Avoid deduplicating tainted strings or ones with instance variables.

Author: aardvark179

spec/ruby/core/string/uminus_spec.rb

@@ -42,6 +42,20 @@
       (-dynamic).should_not equal("this string is frozen".freeze)
       (-dynamic).should_not equal(-"this string is frozen".freeze)
     end
+
+    it "does not deduplicate tainted strings" do
+      dynamic = %w(this string is frozen).join(' ')
+      dynamic.taint
+      (-dynamic).should_not equal("this string is frozen".freeze)
+      (-dynamic).should_not equal(-"this string is frozen".freeze)
+    end
+
+    it "does not deduplicate strings with additional instance variables" do
+      dynamic = %w(this string is frozen).join(' ')
+      dynamic.instance_variable_set(:@foo, :bar)
+      (-dynamic).should_not equal("this string is frozen".freeze)
+      (-dynamic).should_not equal(-"this string is frozen".freeze)
+    end
   end
 
   ruby_version_is "2.6" do

src/main/ruby/core/string.rb

@@ -1591,8 +1591,10 @@ def +@
 
   def -@
     str = frozen? ? self : dup.freeze
-    Truffle::Ropes.flatten_rope(str)
-    Truffle.invoke_primitive(:string_intern, str)
+    unless str.tainted? || !(str.instance_variables).empty?
+      Truffle::Ropes.flatten_rope(str)
+      Truffle.invoke_primitive(:string_intern, str)
+    end
   end
 
   def encoding