Given how much research there is into this topic, I am sure that I just don't understand the complexity of it. But to me it seems like you could have a function that associates one value with another in the GC. Something like `gc_borrows_from`. You would than write the problematic code like this:

    function blob_contents(blob)
       local len_out = ffi.new('unsigned int')
       local contents = gc_borrows_from(blob, hb.hb_blob_get_data(blob, len_out))
       local len = len_out[0];
       return ffi.string(contents, len)
    end

This would tell the GC that the data returned by `hb_blob_get_data` is borrowed from blob, and it can't collect `blob` until `contents` is also unreachable. How to implement that would be up to the runtime, but it seems reasonable to have a wrapper type that holds a traceable reference back to blob.