Provide clear error message when col_select includes no columns

[LukasWallrich]

When col_select contains no columns, it would be great to get a clear error message (or possibly just an empty dataframe) ... in any case, the current cryptic error message made debugging rather hard when this happened in a real context.

library(readr)
temp <- tempfile()
write_csv(iris, temp)
read_csv(temp, col_select = all_of(""))
#> Error in UseMethod("collector_value"): no applicable method for 'collector_value' applied to an object of class "c('collector_skip', 'collector')"

^{Created on 2022-08-21 by the reprex package (v2.0.1)}

[hadley]

Looks like we now get nice messages for free thanks to tidyselect improvements:

library(readr)
read_csv(readr_example("mtcars.csv"), col_select = all_of("x"))
#> Error in `all_of()`:
#> ! Can't subset columns that don't exist.
#> ✖ Column `x` doesn't exist.

^{Created on 2023-07-31 with reprex v2.0.2}

[LukasWallrich]

In the case I struggled with (where the selection is empty) the error message is not quite as nice as it is not clear what the subscript is - but certainly much better, and probably good enough.

library(readr)
read_csv(readr_example("mtcars.csv"), col_select = all_of(""))
#> Error in `all_of()`:
#> ! Can't subset columns.
#> ✖ Subscript can't contain the empty string.
#> ✖ It has an empty string at location 1.
#> Backtrace:
#>      ▆
#>   1. ├─readr::read_csv(readr_example("mtcars.csv"), col_select = all_of(""))
#>   2. │ └─vroom::vroom(...)
#>   3. │   └─vroom:::vroom_(...)
#>   4. ├─vroom (local) `<fn>`(`<col_spec>`, 11L, `<chr>`, all_of(""), "unique")
#>   5. │ ├─names(spec$cols) %in% ...
#>   6. │ └─tidyselect::vars_select(names(spec$cols), !!col_select, .strict = FALSE)
#>   7. │   └─tidyselect:::eval_select_impl(...)
#>   8. │     ├─tidyselect:::with_subscript_errors(...)
#>   9. │     │ └─rlang::try_fetch(...)
#>  10. │     │   └─base::withCallingHandlers(...)
#>  11. │     └─tidyselect:::vars_select_eval(...)
#>  12. │       └─tidyselect:::walk_data_tree(expr, data_mask, context_mask)
#>  13. │         └─tidyselect:::eval_c(expr, data_mask, context_mask)
#>  14. │           └─tidyselect:::reduce_sels(node, data_mask, context_mask, init = init)
#>  15. │             └─tidyselect:::walk_data_tree(new, data_mask, context_mask)
#>  16. │               └─tidyselect:::eval_context(expr, context_mask, call = error_call)
#>  17. │                 ├─tidyselect:::with_chained_errors(...)
#>  18. │                 │ └─rlang::try_fetch(...)
#>  19. │                 │   ├─base::tryCatch(...)
#>  20. │                 │   │ └─base (local) tryCatchList(expr, classes, parentenv, handlers)
#>  21. │                 │   │   └─base (local) tryCatchOne(expr, names, parentenv, handlers[[1L]])
#>  22. │                 │   │     └─base (local) doTryCatch(return(expr), name, parentenv, handler)
#>  23. │                 │   └─base::withCallingHandlers(...)
#>  24. │                 └─rlang::eval_tidy(as_quosure(expr, env), context_mask)
#>  25. ├─tidyselect::all_of("")
#>  26. │ └─tidyselect:::as_indices_impl(x, vars = vars, strict = TRUE)
#>  27. │   └─tidyselect:::chr_as_locations(x, vars, call = call, arg = arg)
#>  28. │     └─vctrs::vec_as_location(...)
#>  29. └─vctrs (local) `<fn>`()
#>  30.   └─vctrs:::stop_subscript_empty(i = i, call = call)
#>  31.     └─rlang::cnd_signal(...)

^{Created on 2023-08-02 with reprex v2.0.2}

[hadley]

It says all_of() in the error, which should hopefully get you to the point pretty quickly.