pub fn derive_prefer_externalptr(input: DeriveInput) -> Result<TokenStream>Expand description
Derive PreferExternalPtr: emits an IntoR impl that wraps the value in
ExternalPtr::new before converting to SEXP.
The type must have a hand-written TypedExternal impl; ExternalPtr derive
already supplies IntoR and must not be combined with this derive.
The generated IntoR::Error is Infallible.