Revert "[EXPERIMENTAL] Make Symbol#to_s return a frozen String [Feature #16150]"

This reverts commit 6ffc045a81.

NEWS

@@ -371,14 +371,6 @@ RubyVM::
     * +RubyVM.resolve_feature_path+ moved to
       <code>$LOAD_PATH.resolve_feature_path</code>.  [Feature #15903] [Feature #15230]
 
-Symbol::
-
-  Modified method::
-
-    * Symbol#to_s now always returns a frozen String. The returned String
-      is always the same for a given Symbol.  This change is
-      experimental.  [Feature #16150]
-
 String::
 
   Unicode::

spec/ruby/core/symbol/shared/id2name.rb

@@ -6,21 +6,4 @@ describe :symbol_id2name, shared: true do
     :@ruby.send(@method).should == "@ruby"
     :@@ruby.send(@method).should == "@@ruby"
   end
-
-  ruby_version_is "2.7" do
-    it "returns a frozen String" do
-      :my_symbol.to_s.frozen?.should == true
-      :"dynamic symbol #{6 * 7}".to_s.frozen?.should == true
-    end
-
-    it "always returns the same String for a given Symbol" do
-      s1 = :my_symbol.to_s
-      s2 = :my_symbol.to_s
-      s1.should equal(s2)
-
-      s1 = :"dynamic symbol #{6 * 7}".to_s
-      s2 = :"dynamic symbol #{2 * 3 * 7}".to_s
-      s1.should equal(s2)
-    end
-  end
 end

string.c

@@ -10880,8 +10880,7 @@ sym_inspect(VALUE sym)
  *     sym.id2name   -> string
  *     sym.to_s      -> string
  *
- *  Returns a frozen string corresponding to <i>sym</i>.
- *  The returned String is always the same String instance for a given Symbol.
+ *  Returns the name or string corresponding to <i>sym</i>.
  *
  *     :fred.id2name   #=> "fred"
  *     :ginger.to_s    #=> "ginger"
@@ -10891,7 +10890,7 @@ sym_inspect(VALUE sym)
 VALUE
 rb_sym_to_s(VALUE sym)
 {
-    return rb_sym2str(sym);
+    return str_new_shared(rb_cString, rb_sym2str(sym));
 }
 
 

test/-ext-/string/test_capacity.rb

@@ -14,9 +14,7 @@ class Test_StringCapacity < Test::Unit::TestCase
   end
 
   def test_capacity_shared
-    str = :abcdefghijklmnopqrstuvwxyz.to_s.dup
-    assert Bug::String.shared_string? str
-    assert_equal 0, capa(str)
+    assert_equal 0, capa(:abcdefghijklmnopqrstuvwxyz.to_s)
   end
 
   def test_capacity_normal