rust-lang/cargo · error

invalid `package.build` file name

Error message

invalid `package.build` file name

What it means

`validate_build_name` checks that each path in `package.build` has a non-None `file_stem()`. A path with no stem (`""`, `"."`, `"/"`, trailing separator only) is meaningless as a build script name and is rejected before target generation.

Solutions

1. Set `build` to a real script path such as `"build.rs"`.
2. Remove the `build` key entirely to use the default `build.rs` convention.
3. Validate the path is non-empty before writing the manifest.

Example fix

# before
[package]
build = ""

# after
[package]
build = "build.rs"

fn valid_build_name(build: &str) -> bool {
    std::path::Path::new(build).file_stem().is_some()
}
// reject empty/dot/slash-only values before writing the manifest

When it happens

Trigger: `[package] build = ""`, `build = "."`, or any value whose `Path::file_stem()` returns `None`. The guard at targets.rs:1138 fires.

Common situations: Empty string from a templating/env-substitution mistake. Accidentally leaving `build` with a placeholder. Misconfigured build tooling that injects an empty path.

Related errors

• found build scripts with duplicate file stems, but all…
• cannot specify both `metabuild` and `build`
• found duplicate name , but all targets must have a unique…
• target .name is required
• target names cannot be empty

AI-assisted analysis of rust-lang/cargo@eb98b54bc9 (2026-08-11). Data as JSON: /api/errors/e00d62de0e56e950. Report an issue: GitHub.

Appendix: source

Thrown at src/workspace/parser/targets.rs:1136

                "`build_file` started as a String and `normalize_path` shouldn't have changed that",
            );
            Ok(Some(TomlPackageBuild::SingleScript(build)))
        }
        Some(TomlPackageBuild::Auto(true)) => {
            Ok(Some(TomlPackageBuild::SingleScript(BUILD_RS.to_owned())))
        }
        Some(TomlPackageBuild::MultipleScript(scripts)) => {
            for script in scripts {
                validate_build_name(script)?;
            }
            Ok(build.cloned())
        }
    }
}

fn validate_build_name(build: &str) -> CargoResult<()> {
    if Path::new(build).file_stem().is_none() {
        anyhow::bail!("invalid `package.build` file name");
    }
    Ok(())
}

fn name_or_panic(target: &TomlTarget) -> &str {
    target
        .name
        .as_deref()
        .unwrap_or_else(|| panic!("target name is required"))
}

fn validate_lib_name(target: &TomlTarget, warnings: &mut Vec<String>) -> CargoResult<()> {
    validate_target_name(target, TARGET_KIND_HUMAN_LIB, TARGET_KIND_LIB, warnings)?;
    let name = name_or_panic(target);
    if name.contains('-') {
        anyhow::bail!("library target names cannot contain hyphens: {}", name)
    }

View on GitHub (pinned to eb98b54bc9)