ZJIT: prepare options whenever the zjit feature is enabled

--enable=zjit sets the zjit feature bit without parsing a --zjit option,
so OPTIONS is never initialized and rb_zjit_option_enable() returns
false.

ZJIT then stays disabled at boot. The same happens with --enable-zjit
and RUBYOPT=--enable=zjit, and with --jit, --enable=jit and --enable=all
on ZJIT-only builds.

In this commit we call rb_zjit_prepare_options() whenever the zjit
feature is set, regardless of where the bit came from. It's a no-op
when the --zjit option parser has already initialized OPTIONS, so
--zjit-disable and the other ZJIT options still apply.

[Bug #22427]

ruby.c

@@ -2425,12 +2425,6 @@ process_options(int argc, char **argv, ruby_cmdline_options_t *opt)
 #if USE_ZJIT
         if (!FEATURE_USED_P(opt->features, zjit) && env_var_truthy("RUBY_ZJIT_ENABLE")) {
             FEATURE_SET(opt->features, FEATURE_BIT(zjit));
-
-            // When the --zjit flag is specified, we would have call setup_zjit_options(""),
-            // which would have called rb_zjit_prepare_options() internally. This ensures we
-            // go through the same set up but with less overhead than setup_zjit_options("").
-            extern void rb_zjit_prepare_options();
-            rb_zjit_prepare_options();
         }
 #endif
     }
@@ -2447,6 +2441,9 @@ process_options(int argc, char **argv, ruby_cmdline_options_t *opt)
 #endif
 #if USE_ZJIT
     if (FEATURE_SET_P(opt->features, zjit)) {
+        // --enable=zjit, RUBY_ZJIT_ENABLE and --jit in ZJIT-only builds set the bit without parsing a --zjit* option.
+        extern void rb_zjit_prepare_options(void);
+        rb_zjit_prepare_options();
         bool rb_zjit_option_enable(void);
         opt->zjit = rb_zjit_option_enable(); // set opt->zjit for Init_ruby_description() and calling rb_zjit_init()
     }

test/ruby/test_zjit_cli.rb

@@ -110,6 +110,29 @@ class TestZJITCLI < Test::Unit::TestCase
     end
   end
 
+  def test_enable_through_feature_flag
+    child_env = {'RUBY_YJIT_ENABLE' => nil, 'RUBY_ZJIT_ENABLE' => nil, 'RUBYOPT' => nil}
+    args_list = [
+      %w[--enable=zjit],
+      %w[--enable-zjit],
+      %w[--disable=zjit --enable=zjit],
+      %w[--zjit --zjit-disable --enable=zjit],
+    ]
+    args_list += [%w[--jit], %w[--enable=jit]] unless JITSupport.yjit_supported?
+    args_list.each do |args|
+      assert_in_out_err([child_env, '--disable-all', *args, '-e', 'p RubyVM::ZJIT.enabled?']) do |stdout, stderr|
+        expected = args.include?('--zjit-disable') ? 'false' : 'true'
+        assert_equal([expected], stdout, args.inspect)
+        assert_equal([], stderr, args.inspect)
+      end
+    end
+
+    assert_in_out_err([child_env.merge('RUBYOPT' => '--enable=zjit'), '--disable=gems,yjit', '-v']) do |stdout, stderr|
+      assert_include(stdout.first, '+ZJIT')
+      assert_equal([], stderr)
+    end
+  end
+
   def test_zjit_enable
     # --disable-all is important in case the build/environment has YJIT enabled by
     # default through e.g. -DYJIT_FORCE_ENABLE. Can't enable ZJIT when YJIT is on.