RDoc formatter hardcoded to darkfish for RubyGems

It seems that RDoc is not currently designed to allow a custom formatter when running gem rdoc. Even when using .gemrc and setting the --format option to a custom formatter (like horo) seems to not work. In fact, in the latest version of RDoc (6.0.4), any attempt to use a custom formatter via .gemrc causes a crash:

$ gem rdoc --rdoc --overwrite rake
Parsing documentation for rake-12.3.1
Installing ri documentation for rake-12.3.1
ERROR:  While executing gem ... (TypeError)
    no implicit conversion of nil into String

My .gemrc file:

rdoc: -f horo

I traced the TypeError exception to here: https://github.com/ruby/rdoc/blob/master/lib/rdoc/generator/darkfish.rb#L167

Thanks in advance.

從 lib/rdoc/generator/darkfish.rb 第 167 行回報的失敗開始，透過在將 .gemrc 設定為 rdoc: -f horo 的情況下執行 gem rdoc --rdoc --overwrite rake 來重現該失敗。追蹤自訂 formatter 選項如何傳遞至 RDoc，然後驗證 gem rdoc 能接受該 formatter，而不會出現回報的 TypeError，並使用要求的 formatter。