mruby-regexp: fix String#sub/#gsub argument handling - #6989

String#sub and String#gsub in mruby-regexp branched on whether a block was given rather than on how many arguments were actually passed. Two behaviours diverged from CRuby as a result.

1. The block won over the replacement argument

"abc".sub(/b/, "X") { "Y" }   #=> "aYc"  (CRuby: "aXc")
"abcb".gsub(/b/, "X") { "Y" } #=> "aYcY" (CRuby: "aXcX")

CRuby ignores the block whenever a replacement argument is present. Both methods now take a splat, so an omitted replacement can be told apart from an explicit one, and dispatch on the argument count instead.

This also lets them reject a bad argument count with the CRuby message. gsub always reports 1..2; sub reports 1..2 with a block and exactly 2 without one, matching CRuby.

2. gsub without a block deleted every match

"abcb".gsub(/b/) #=> "ac"  (CRuby: #<Enumerator: "abcb":gsub(/b/)>)

It fell through to __gsub_str with nil.to_s as the replacement. It now returns an enumerator over the matched substrings, so gsub(pattern).each { ... } performs the substitution as in CRuby.

Enumerator lives in mruby-enumerator, which mruby-regexp does not require at runtime — without it, to_enum is core Kernel#to_enum, which raises NotImplementedError, exactly as with Kernel#loop and String#each_char in mruby-string-ext. The gem declares the dependency only when the build already contains it, so no build gains Enumerator (and thus Fiber) it did not ask for. This is what lets the enumerator test run: mrbtest gives each gem's tests an mrb_open_core() state holding just its declared dependencies. When Enumerator is undefined the test skips itself. Both gems are in stdlib.gembox, so a normal build runs it.

Notes

• "abc".gsub(/b/, nil) returns "ac" where CRuby raises TypeError. That predates this change and is left alone.
• rake test passes (1890 OK / 0 KO / 0 Crash).
• Checked against three build configs: default (test runs), mruby-regexp + mruby-string-ext only (test skips, neither Enumerator nor Fiber linked), and mruby-regexp + mruby-set where mruby-enumerator arrives transitively (test skips).

Summary by CodeRabbit

• Bug Fixes
  • Improved String#sub and String#gsub argument-count validation.
  • Ensured replacement arguments take precedence over blocks.
  • Preserved blockless gsub enumeration behavior.
  • Continued support for escaping string patterns.
  • Improved handling of invalid argument combinations with clear errors.