Function route_lowerable 

pub fn route_lowerable(
    params: &[(Symbol, &TypeExpr<'_>)],
    return_type: Option<&TypeExpr<'_>>,
    body: &RouteBody<'_>,
    interner: &Interner,
    is_record: &dyn Fn(Symbol) -> bool,
    components: &[ComponentDef<'_>],
    at: Span,
) -> Result<(), NotLowerable>