Function meta_index 

pub fn meta_index(
    cx: &mut Cx,
    proto: &dyn MetaObjectProtocol,
    receiver: &Value,
    key: &Value,
    index_slot: &Symbol,
) -> Result<Option<Value>>

Performs an indexed read with a caller-selected metaobject fallback slot.

Raw values win. If raw access misses, the protocol looks up index_slot on receiver and applies that meta value through MetaObjectProtocol::apply_meta.