rust-lang/cargo · error

failed to find rmeta dep for pipelined dep

Error message

failed to find rmeta dep for pipelined dep

What it means

When passing `--extern` for a pipelined dependency that only needs metadata, Cargo finds the `FileFlavor::Rmeta` output via `.find(...).expect("failed to find rmeta dep for pipelined dep")`. A dependency whose edge is `only_requires_rmeta` (or in check mode) is expected to have produced an `.rmeta` file; absence means the outputs list contains no Rmeta-flavored entry.

Source

Thrown at src/compiler/mod.rs:1858

        }
        value.push(extern_crate_name.as_str());
        value.push("=");

        let mut pass = |file| {
            let mut value = value.clone();
            value.push(file);
            result.push(OsString::from("--extern"));
            result.push(value);
        };

        let outputs = build_runner.outputs(&dep.unit)?;

        if build_runner.only_requires_rmeta(unit, &dep.unit) || dep.unit.mode.is_check() {
            // Example: rlib dependency for an rlib, rmeta is all that is required.
            let output = outputs
                .iter()
                .find(|output| output.flavor == FileFlavor::Rmeta)
                .expect("failed to find rmeta dep for pipelined dep");
            pass(&output.path);
        } else {
            // Example: a bin needs `rlib` for dependencies, it cannot use rmeta.
            for output in outputs.iter() {
                if output.flavor == FileFlavor::Linkable {
                    pass(&output.path);
                }
                // If we use -Zembed-metadata=no, we also need to pass the path to the
                // corresponding .rmeta file to the linkable artifact, because the
                // normal dependency (rlib) doesn't contain the full metadata.
                else if no_embed_metadata && output.flavor == FileFlavor::Rmeta {
                    pass(&output.path);
                }
            }
        }
        Ok(())
    };

View on GitHub (pinned to 0e07a15537)

Solutions

1. `cargo clean` then rebuild — corrupted/incomplete pipelined outputs are the usual cause.
2. Disable pipelined compilation temporarily (`CARGO_BUILD_PIPELINED=false`) to confirm, then re-enable after a clean.
3. Free disk space and ensure no process deletes from `target/`.

Example fix

// before
let output = outputs
    .iter()
    .find(|output| output.flavor == FileFlavor::Rmeta)
    .expect("failed to find rmeta dep for pipelined dep");
// after
let output = outputs
    .iter()
    .find(|output| output.flavor == FileFlavor::Rmeta)
    .ok_or_else(|| anyhow::anyhow!("pipelined dep `{}` produced no .rmeta output", dep.unit.pkg.package_id()))?;

// Confirm pipelined rmeta files exist before incremental builds
fn pipelined_rmeta_present(deps: &std::path::Path) -> bool {
    deps.read_dir().map(|mut it| it.any(|e| e.ok()
        .and_then(|e| e.path().extension().and_then(|s| s.to_str()).map(|s| s == "rmeta"))
        .unwrap_or(false))).unwrap_or(false)
}

When it happens

Trigger: A pipelined compilation whose upstream unit did not emit an rmeta (rustc crashed, was killed, or run with a mode that suppresses rmeta) while the downstream unit was scheduled as if it had; switching rustc versions across a partial build where rmeta-emission rules changed; target-dir tampering removing the rmeta file after outputs were computed.

Common situations: Interrupted build resumed without `cargo clean`; concurrent `cargo`/`rm` on `target/`; disk-full mid-build; mixing pipelined vs non-pipelined builds across toolchain changes.

Related errors

• failed to find rmeta
• required(true)
• a file should always have a parent
• already loaded without errors
• set_verify_owner_validation should never fail

AI-assisted analysis of rust-lang/cargo@0e07a15537 (2026-08-06). Data as JSON: /data/errors/1f12eebb5a0afb8d.json. Report an issue: GitHub.