Function sidecar_fallback_c_name 

pub(crate) fn sidecar_fallback_c_name(type_name: &str) -> String

C_<crate>__mx_rdata_fallback_<Type> — the field fallback of an S3, S4 or env class with r_data_accessors (fallback = "Self::f", #2150), which the impl block that has it emits.