Expose GC.config[:implementation], to query the currently active GC

And a default and readonly key to the GC.config hash that names the
current GC implementation.

This is provided by each implementation by the API function
rb_gc_impl_active_gc_name

gc.c

@@ -642,6 +642,8 @@ typedef struct gc_function_map {
     bool (*garbage_object_p)(void *objspace_ptr, VALUE obj);
     void (*set_event_hook)(void *objspace_ptr, const rb_event_flag_t event);
     void (*copy_attributes)(void *objspace_ptr, VALUE dest, VALUE obj);
+    // GC Identification
+    const char *(*active_gc_name)(void);
 } rb_gc_function_map_t;
 
 static rb_gc_function_map_t rb_gc_functions;
@@ -785,6 +787,8 @@ ruby_external_gc_init(void)
     load_external_gc_func(garbage_object_p);
     load_external_gc_func(set_event_hook);
     load_external_gc_func(copy_attributes);
+    //GC Identification
+    load_external_gc_func(active_gc_name);
 
 # undef load_external_gc_func
 
@@ -864,6 +868,8 @@ ruby_external_gc_init(void)
 # define rb_gc_impl_garbage_object_p rb_gc_functions.garbage_object_p
 # define rb_gc_impl_set_event_hook rb_gc_functions.set_event_hook
 # define rb_gc_impl_copy_attributes rb_gc_functions.copy_attributes
+// GC Identification
+# define rb_gc_impl_active_gc_name rb_gc_functions.active_gc_name
 #endif
 
 static VALUE initial_stress = Qfalse;
@@ -2761,6 +2767,12 @@ rb_gc_copy_attributes(VALUE dest, VALUE obj)
     rb_gc_impl_copy_attributes(rb_gc_get_objspace(), dest, obj);
 }
 
+const char *
+rb_gc_active_gc_name(void)
+{
+    return rb_gc_impl_active_gc_name();
+}
+
 // TODO: rearchitect this function to work for a generic GC
 size_t
 rb_obj_gc_flags(VALUE obj, ID* flags, size_t max)
@@ -3489,7 +3501,10 @@ gc_stat_heap(rb_execution_context_t *ec, VALUE self, VALUE heap_name, VALUE arg)
 static VALUE
 gc_config_get(rb_execution_context_t *ec, VALUE self)
 {
-    return rb_gc_impl_config_get(rb_gc_get_objspace());
+    VALUE cfg_hash = rb_gc_impl_config_get(rb_gc_get_objspace());
+    rb_hash_aset(cfg_hash, sym("implementation"), rb_fstring_cstr(rb_gc_impl_active_gc_name()));
+
+    return cfg_hash;
 }
 
 static VALUE
@@ -3582,8 +3597,6 @@ gc_disable(rb_execution_context_t *ec, VALUE _)
     return rb_gc_disable();
 }
 
-
-
 // TODO: think about moving ruby_gc_set_params into Init_heap or Init_gc
 void
 ruby_gc_set_params(void)

gc/default.c

@@ -6156,6 +6156,12 @@ rb_gc_impl_copy_attributes(void *objspace_ptr, VALUE dest, VALUE obj)
     rb_gc_impl_copy_finalizer(objspace, dest, obj);
 }
 
+const char *
+rb_gc_impl_active_gc_name(void)
+{
+    return "default";
+}
+
 void
 rb_gc_impl_writebarrier_remember(void *objspace_ptr, VALUE obj)
 {

test/ruby/test_gc.rb

@@ -117,6 +117,21 @@ class TestGc < Test::Unit::TestCase
     GC.start
   end
 
+  def test_gc_config_implementation
+    omit unless /darwin|linux/.match(RUBY_PLATFORM)
+
+    gc_name = (ENV['RUBY_GC_LIBRARY'] || "default")
+    assert_equal gc_name, GC.config[:implementation]
+  end
+
+  def test_gc_config_implementation_is_readonly
+    omit unless /darwin|linux/.match(RUBY_PLATFORM)
+
+    impl = GC.config[:implementation]
+    GC.config(implementation: "somethingelse")
+    assert_equal(impl, GC.config[:implementation])
+  end
+
   def test_start_full_mark
     return unless use_rgengc?
     omit 'stress' if GC.stress