Function max_universe_of_infer_vars 

pub fn max_universe_of_infer_vars<Infcx, I, T>(
    infcx: &Infcx,
    t: T,
) -> UniverseIndex
where
    Infcx: InferCtxtLike<Interner = I>,
    I: Interner,
    T: TypeFoldable<I>,

The largest universe a variable was from in t