New unnecessary_placeholder_linter

[MichaelChirico]

Part of #884

Originally named MagrittrDotLinter(), I think the new name is an improvement (and paves the way for base's _ to be included), but open to further refinement here.

[codecov-commenter]

Codecov Report

Merging #1656 (bc78d68) into main (e05d994) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1656   +/-   ##
=======================================
  Coverage   98.49%   98.50%           
=======================================
  Files         101      102    +1     
  Lines        4456     4478   +22     
=======================================
+ Hits         4389     4411   +22     
  Misses         67       67           

Impacted Files	Coverage Δ
R/unnecessary_placeholder_linter.R	100.00% <100.00%> (ø)

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[IndrajeetPatil]

Do you think we should also expand this linter to cover other operators from {magrittr}?

IINM, %<>% and %T>% should work out of the box if we expand text() in XPath.

The %$% is a bit more tricky and can possibly be dealt with separately.

WDYT?

library(magrittr)

y <- 1
y %<>% sum(., 1) # lint
y
#> [1] 2

z <- 1
z %<>% sum(1)
z
#> [1] 2

# N.B. removing plots output manually to keep the reprex clean
1 %T>% plot %>% sum(1)
#> [1] 2
1 %T>% plot() %>% sum(1)
#> [1] 2
1 %T>% plot(.) %>% sum(1) # lint
#> [1] 2

mtcars %$% cor(wt, mpg)
#> [1] -0.8676594
mtcars %$% cor(.$wt, .$mpg) # lint
#> [1] -0.8676594

<sup>Created on 2022-10-10 with [reprex v2.0.2](https://reprex.tidyverse.org)</sup>

[MichaelChirico]

Agree we could add those, thanks for the suggestion! let's file follow-ups for those