mutate() now supports the argument .keep = "transmute"

[apalacio9502]

Hi @krlmlr,

This merge request seeks to support the new .keep = "transmute" argument in the mutate, based on the dplyr pull request tidyverse/dplyr#7038

Below I leave an example comparing transmute with mutate(.keep = "transmute")

example <- tibble(
  group = c("a", "a", "b", "b"),
  old = c(1, 2, 3, 4)
)

example <- duckplyr::as_duckplyr_df(example)

# transmute --------------------------------------------------------------------

example %>% transmute(
  group,
  new = old * 2,
  old
)

materializing:
---------------------
--- Relation Tree ---
---------------------
Projection ["group" as group, *("old", 2.0) as new, "old" as old]
r_dataframe_scan(0x7fbd82db4148)

---------------------
-- Result Columns  --
---------------------
- group (VARCHAR)
- new (DOUBLE)
- old (DOUBLE)

# A tibble: 4 × 3
group   new   old
<chr> <dbl> <dbl>
1 a         2     1
2 a         4     2
3 b         6     3
4 b         8     4

# mutate(.keep = "transmute") --------------------------------------------------

example %>% mutate(
  .keep = "transmute",
  group,
  new = old * 2,
  old
) 

materializing:
---------------------
--- Relation Tree ---
---------------------
Projection ["group" as group, "new" as new, "old" as old]
Projection ["group" as group, "old" as old, "new" as new]
Projection ["group" as group, "old" as old, *("old", 2.0) as new]
Projection ["group" as group, "old" as old]
r_dataframe_scan(0x7fbd6485f588)

---------------------
-- Result Columns  --
---------------------
- group (VARCHAR)
- new (DOUBLE)
- old (DOUBLE)

# A tibble: 4 × 3
group   new   old
<chr> <dbl> <dbl>
1 a         2     1
2 a         4     2
3 b         6     3
4 b         8     4

Regards,

[krlmlr]

Thanks for the PR! It's a bit early for me to review it, let's wait on what happens in dplyr.