Function cs_foldr 

pub(crate) fn cs_foldr(
    cx: &ExtCtxt<'_>,
    trait_span: Span,
    substructure: Substructure<'_>,
    single: impl Fn(FieldInfo) -> Box<Expr>,
    combine: impl Fn(Span, Box<Expr>, Box<Expr>) -> Box<Expr>,
    fieldless: impl Fn() -> Box<Expr>,
) -> Box<Expr>

Folds over fields, combining the expressions for each field in a sequence. Statics may not be folded over.