Add tests about String's internal capacity

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@53671 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/-test-/string/capacity.c

@@ -0,0 +1,17 @@
+#include "ruby.h"
+#include "internal.h"
+
+static VALUE
+bug_str_capacity(VALUE klass, VALUE str)
+{
+    return
+	STR_EMBED_P(str) ? INT2FIX(RSTRING_EMBED_LEN_MAX) : \
+	STR_SHARED_P(str) ? INT2FIX(0) : \
+	LONG2FIX(RSTRING(str)->as.heap.aux.capa);
+}
+
+void
+Init_capacity(VALUE klass)
+{
+    rb_define_singleton_method(klass, "capacity", bug_str_capacity, 1);
+}

test/-ext-/string/test_capacity.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+require 'test/unit'
+require '-test-/string'
+
+class Test_StringCapacity < Test::Unit::TestCase
+  def test_capacity_embeded
+    size = RbConfig::SIZEOF['void*'] * 3 - 1
+    assert_equal size, Bug::String.capacity('foo')
+  end
+
+  def test_capacity_shared
+    assert_equal 0, Bug::String.capacity(:abcdefghijklmnopqrstuvwxyz.to_s)
+  end
+
+  def test_capacity_normal
+    assert_equal 128, Bug::String.capacity('1'*128)
+  end
+end