`tbl_vars()` accesses `nrow()` unnecessarily

[krlmlr]

This currently affects joins with library(duckplyr) . I can work around, perhaps we find a nicer way.

Difficult to show with current duckplyr because we have our group_vars() method. The calling sequence goes as follows:

• tbl_vars() calls group_vars() in the generic
• group_vars.data.frame() calls names(group_data())
• group_data() needs nrow()

Perhaps we can implement an ALTREP solution for group_vars() ? Or add an argument to group_data() that indicates we really only care about the names?

Relevant backtrace from a debugging session:

Backtrace:
     ▆
  1. ├─testthat::expect_error(...) at test-tpch.R:245:3
  2. │ └─testthat:::expect_condition_matching(...)
  3. │   └─testthat:::quasi_capture(...)
  4. │     ├─testthat (local) .capture(...)
  5. │     │ └─base::withCallingHandlers(...)
  6. │     └─rlang::eval_bare(quo_get_expr(.quo), quo_get_env(.quo))
  7. └─duckplyr:::tpch_02() at test-tpch.R:247:5
  8.   ├─dplyr::inner_join(na_matches = TPCH_NA_MATCHES, p, ps, by = c(p_partkey = "ps_partkey")) at duckplyr/R/tpch.R:35:3
  9.   └─duckplyr (local) inner_join.data.frame(na_matches = TPCH_NA_MATCHES, p, ps, by = c(p_partkey = "ps_partkey"))
 10.     ├─duckplyr:::rel_try(...) at duckplyr/R/inner_join.R:10:3
 11.     │ └─base::force(rel) at duckplyr/R/relational.R:62:5
 12.     └─duckplyr:::rel_join_impl(...) at duckplyr/R/inner_join.R:22:7
 13.       └─duckplyr:::tbl_vars_safe(x) at duckplyr/R/join.R:19:3
 14.         └─dplyr::tbl_vars(x) at duckplyr/R/join.R:142:3
 15.           ├─dplyr:::new_sel_vars(tbl_vars_dispatch(x), group_vars(x))
 16.           │ └─base::structure(...)
 17.           ├─dplyr::group_vars(x)
 18.           └─dplyr:::group_vars.data.frame(x)
 19.             ├─generics::setdiff(names(group_data(x)), ".rows")
 20.             ├─dplyr::group_data(x)
 21.             └─dplyr:::group_data.data.frame(x)
 22.               └─base::nrow(.data)
 23.                 ├─base::dim(x)
 24.                 └─base::dim.data.frame(x)
 25.                   └─base::.row_names_info(x, 2L)