Advice for long formulas

[MichaelChirico]

Is there any advice we want to encode for long formulas?

Currently the guide only discusses formulas w.r.t. operator spacing, and only in the context of one-sided formulas:

https://style.tidyverse.org/syntax.html#infix-operators

It's quite common for us to see formulas stretch >1 line, even up to 4-5 lines or more. I am not sure what rules to apply for best readability in this case.

1. Should a terminal ~ always induce indentation (so that the RHS is always outdented vs. the LHS)?
2. Should a terminal + (or occasionally -) induce indentation on the next line?

Originally raised here: https://github.com/r-lib/styler/issues/900

Key example -- how should the following be formatted (as might come up, e.g., in a dcast() reshaping):

long_y_variable1 + long_y_variable_2 +
  long_y_variable_3 ~
    long_x_variable_1 + long_x_variable_2 +
      long_x_variable_3

Can any of those outdentations be removed?

Perhaps the problem is more generally about which operators do/don't induce indentation on the next line -- currently there's no discussion of long arithmetic statements either. It seems to me splitting on higher-precedence operators is a good general rule (so split on +, not *, where possible).