Function real_slice_no_na 

fn real_slice_no_na(s: &[f64]) -> bool

no_na for a double slice: true only when no element is NA or NaN.

This is R’s ISNAN test, not R_IsNA: anyNA() returns FALSE as soon as REAL_NO_NA() reports true (its own scan tests ISNAN, so anyNA(c(1, NaN)) is TRUE), and sort() uses the same hint to skip its NA handling, which drops NaN too.