obj_maybe_translate_encoding causes a crash

[TylerGrantSmith]

Running the following code causes R to crash

# no error
x <-  vctrs::new_rcrd(list(foo = 1))
vctrs:::obj_maybe_translate_encoding(x)

# crash
x <-  vctrs::new_rcrd(list(foo = 1:2))
vctrs:::obj_maybe_translate_encoding(x)

I was lead to this trying to debug the following error: tidyverse/tidyr#1004

[DavisVaughan]

Slightly lower level reprex:

library(vctrs)

x <- data.frame(a = new_rcrd(list(foo = 1:2)), b = 1:2)
y <- list(x)

x_proxy <- vec_proxy_equal(x)
y_proxy <- vec_proxy_equal(y)

# proxies the rcrd into a data frame (which is then flattened)
x_proxy
#>   foo b
#> 1   1 1
#> 2   2 2

# proxies the wrapper list, but not the elements
y_proxy
#> [[1]]
#> Error: `format.vctrs_rcrd()` not implemented.

# works
vctrs:::obj_maybe_translate_encoding(x_proxy)
#>   foo b
#> 1   1 1
#> 2   2 2

# crashes because when it gets to `y_proxy[[1]]`, which is a data frame,
# it passes on `size = 2` to each column of that data frame. This is an
# issue because `y_proxy[[1]][[1]]` is a rcrd which internally has length 1
# even though it has size 2. It hasn't been proxied here, so we index OOB
vctrs:::obj_maybe_translate_encoding(y_proxy)