Include the currently active GC in RUBY_DESCRIPTION

This will add +MOD_GC to the version string and Ruby description when
Ruby is compiled with shared gc support.

When shared GC support is compiled in and a GC module has been loaded
using RUBY_GC_LIBRARY, the version string will include the name of
the currently active GC as reported by the rb_gc_active_gc_name function
in the form

+MOD_GC[gc_name]

common.mk

@@ -19516,6 +19516,7 @@ version.$(OBJEXT): $(CCAN_DIR)/str/str.h
 version.$(OBJEXT): $(hdrdir)/ruby.h
 version.$(OBJEXT): $(hdrdir)/ruby/ruby.h
 version.$(OBJEXT): $(hdrdir)/ruby/version.h
+version.$(OBJEXT): $(top_srcdir)/gc/gc.h
 version.$(OBJEXT): $(top_srcdir)/internal/array.h
 version.$(OBJEXT): $(top_srcdir)/internal/basic_operators.h
 version.$(OBJEXT): $(top_srcdir)/internal/cmdlineopt.h

gc/gc.h

@@ -42,6 +42,7 @@ uint32_t rb_gc_get_shape(VALUE obj);
 void rb_gc_set_shape(VALUE obj, uint32_t shape_id);
 uint32_t rb_gc_rebuild_shape(VALUE obj, size_t heap_id);
 size_t rb_obj_memsize_of(VALUE obj);
+const char * rb_gc_active_gc_name(void);
 RUBY_SYMBOL_EXPORT_END
 
 void rb_ractor_finish_marking(void);

test/ruby/test_rubyoptions.rb

@@ -307,6 +307,14 @@ def test_rjit_version
     end
   end
 
+  def test_enabled_gc
+    if RbConfig::CONFIG['shared_gc_dir'].length > 0
+      assert_match(/\+MOD_GC/, RUBY_DESCRIPTION)
+    else
+      assert_match(/^((?!\+MOD_GC).)*$/, RUBY_DESCRIPTION)
+    end
+  end
+
   def test_parser_flag
     assert_in_out_err(%w(--parser=prism -e) + ["puts :hi"], "", %w(hi), [])
     assert_in_out_err(%w(--parser=prism --dump=parsetree -e _=:hi), "", /"hi"/, [])

version.c

@@ -16,6 +16,7 @@
 #include "vm_core.h"
 #include "rjit.h"
 #include "yjit.h"
+#include "gc/gc.h"
 #include <stdio.h>
 
 #ifndef EXIT_SUCCESS
@@ -61,6 +62,11 @@ const int ruby_api_version[] = {
 #else
 #define YJIT_DESCRIPTION " +YJIT"
 #endif
+#if USE_SHARED_GC
+#define GC_DESCRIPTION " +MOD_GC"
+#else
+#define GC_DESCRIPTION ""
+#endif
 const char ruby_version[] = RUBY_VERSION;
 const char ruby_revision[] = RUBY_FULL_REVISION;
 const char ruby_release_date[] = RUBY_RELEASE_DATE;
@@ -167,6 +173,7 @@ define_ruby_description(const char *const jit_opt)
         + rb_strlen_lit(YJIT_DESCRIPTION)
         + rb_strlen_lit(" +MN")
         + rb_strlen_lit(" +PRISM")
+        + rb_strlen_lit(GC_DESCRIPTION)
     ];
 
     int n = ruby_description_opt_point;
@@ -176,6 +183,14 @@ define_ruby_description(const char *const jit_opt)
     RUBY_ASSERT(n <= ruby_description_opt_point + (int)rb_strlen_lit(YJIT_DESCRIPTION));
     if (ruby_mn_threads_enabled) append(" +MN");
     if (rb_ruby_prism_p()) append(" +PRISM");
+#ifdef USE_SHARED_GC
+    append(GC_DESCRIPTION);
+    if (getenv("RUBY_GC_LIBRARY")) {
+        append("[");
+        append(rb_gc_active_gc_name());
+        append("]");
+    }
+#endif
     append(ruby_description + ruby_description_opt_point);
 # undef append