Function rustc_builtin_macros::deriving::generic::cs_fold

pub fn cs_fold<F>(
    use_foldl: bool, 
    f: F, 
    base: P<Expr>, 
    enum_nonmatch_f: Box<dyn FnMut(&mut ExtCtxt<'_>, Span, (&[Ident], &[Ident]), &[P<Expr>]) -> P<Expr> + '_>, 
    cx: &mut ExtCtxt<'_>, 
    trait_span: Span, 
    substructure: &Substructure<'_>
) -> P<Expr> where
    F: FnMut(&mut ExtCtxt<'_>, Span, P<Expr>, P<Expr>, &[P<Expr>]) -> P<Expr>, 

Fold the fields. use_foldl controls whether this is done left-to-right (true) or right-to-left (false).