SIFR-ASYNC-0005: Blocking offload target is not classified as blocking I/O or CPU-heavy work. - Sifr

SIFR-ASYNC-0005 belongs to the Async effect and awaitability diagnostic family. It means: blocking offload target is not classified as blocking I/O or CPU-heavy work.

Why It Happens

Sifr tracks suspension and blocking effects statically. This diagnostic fires when async code either does not really suspend, awaits work that cannot suspend, or calls blocking/CPU-heavy work without an explicit offload boundary.

Use sifr --explain SIFR-ASYNC-0005 locally to see the renderer’s exact message template and any machine-applicable suggestions for the compiler version you are running.

Erroneous Code

async def main() -> int:
    return await task.spawn_blocking(lambda: 42)

How To Fix It

Only offload functions classified as blocking I/O or CPU-heavy work; otherwise leave ordinary synchronous helpers synchronous.

Fixed Code

def blocking_read() -> str:
    return read_text("config.txt")

async def main() -> str:
    return await task.spawn_blocking(blocking_read)

Details

Field	Value
Code	`SIFR-ASYNC-0005`
Family	`ASYNC`
Severity	Error
Stability	stable
Owner	`sifr_lowering::lower::workload_annotations`
Representative fixture	`crates/sifr/tests/e2e/fail/spawn_blocking_unannotated_rejected.sifr`

See the Error Codes index for the complete catalog.