Fix `--debug-frozen-string-literal` to not apply `--disable-frozen-string-literal`

[Feature #20205]

This was an undesired side effect. Now that this value is a triplet, we can't
assume it's disabled by default.

ruby.c

@@ -117,6 +117,7 @@ void rb_warning_category_update(unsigned int mask, unsigned int bits);
 #define DEFINE_DEBUG_FEATURE(bit) feature_debug_##bit
 enum feature_flag_bits {
     EACH_FEATURES(DEFINE_FEATURE, COMMA),
+    DEFINE_FEATURE(frozen_string_literal_set),
     feature_debug_flag_first,
 #if defined(RJIT_FORCE_ENABLE) || !USE_YJIT
     DEFINE_FEATURE(jit) = feature_rjit,
@@ -189,6 +190,7 @@ enum {
     COMPILATION_FEATURES = (
         0
         | FEATURE_BIT(frozen_string_literal)
+        | FEATURE_BIT(frozen_string_literal_set)
         | FEATURE_BIT(debug_frozen_string_literal)
         ),
     DEFAULT_FEATURES = (
@@ -197,6 +199,7 @@ enum {
         & ~FEATURE_BIT(gems)
 #endif
         & ~FEATURE_BIT(frozen_string_literal)
+        & ~FEATURE_BIT(frozen_string_literal_set)
         & ~feature_jit_mask
         )
 };
@@ -1033,6 +1036,9 @@ feature_option(const char *str, int len, void *arg, const unsigned int enable)
 
   found:
     FEATURE_SET_TO(*argp, mask, (mask & enable));
+    if (NAME_MATCH_P("frozen_string_literal", str, len)) {
+        FEATURE_SET_TO(*argp, FEATURE_BIT(frozen_string_literal_set), FEATURE_BIT(frozen_string_literal_set));
+    }
     return;
 }
 
@@ -2437,7 +2443,10 @@ process_options(int argc, char **argv, ruby_cmdline_options_t *opt)
 #define SET_COMPILE_OPTION(h, o, name) \
         rb_hash_aset((h), ID2SYM(rb_intern_const(#name)), \
                      RBOOL(FEATURE_SET_P(o->features, name)))
-        SET_COMPILE_OPTION(option, opt, frozen_string_literal);
+
+        if (FEATURE_SET_P(opt->features, frozen_string_literal_set)) {
+            SET_COMPILE_OPTION(option, opt, frozen_string_literal);
+        }
         SET_COMPILE_OPTION(option, opt, debug_frozen_string_literal);
         rb_funcallv(rb_cISeq, rb_intern_const("compile_option="), 1, &option);
 #undef SET_COMPILE_OPTION

test/ruby/test_rubyoptions.rb

@@ -1233,6 +1233,17 @@ class TestRubyOptions < Test::Unit::TestCase
     end
   end
 
+  def test_frozen_string_literal_debug_chilled_strings
+    code = <<~RUBY
+      "foo" << "bar"
+    RUBY
+    warning = ["-:1: warning: literal string will be frozen in the future"]
+    assert_in_out_err(["-W:deprecated"], code, [], warning)
+    assert_in_out_err(["-W:deprecated", "--debug-frozen-string-literal"], code, [], warning)
+    assert_in_out_err(["-W:deprecated", "--disable-frozen-string-literal", "--debug-frozen-string-literal"], code, [], [])
+    assert_in_out_err(["-W:deprecated", "--enable-frozen-string-literal", "--debug-frozen-string-literal"], code, [], ["-:1:in '<main>': can't modify frozen String: \"foo\", created at -:1 (FrozenError)"])
+  end
+
   def test___dir__encoding
     lang = {"LC_ALL"=>ENV["LC_ALL"]||ENV["LANG"]}
     with_tmpchdir do