[R]: `vctrs` custom class errors on roundtrip

[jonkeane]

Describe the bug, including details regarding any error messages, version, and platform.

I think that this is fallout from #41969 but when making some examples for how to make a custom vctrs class I ran into an error on round trip Error in '[<-': ! Can't convert 'value' <list> to <special>..

library(arrow, quietly = TRUE, warn.conflicts = FALSE)
#> Warning: package 'arrow' was built under R version 4.4.1

library(vctrs, quietly = TRUE, warn.conflicts = FALSE)

special <- function(n = integer()) {
  new_rcrd(
    fields = list(special = n),
    class = "special"
  )
}

frac_df <- data.frame(
  total = c(2, 2, 7, 9)
)

frac_df$special = special(frac_df$total)

as_arrow_table(frac_df)
#> Error in `[<-`:
#> ! Can't convert `value` <list> to <special>.

^{Created on 2024-06-20 with reprex v2.1.0}

Component(s)

R

[nealrichardson]

This is in the VctrsExtensionType code path, which as it turns out serializes a vec_ptype and not just a list of attributes. So that will need to be handled differently (here, specifically).

[thisisnic]

Issue resolved by pull request 42226
#42226