Implement nest(.by = ) and revive .key

[DavisVaughan]

Closes #1458

df <- tibble(x = c(1, 1, 1, 2, 2, 3), y = 1:6, z = 6:1)

# Can now do this
nest(df, .by = x)
#> # A tibble: 3 × 2
#>       x data            
#>   <dbl> <list>          
#> 1     1 <tibble [3 × 2]>
#> 2     2 <tibble [2 × 2]>
#> 3     3 <tibble [1 × 2]>

# Rather than either of these:
# nest(df, data = -x)
# nest(df, data = c(y, z))

Also has these two cool new features:

# Implicit `select(df, -z)`
nest(df, data = y, .by = x)
#> # A tibble: 3 × 2
#>       x data            
#>   <dbl> <list>          
#> 1     1 <tibble [3 × 1]>
#> 2     2 <tibble [2 × 1]>
#> 3     3 <tibble [1 × 1]>

# Include `x` in the inner nesting as well
nest(df, data = everything(), .by = x)
#> # A tibble: 3 × 2
#>       x data            
#>   <dbl> <list>          
#> 1     1 <tibble [3 × 3]>
#> 2     2 <tibble [2 × 3]>
#> 3     3 <tibble [1 × 3]>

This also revives the .key argument, with a default of NULL. This is a bit tricky because it used to be deprecated(), and if S3 method authors updated their methods to also say .key = deprecated() as the default, then their code may break. I've handled this by silently converting deprecated() to NULL.

.key is used whenever ... isn't specified. This happens in 3 ways:

• nest(df) where we nest everything
• nest(df, .by = x) where we specify columns to nest by
• df %>% group_by(x) %>% nest() where columns to nest by are implied by the groups

If both ... and .key are specified, a warning is thrown saying .key is ignored.

---

The only revdep that breaks because of these changes is panelr, because they have sticky [ methods and we now use df[cols] in nest(). I can send them a PR after we merge this.

[DavisVaughan]

Supports df %>% nest(y, .by = x) where we would get the warning about forgetting data = y but then it would continue as expected

[DavisVaughan]

vec_cbind() with a grouped data frame will drop the <grouped_df> class and return a bare tibble. So we use reconstruct_tibble() after that to restore the grouped class if possible. There are tests for this.

Both pack() and chop() also do reconstruct_tibble() on the way out so we don't need it after them

[DavisVaughan]

This was a bit unfortunate. Adding a tidy-select .by argument means that you can't call NextMethod() anymore, you have to recall the generic (or the method directly, as is done here).

This did not break any S3 methods out there as far as I can tell.

[DavisVaughan]

This was previously not being caught, and was giving an even more obscure error from chop()

[DavisVaughan]

My "hack" to catch .key = deprecated() from other S3 methods

[hadley]

Maybe we've finally gotten the nesting interface right!

[hadley]

Should we switch this to lifecycle_warn(always = TRUE)? Or maybe even make it defunct? It was deprecated in tidyr 1.0.0 (August 2019), so it's been >3 years with a warning on every use.

[DavisVaughan]

Hmm I'll fiddle with that in a follow up PR