Function eval_to_valtree 

pub(crate) fn eval_to_valtree<'tcx>(
    tcx: TyCtxt<'tcx>,
    typing_env: TypingEnv<'tcx>,
    cid: GlobalId<'tcx>,
) -> EvalToValTreeResult<'tcx>

Evaluates a constant and turns it into a type-level constant value.