Function build_expr_selectors 

pub async fn build_expr_selectors<ResolveEnd, Fut>(
    spec: ExprSelectorSpec<'_>,
    resolve_end: ResolveEnd,
) -> Result<Vec<SliceSelector>, RuntimeError>
where
    ResolveEnd: FnMut(usize, &EndExpr) -> Fut,
    Fut: Future<Output = Result<i64, RuntimeError>>,