bevyengine/bevy · error

Failed to create {file_path:?}

Error message

Failed to create {file_path:?}

What it means

Panic during auto-registration data export: the derive macro's generated code failed to create/open the per-crate registration-fns file under the build directory (keyed by CARGO_CRATE_NAME). Indicates a filesystem/environment problem at compile time (permissions, missing OUT_DIR).

Source

Thrown at crates/bevy_reflect/derive/src/impls/common.rs:220

        // To allow writing to this file from multiple threads during compilation it is protected by mutex.
        // This static is valid for the duration of compilation of one crate and we have one file per crate,
        // so it is enough to protect compilation threads from overwriting each other.
        // This file is reset on every crate recompilation.
        //
        // It might make sense to replace the mutex with File::lock when file_lock feature becomes stable.
        static REGISTRATION_FNS_EXPORT: LazyLock<Mutex<fs::File>> = LazyLock::new(|| {
            let path = PathBuf::from("target").join("bevy_reflect_type_registrations");
            fs::DirBuilder::new()
                .recursive(true)
                .create(&path)
                .unwrap_or_else(|_| panic!("Failed to create {path:?}"));
            let file_path = path.join(env::var("CARGO_CRATE_NAME").unwrap());
            let file = fs::OpenOptions::new()
                .create(true)
                .write(true)
                .truncate(true)
                .open(&file_path)
                .unwrap_or_else(|_| panic!("Failed to create {file_path:?}"));
            Mutex::new(file)
        });

        let export_name = format!("_bevy_reflect_register_{}", uuid::Uuid::new_v4().as_u128());

        {
            let mut file = REGISTRATION_FNS_EXPORT.lock().unwrap();
            writeln!(file, "{export_name}")
                .unwrap_or_else(|_| panic!("Failed to write registration function {export_name}"));
            // We must sync_data to ensure all content is written before releasing the lock.
            file.sync_data().unwrap();
        };

        Some(quote! {
            /// # Safety
            /// This function must only be used by the `load_type_registrations` macro.
            #[unsafe(export_name=#export_name)]
            pub unsafe extern "Rust" fn bevy_register_type(registry: &mut #bevy_reflect_path::TypeRegistry) {

View on GitHub (pinned to 396ca72708)

Solutions

1. Check filesystem permissions and disk space for the build directory
2. Clean the target directory and rebuild
3. Remove stale lock/scratch files left by a previous build

When it happens

Trigger: Thrown at crates/bevy_reflect/derive/src/impls/common.rs:220 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of bevyengine/bevy@396ca72708 (2026-08-20). Data as JSON: /api/errors/ed413f34b74b6800. Report an issue: GitHub.