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

crate types to rustc can only be passed to one target…

Error message

crate types to rustc can only be passed to one target, consider filtering
the package by passing, e.g., `--lib` or `--example` to specify a single target

What it means

Thrown by override_rustc_crate_types (`cargo rustc --crate-type ...`) when the unit slice length is not exactly 1. Crate-type overrides apply to a single target, so ambiguity (zero or many units) is rejected up front before any target kind is rewritten.

Solutions

1. Add a target selector to choose exactly one unit: `--lib`, `--example NAME`, `--bin NAME`.
2. Narrow the package: `-p <pkg> --lib --crate-type cdylib`.
3. Confirm the target you expect is actually defined in the manifest.

Example fix

# before
cargo rustc --crate-type cdylib --release
# after
cargo rustc --lib --crate-type cdylib --release

// Ensure exactly one target selector before `--crate-type`.
fn exactly_one_target(args: &[String]) -> bool {
    args.iter().filter(|a| a.starts_with("--lib") || a.starts_with("--bin") || a.starts_with("--example")).count() == 1
}

When it happens

Trigger: Running `cargo rustc --crate-type cdylib` without a target filter when the package has multiple buildable units (lib+bin, multiple bins), or when no unit is selected at all.

Common situations: Forgetting `--lib`/`--example NAME` when experimenting with crate types; running --crate-type on a workspace default that resolves to several roots; CI invoking cargo rustc with extra args on a multi-target package.

Related errors

• crate types can only be specified for libraries and example…
• extra arguments to ` ` can only be passed to one target…
• malformed output when learning about crate-type
• no library targets found in package
• no library targets found in packages

AI-assisted analysis of rust-lang/cargo@495c385d08 (2026-08-11). Data as JSON: /api/errors/b54ab499fa4e4f73. Report an issue: GitHub.

Appendix: source

Thrown at src/ops/cargo_compile/mod.rs:1151

            visit(&dep.unit, graph, visited);
        }
    }
    for unit in root_units {
        visit(unit, unit_graph, &mut visited);
    }
    unit_graph.retain(|unit, _| visited.contains(unit));
}

/// Override crate types for given units.
///
/// This is primarily used by `cargo rustc --crate-type`.
fn override_rustc_crate_types(
    units: &mut [Unit],
    args: &[String],
    interner: &UnitInterner,
) -> CargoResult<()> {
    if units.len() != 1 {
        anyhow::bail!(
            "crate types to rustc can only be passed to one \
            target, consider filtering\nthe package by passing, \
            e.g., `--lib` or `--example` to specify a single target"
        );
    }

    let unit = &units[0];
    let override_unit = |f: fn(Vec<CrateType>) -> TargetKind| {
        let crate_types = args.iter().map(|s| s.into()).collect();
        let mut target = unit.target.clone();
        target.set_kind(f(crate_types));
        interner.intern(
            &unit.pkg,
            &target,
            unit.profile.clone(),
            unit.kind,
            unit.mode,
            unit.features.clone(),

View on GitHub (pinned to 495c385d08)