Fix source location of autoloaded constant [Bug #16764]

test/ruby/test_autoload.rb

@@ -427,6 +427,19 @@ p Foo::Bar
     end
   end
 
+  def test_source_location
+    klass = self.class
+    bug = "Bug16764"
+    Dir.mktmpdir('autoload') do |tmpdir|
+      path = "#{tmpdir}/test-#{bug}.rb"
+      File.write(path, "#{klass}::#{bug} = __FILE__\n")
+      klass.autoload(:Bug16764, path)
+      assert_equal [__FILE__, __LINE__-1], klass.const_source_location(bug)
+      assert_equal path, klass.const_get(bug)
+      assert_equal [path, 1], klass.const_source_location(bug)
+    end
+  end
+
   def test_no_leak
     assert_no_memory_leak([], '', <<~'end;', 'many autoloads', timeout: 60)
       200000.times do |i|

variable.c

@@ -1807,8 +1807,10 @@ struct autoload_const {
     VALUE mod;
     VALUE ad; /* autoload_data_i */
     VALUE value;
+    VALUE file;
     ID id;
     rb_const_flag_t flag;
+    int line;
 };
 
 /* always on stack, no need to mark */
@@ -1877,6 +1879,7 @@ autoload_c_compact(void *ptr)
     ac->mod = rb_gc_location(ac->mod);
     ac->ad = rb_gc_location(ac->ad);
     ac->value = rb_gc_location(ac->value);
+    ac->file = rb_gc_location(ac->file);
 }
 
 static void
@@ -1887,6 +1890,7 @@ autoload_c_mark(void *ptr)
     rb_gc_mark_movable(ac->mod);
     rb_gc_mark_movable(ac->ad);
     rb_gc_mark_movable(ac->value);
+    rb_gc_mark_movable(ac->file);
 }
 
 static void
@@ -2772,11 +2776,11 @@ rb_const_set(VALUE klass, ID id, VALUE val)
 	setup_const_entry(ce, klass, val, CONST_PUBLIC);
     }
     else {
-	struct autoload_const ac;
-	ac.mod = klass;
-	ac.id = id;
-	ac.value = val;
-	ac.flag = CONST_PUBLIC;
+        struct autoload_const ac = {
+            .mod = klass, .id = id,
+            .value = val, .flag = CONST_PUBLIC,
+            /* fill the rest with 0 */
+        };
 	const_tbl_update(&ac);
     }
     /*
@@ -2841,10 +2845,17 @@ const_tbl_update(struct autoload_const *ac)
 		rb_clear_constant_cache();
 
 		ac->value = val; /* autoload_i is non-WB-protected */
-		return;
+                ac->file = rb_source_location(&ac->line);
 	    }
-	    /* otherwise, allow to override */
-	    autoload_delete(klass, id);
+            else {
+                /* otherwise autoloaded constant, allow to override */
+                autoload_delete(klass, id);
+                ce->flag = visibility;
+                RB_OBJ_WRITE(klass, &ce->value, val);
+                RB_OBJ_WRITE(klass, &ce->file, ac->file);
+                ce->line = ac->line;
+            }
+            return;
 	}
 	else {
 	    VALUE name = QUOTE_ID(id);