Function gen_opaque_impl_block 

pub fn gen_opaque_impl_block(
    typ: &TypeDef,
    mapper: &dyn TypeMapper,
    cfg: &RustBindingConfig<'_>,
    opaque_types: &AHashSet<String>,
    adapter_bodies: &AdapterBodies,
) -> String

Generate a full impl block for an opaque type, delegating methods to self.inner.

opaque_types is the set of type names that are opaque wrappers (use Arc<inner>). This is needed so that return-type wrapping uses the correct pattern for cross-type returns.