Function altrep_data2_as_unchecked 

pub unsafe fn altrep_data2_as_unchecked<T: TypedExternal>(
    x: SEXP,
) -> Option<ExternalPtr<T>>

Extract the ALTREP data2 slot (unchecked version).

Skips thread safety checks for performance-critical ALTREP callbacks.

Safety

• x must be a valid ALTREP SEXP
• Must be called from the R main thread (guaranteed in ALTREP callbacks)