Define unsupported GC compaction methods as rb_f_notimplement

Fixes [Bug #18779]

Define the following methods as `rb_f_notimplement` on unsupported
platforms:

- GC.compact
- GC.auto_compact
- GC.auto_compact=
- GC.latest_compact_info
- GC.verify_compaction_references

This change allows users to call `GC.respond_to?(:compact)` to
properly test for compaction support. Previously, it was necessary to
invoke `GC.compact` or `GC.verify_compaction_references` and check if
those methods raised `NotImplementedError` to determine if compaction
was supported.

This follows the precedent set for other platform-specific
methods. For example, in `process.c` for methods such as
`Process.fork`, `Process.setpgid`, and `Process.getpriority`.

gc.c

@@ -9671,6 +9671,7 @@ gc_move(rb_objspace_t *objspace, VALUE scan, VALUE free, size_t slot_size)
     return (VALUE)src;
 }
 
+#if GC_COMPACTION_SUPPORTED
 static int
 compare_free_slots(const void *left, const void *right, void *dummy)
 {
@@ -9719,6 +9720,7 @@ gc_sort_heap_by_empty_slots(rb_objspace_t *objspace)
         free(page_list);
     }
 }
+#endif
 
 static void
 gc_ref_update_array(rb_objspace_t * objspace, VALUE v)
@@ -10410,6 +10412,7 @@ gc_update_references(rb_objspace_t *objspace)
     gc_update_table_refs(objspace, finalizer_table);
 }
 
+#if GC_COMPACTION_SUPPORTED
 /*
  *  call-seq:
  *     GC.latest_compact_info -> {:considered=>{:T_CLASS=>11}, :moved=>{:T_CLASS=>11}}
@@ -10446,7 +10449,11 @@ gc_compact_stats(VALUE self)
 
     return h;
 }
+#else
+#  define gc_compact_stats rb_f_notimplement
+#endif
 
+#if GC_COMPACTION_SUPPORTED
 static void
 root_obj_check_moved_i(const char *category, VALUE obj, void *data)
 {
@@ -10508,6 +10515,10 @@ heap_check_moved_i(void *vstart, void *vend, size_t stride, void *data)
  *
  * This method is implementation specific and not expected to be implemented
  * in any implementation besides MRI.
+ *
+ * To test whether GC compaction is supported, use the idiom:
+ *
+ *   GC.respond_to?(:compact)
  */
 static VALUE
 gc_compact(VALUE self)
@@ -10517,7 +10528,11 @@ gc_compact(VALUE self)
 
     return gc_compact_stats(self);
 }
+#else
+#  define gc_compact rb_f_notimplement
+#endif
 
+#if GC_COMPACTION_SUPPORTED
 /*
  * call-seq:
  *    GC.verify_compaction_references(toward: nil, double_heap: false) -> hash
@@ -10586,6 +10601,9 @@ gc_verify_compaction_references(int argc, VALUE *argv, VALUE self)
 
     return gc_compact_stats(self);
 }
+#else
+#  define gc_verify_compaction_references rb_f_notimplement
+#endif
 
 VALUE
 rb_gc_start(void)
@@ -11173,6 +11191,7 @@ gc_disable(rb_execution_context_t *ec, VALUE _)
     return rb_gc_disable();
 }
 
+#if GC_COMPACTION_SUPPORTED
 /*
  *  call-seq:
  *     GC.auto_compact = flag
@@ -11194,14 +11213,14 @@ gc_set_auto_compact(VALUE _, VALUE v)
     }
 #endif
 
-#if !GC_COMPACTION_SUPPORTED
-    rb_raise(rb_eNotImpError, "Automatic compaction isn't available on this platform");
-#endif
-
     ruby_enable_autocompact = RTEST(v);
     return v;
 }
+#else
+#  define gc_set_auto_compact rb_f_notimplement
+#endif
 
+#if GC_COMPACTION_SUPPORTED
 /*
  *  call-seq:
  *     GC.auto_compact    -> true or false
@@ -11213,6 +11232,9 @@ gc_get_auto_compact(VALUE _)
 {
     return RBOOL(ruby_enable_autocompact);
 }
+#else
+#  define gc_get_auto_compact rb_f_notimplement
+#endif
 
 static int
 get_envparam_size(const char *name, size_t *default_value, size_t lower_bound)

test/ruby/test_gc_compact.rb

@@ -9,14 +9,7 @@
 end
 
 class TestGCCompact < Test::Unit::TestCase
-  module SupportsCompact
-    def setup
-      omit "autocompact not supported on this platform" unless supports_auto_compact?
-      super
-    end
-
-    private
-
+  module CompactionSupportInspector
     def supports_auto_compact?
       return false if /wasm/ =~ RUBY_PLATFORM
       return true unless defined?(Etc::SC_PAGE_SIZE)
@@ -31,10 +24,19 @@ def supports_auto_compact?
     end
   end
 
-  include SupportsCompact
+  module OmitUnlessCompactSupported
+    include CompactionSupportInspector
+
+    def setup
+      omit "autocompact not supported on this platform" unless supports_auto_compact?
+      super
+    end
+  end
+
+  include OmitUnlessCompactSupported
 
   class AutoCompact < Test::Unit::TestCase
-    include SupportsCompact
+    include OmitUnlessCompactSupported
 
     def test_enable_autocompact
       before = GC.auto_compact
@@ -88,13 +90,39 @@ def test_implicit_compaction_does_something
     end
   end
 
-  def os_page_size
-    return true unless defined?(Etc::SC_PAGE_SIZE)
+  class CompactMethodsNotImplemented < Test::Unit::TestCase
+    include CompactionSupportInspector
+
+    def assert_not_implemented(method, *args)
+      omit "autocompact is supported on this platform" if supports_auto_compact?
+
+      assert_raise(NotImplementedError) { GC.send(method, *args) }
+      refute(GC.respond_to?(method), "GC.#{method} should be defined as rb_f_notimplement")
+    end
+
+    def test_gc_compact_not_implemented
+      assert_not_implemented(:compact)
+    end
+
+    def test_gc_auto_compact_get_not_implemented
+      assert_not_implemented(:auto_compact)
+    end
+
+    def test_gc_auto_compact_set_not_implemented
+      assert_not_implemented(:auto_compact=, true)
+    end
+
+    def test_gc_latest_compact_info_not_implemented
+      assert_not_implemented(:latest_compact_info)
+    end
+
+    def test_gc_verify_compaction_references_not_implemented
+      assert_not_implemented(:verify_compaction_references)
+    end
   end
 
-  def setup
-    omit "autocompact not supported on this platform" unless supports_auto_compact?
-    super
+  def os_page_size
+    return true unless defined?(Etc::SC_PAGE_SIZE)
   end
 
   def test_gc_compact_stats