#[derive(PreferExternalPtr)]Expand description
Derive PreferExternalPtr: when a type implements both ExternalPtr and
other conversion paths (e.g., IntoList), this selects ExternalPtr wrapping
as the default IntoR conversion.
§Example
ⓘ
#[derive(ExternalPtr, IntoList, PreferExternalPtr)]
struct Model { weights: Vec<f64> }
// IntoR wraps as ExternalPtr (opaque R object), not list