Function defaults_to_composefs_backend 

pub(crate) fn defaults_to_composefs_backend(
    root: &Dir,
    has_ostree_prepareroot: bool,
) -> Result<bool>

Whether bootc install should default to the composefs backend for this root: it is composefs-native, and it has no ostree prepare-root.conf (has_ostree_prepareroot, which the caller loads anyway), without which it can’t be installed with the ostree backend. An image that has both configurations still defaults to ostree.