rust-lang/cargo · error

cannot mix `proc-macro` crate type with others

Error message

cannot mix `proc-macro` crate type with others

What it means

When `crate-type` contains `proc-macro`, it must be the sole entry. A proc-macro crate must export only `#[proc_macro_derive]`/`#[proc_macro_attribute]` items at the crate root and cannot coexist with other crate types. The match arm at targets.rs:257-259 emits a warning suggesting `proc-macro = true`, then bails if `kinds.len() > 1`.

Solutions

1. Replace `crate-type = ["proc-macro", ...]` with the boolean `proc-macro = true` (the recommended form).
2. If you must use `crate-type`, list only `["proc-macro"]`.
3. Move non-macro code into a separate companion crate.

Example fix

# before
[lib]
crate-type = ["proc-macro", "rlib"]

# after
[lib]
proc-macro = true

fn proc_macro_mixed(crate_types: &[String]) -> bool {
    crate_types.iter().any(|t| t == "proc-macro") && crate_types.len() > 1
}
// prefer `[lib] proc-macro = true` over explicit crate-type

When it happens

Trigger: Cargo.toml has `[lib] crate-type = ["proc-macro", "rlib"]` or any list where `proc-macro` appears alongside another type. The `if kinds.len() > 1` check inside the proc-macro arm fires.

Common situations: Trying to ship a proc-macro and a normal library from the same crate. Migrating from `proc-macro = true` to explicit crate-type and leaving extra entries. IDE/template scaffolding that pre-fills multiple crate types.

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AI-assisted analysis of rust-lang/cargo@eb98b54bc9 (2026-08-11). Data as JSON: /api/errors/cc9e26621beeef43. Report an issue: GitHub.

Appendix: source

Thrown at src/workspace/parser/targets.rs:258

    // A plugin requires exporting plugin_registrar so a crate cannot be
    // both at once.
    let crate_types = match (lib.crate_types(), lib.proc_macro()) {
        (Some(kinds), _)
            if kinds.contains(&CrateType::Dylib.as_str().to_owned())
                && kinds.contains(&CrateType::Cdylib.as_str().to_owned()) =>
        {
            anyhow::bail!(format!(
                "library `{}` cannot set the crate type of both `dylib` and `cdylib`",
                name_or_panic(lib)
            ));
        }
        (Some(kinds), _) if kinds.contains(&"proc-macro".to_string()) => {
            warnings.push(format!(
                "library `{}` should only specify `proc-macro = true` instead of setting `crate-type`",
                name_or_panic(lib)
            ));
            if kinds.len() > 1 {
                anyhow::bail!("cannot mix `proc-macro` crate type with others");
            }
            vec![CrateType::ProcMacro]
        }
        (Some(kinds), _) => kinds.iter().map(|s| s.into()).collect(),
        (None, Some(true)) => vec![CrateType::ProcMacro],
        (None, _) => vec![CrateType::Lib],
    };

    let mut target = Target::lib_target(name_or_panic(lib), crate_types, path, edition);
    configure(lib, &mut target, TARGET_KIND_HUMAN_LIB, warnings)?;
    target.set_name_inferred(original_lib.map_or(true, |v| v.name.is_none()));
    Ok(Some(target))
}

#[tracing::instrument(skip_all)]
pub fn normalize_bins(
    toml_bins: Option<&Vec<TomlBinTarget>>,
    package_root: &Path,

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