assignment_linter() gains operator= argument to allow, e.g. '=' assignments

[MichaelChirico]

Supersedes #2698 and #2521. h/t again @J-Moravec for getting the ball rolling here.

Closes #2441.

This also un-couples the linted operator from the allow_trailing behavior; lints generated by the operator used are now independent of lints generated by line-trailing assignment operators:

## current 'main'
lint("a <-
1", assignment_linter(allow_trailing=FALSE))
# <text>:1:3: style: [assignment_linter] Assignment <- should not be trailing at the end of a line.
# a <-
#   ^~
lint("a =
1", assignment_linter(allow_trailing=FALSE))
# <text>:1:3: style: [assignment_linter] Use <-, not =, for assignment.
# a =
#   ^

## current branch (assignment-operator)
lint("a <-
1", assignment_linter(allow_trailing=FALSE))
# <text>:1:3: style: [assignment_linter] Assignment <- should not be trailing at the end of a line.
# a <-
#   ^~
lint("a =
1", assignment_linter(allow_trailing=FALSE))
# <text>:1:3: style: [assignment_linter] Use one of <-, <<- for assignment, not =.
# a =
#   ^
# <text>:1:3: style: [assignment_linter] Assignment = should not be trailing at the end of a line.
# a =
#   ^

Having <<- allowed by default is back-compatible, but does strike me as odd -- should we also plan to remove this from the defaults?

[MichaelChirico]

Thanks @IndrajeetPatil! I'd also like to get feedback from @AshesITR in this case since he commented a lot on the earlier PRs & this will be released soon afterwards.

[IndrajeetPatil]

Having <<- allowed by default is back-compatible, but does strike me as odd -- should we also plan to remove this from the defaults?

Yeah, I think it'll be good to remove it from the defaults.

[AshesITR]

Good stuff, just a few kinks that could be ironed out.

[AshesITR]

Basically GTG. Nice work, thanks!

[AshesITR]

Having <<- allowed by default is back-compatible, but does strike me as odd -- should we also plan to remove this from the defaults?

Yeah, I think it'll be good to remove it from the defaults.

Is there advice in the style guide that motivates this change? If so, I'd be fine with shipping another breaking change related to this linter, given that we do impact analysis first.
Otherwise I see no clear cause to change our default behavior.

[MichaelChirico]

Having <<- allowed by default is back-compatible, but does strike me as odd -- should we also plan to remove this from the defaults?

Yeah, I think it'll be good to remove it from the defaults.

Is there advice in the style guide that motivates this change? If so, I'd be fine with shipping another breaking change related to this linter, given that we do impact analysis first. Otherwise I see no clear cause to change our default behavior.

Ah, I always assumed there was such advice, but there's not. I think there should be:

tidyverse/style#239

But you're right we shouldn't introduce a breaking change at this late hour if it doesn't contradict the style guide.

On the other hand, {lintr}'s own defaults already do discourage <<-: <<- is a default undesirable operator.

So at a minimum, we have some internal contradiction in our defaults.

I filed #2717 as a follow-up -- we should at least resolve the conflict in the next release regardless of if there's movement in the upstream style guide report.