Function rustc_builtin_macros::deriving::generic::cs_fold

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

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