extendr: Handling of missing values

Do we have a systematic way of handling NA values on the Rust side? It’s not clear to me what the current intent is to handling missing values. Most basic Rust data types (e.g., String) cannot handle missing values. Instead, Rust uses options. Maybe we should systematically wrap all incoming data types into options?

As an example, the current implementation silently converts NA_character_ into "NA", which I would argue is incorrect behavior.

library(rextendr)

rust_function(
  code = "fn rust_strings(input: &str) -> String {
    input.to_string()
  }",
  patch.crates_io = c(
    'extendr-api = { git = "https://github.com/extendr/extendr" }',
    'extendr-macros = { git = "https://github.com/extendr/extendr" }'
  )
)

rust_strings("a")
#> [1] "a"
rust_strings(NA_character_)
#> [1] "NA"

^{Created on 2020-12-05 by the reprex package (v0.3.0)}