Function select_frame_rows 

unsafe fn select_frame_rows(frame: SEXP, idx: &[usize]) -> OwnedProtect

The body of DataFrame::select_rows: row-subset the data.frame VECSXP frame at the 0-based rows idx. Shared with packed data.frame columns.

Returns the new frame still protected; the caller roots it before the guard drops.

Safety

R main thread; frame is a data.frame list whose columns all have at least max(idx) + 1 rows.