tracel-ai/burn · error

Could not get cache directory

Error message

Could not get cache directory

What it means

Generic guard panic in get_cache_dir: dirs::cache_dir() returned None, meaning the platform cache directory could not be determined. On Linux this happens when XDG_CACHE_HOME is set but empty or non-absolute (or HOME is unavailable); on other platforms when the OS API for the user cache path fails. The faulty input is the environment/platform configuration, not the code's arguments. It surfaces when load_pretrained_weights tries to place downloaded LPIPS weights.

Source

Thrown at crates/burn-train/src/metric/vision/lpips/weights.rs:47

        LpipsNet::Vgg => LPIPS_VGG_URL,
        LpipsNet::Alex => LPIPS_ALEX_URL,
        LpipsNet::Squeeze => LPIPS_SQUEEZE_URL,
    }
}

/// Get the download URL for backbone ImageNet weights.
pub fn get_backbone_weights_url(net: LpipsNet) -> &'static str {
    match net {
        LpipsNet::Vgg => VGG16_IMAGENET_URL,
        LpipsNet::Alex => ALEXNET_IMAGENET_URL,
        LpipsNet::Squeeze => SQUEEZENET_IMAGENET_URL,
    }
}

/// Get the cache directory for LPIPS weights.
fn get_cache_dir() -> PathBuf {
    let cache_dir = dirs::cache_dir()
        .expect("Could not get cache directory")
        .join("burn-dataset")
        .join("lpips");

    if !cache_dir.exists() {
        create_dir_all(&cache_dir).expect("Failed to create cache directory");
    }

    cache_dir
}

/// Download file if not cached and return the cache path.
fn download_if_needed(url: &str, cache_path: &PathBuf, message: &str) {
    if !cache_path.exists() {
        let bytes = download_file_as_bytes(url, message);
        let mut file = File::create(cache_path).expect("Failed to create cache file");
        file.write_all(&bytes).expect("Failed to write weights");
    }
}

View on GitHub (pinned to d16f7ba2ed)

Solutions

1. Ensure XDG_CACHE_HOME (Linux) or the user home directory is set
2. Run in an environment with a resolvable user cache dir
3. Override by providing weights manually if the environment lacks a cache dir

When it happens

Trigger: Thrown at crates/burn-train/src/metric/vision/lpips/weights.rs:47 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of tracel-ai/burn@d16f7ba2ed (2026-09-05). Data as JSON: /api/errors/ca8ca64f20f0f32f. Report an issue: GitHub.