Make Complex#{inspect,to_s} work correctly if real part #inspect returns frozen string

Make static f_format function take a non-frozen string to append
to.

This does not result in an additional allocation for #inspect,
but it does result in an additional allocation for #to_s.

Fixes [Bug #20337]

complex.c

@@ -1593,16 +1593,15 @@ f_tpositive_p(VALUE x)
 }
 
 static VALUE
-f_format(VALUE self, VALUE (*func)(VALUE))
+f_format(VALUE self, VALUE s, VALUE (*func)(VALUE))
 {
-    VALUE s;
     int impos;
 
     get_dat1(self);
 
     impos = f_tpositive_p(dat->imag);
 
-    s = (*func)(dat->real);
+    rb_str_concat(s, (*func)(dat->real));
     rb_str_cat2(s, !impos ? "-" : "+");
 
     rb_str_concat(s, (*func)(f_abs(dat->imag)));
@@ -1629,7 +1628,7 @@ f_format(VALUE self, VALUE (*func)(VALUE))
 static VALUE
 nucomp_to_s(VALUE self)
 {
-    return f_format(self, rb_String);
+    return f_format(self, rb_usascii_str_new2(""), rb_String);
 }
 
 /*
@@ -1651,7 +1650,7 @@ nucomp_inspect(VALUE self)
     VALUE s;
 
     s = rb_usascii_str_new2("(");
-    rb_str_concat(s, f_format(self, rb_inspect));
+    f_format(self, s, rb_inspect);
     rb_str_cat2(s, ")");
 
     return s;

test/ruby/test_complex.rb

@@ -741,6 +741,17 @@ class Complex_Test < Test::Unit::TestCase
     assert_equal('(1+2i)', c.inspect)
   end
 
+  def test_inspect_to_s_frozen_bug_20337
+    assert_separately([], <<~'RUBY')
+      class Numeric
+        def inspect = super.freeze
+      end
+      c = Complex(Numeric.new, 1)
+      assert_match(/\A\(#<Numeric:/, c.inspect)
+      assert_match(/\A#<Numeric:/, c.to_s)
+    RUBY
+  end
+
   def test_marshal
     c = Complex(1,2)