Fix building native extension for cext-ruby/string/test_capacity.rb

test/mri/tests/cext-c/string/fstring.c

@@ -10,13 +10,15 @@ bug_s_fstring(VALUE self, VALUE str)
     return rb_fstring(str);
 }
 
+#ifndef TRUFFLERUBY
 VALUE
 bug_s_fstring_fake_str(VALUE self)
 {
     static const char literal[] = "abcdefghijklmnopqrstuvwxyz";
     struct RString fake_str;
     return rb_fstring(rb_setup_fake_str(&fake_str, literal, sizeof(literal) - 1, 0));
 }
+#endif
 
 VALUE
 bug_s_rb_enc_interned_str(VALUE self, VALUE encoding)
@@ -34,7 +36,11 @@ void
 Init_string_fstring(VALUE klass)
 {
     rb_define_singleton_method(klass, "fstring", bug_s_fstring, 1);
+// Skip fstring_fake_str because it's a dependency of cext-ruby/string/test_fstring.rb that's excluded
+// and building fails because of not declared/implemented rb_setup_fake_str.
+#ifndef TRUFFLERUBY
     rb_define_singleton_method(klass, "fstring_fake_str", bug_s_fstring_fake_str, 0);
+#endif
     rb_define_singleton_method(klass, "rb_enc_interned_str", bug_s_rb_enc_interned_str, 1);
     rb_define_singleton_method(klass, "rb_enc_str_new", bug_s_rb_enc_str_new, 1);
 }