Macro impl_default_fold 

pub macro impl_default_fold($Src: ty, $Dst: ty) {
    ...
}

pub macro impl_default_fold($Src: ty, $Dst: ty)

Implements Into-based defaults for the required Fold members:

• Fold::fold_annotation where Src::Annotation: Into<Dst::Annotation>
• Fold::fold_macro_id where Src::MacroId: Into<Dst::MacroId>
• Fold::fold_symbol where Src::Symbol: Into<Dst::Symbol>
• Fold::fold_path where Src::Path: Into<Dst::Path>
• Fold::fold_literal where Src::Literal: Into<Dst::Literal>

Examples:

Implements an “identity” folder

struct IdentityFold;
impl<A: AstTypes> Fold<A, A> for IdentityFold {
    type Error = std::convert::Infallible;
    impl_default_fold!(A, A);
}