Example of creating a Gist using Python

init-deep-dive_rust-proc-macro.rs

impl ToTokens for Comprehension {
    fn to_tokens(&self, tokens: &mut TokenStream2) {
        let all_for_if_clauses =
            std::iter::once(&self.for_if_clause).chain(&self.additional_for_if_clauses);
        let mut innermost_to_outermost = all_for_if_clauses.rev();

        let mut output = {
            // innermost is a special case--here we do the mapping
            let innermost = innermost_to_outermost
                .next()
                .expect("We know we have at least one ForIfClause (self.for_if_clause)");
            let ForIfClause {
                pattern,
                sequence,
                conditions,
            } = innermost;

            let Mapping(mapping) = &self.mapping;

            quote! {
                ::core::iter::IntoIterator::into_iter(#sequence).filter_map(move |#pattern| {
                    (true #(&& (#conditions))*).then(|| #mapping)
                })
            }
        };

        // Now we walk through the rest of the ForIfClauses, wrapping the current `output` in a new layer of iteration each time.
        // We also add an extra call to '.flatten()'.
        output = innermost_to_outermost.fold(output, |current_output, next_layer| {
            let ForIfClause {
                pattern,
                sequence,
                conditions,
            } = next_layer;
            quote! {
                ::core::iter::IntoIterator::into_iter(#sequence).filter_map(move |#pattern| {
                    (true #(&& (#conditions))*).then(|| #current_output)
                })
                .flatten()
            }
        });

        tokens.extend(output)
    }
}