Function altrep_data1_mut_unchecked 

pub unsafe fn altrep_data1_mut_unchecked<T: TypedExternal>(
    x: SEXP,
) -> Option<&'static mut T>

Get a mutable reference to data in ALTREP data1 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)
• The caller must ensure no other references to the data exist