Function cs_fold 

pub(crate) fn cs_fold<F>(
    use_foldl: bool,
    cx: &ExtCtxt<'_>,
    trait_span: Span,
    substructure: &Substructure<'_>,
    f: F,
) -> Box<Expr>
where
    F: FnMut(&ExtCtxt<'_>, CsFold<'_>) -> Box<Expr>,

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