b63a (2026-08-10). Data as JSON: /api/errors/15ca21a45ce8f58b. Report an issue: GitHub.