Function missing_to_sexp 

pub unsafe fn missing_to_sexp<T>(x: Missing<T>) -> SEXP
where
    T: IntoR,

Convert a Missing<T> trait-method argument to the SEXP the vtable shim receives: Missing::Absent is R’s missing-argument sentinel, which the shim’s TryFromSexp for Missing<T> reads back as Absent (the same sentinel an R wrapper forwards for an omitted argument), and Present(value) converts value with to_sexp, so T needs IntoR like any other argument type (Missing<AsNumeric> crosses as the sentinel or the number).

Missing<T> has no IntoR impl on purpose: the sentinel is an argument marker, not a value to hand back to R. Only the View’s argument array needs this direction.

Safety

Same as to_sexp.