Function build_suggestion_code 

pub(super) fn build_suggestion_code(
    code_field: &Ident,
    nested: ParseStream<'_>,
    fields: &IndexMap<String, (Ident, TokenStream)>,
    allow_multiple: AllowMultipleAlternatives,
) -> Result<TokenStream, Error>

Constructs the format!() invocation(s) necessary for a #[suggestion*(code = "foo")] or #[suggestion*(code("foo", "bar"))] attribute field