prism_compile.c: X_STRING should be frozen

The backtick method recieves a frozen string unless it is interpolated.

Otherwise the string held in the ISeq could be mutated by a custom
backtick method.

prism_compile.c

@@ -8352,7 +8352,7 @@ pm_compile_node(rb_iseq_t *iseq, const pm_node_t *node, LINK_ANCHOR *const ret,
         // `foo`
         // ^^^^^
         const pm_x_string_node_t *cast = (const pm_x_string_node_t *) node;
-        VALUE value = parse_string_encoded(scope_node, node, &cast->unescaped);
+        VALUE value = rb_fstring(parse_string_encoded(scope_node, node, &cast->unescaped));
 
         PUSH_INSN(ret, location, putself);
         PUSH_INSN1(ret, location, putobject, value);

spec/ruby/language/execution_spec.rb

@@ -5,6 +5,45 @@ describe "``" do
     ip = 'world'
     `echo disc #{ip}`.should == "disc world\n"
   end
+
+  it "can be redefined and receive a frozen string as argument" do
+    called = false
+    runner = Object.new
+
+    runner.singleton_class.define_method(:`) do |str|
+      called = true
+
+      str.should == "test command"
+      str.frozen?.should == true
+    end
+
+    runner.instance_exec do
+      `test command`
+    end
+
+    called.should == true
+  end
+
+  it "the argument isn't frozen if it contains interpolation" do
+    called = false
+    runner = Object.new
+
+    runner.singleton_class.define_method(:`) do |str|
+      called = true
+
+      str.should == "test command"
+      str.frozen?.should == false
+      str << "mutated"
+    end
+
+    2.times do
+      runner.instance_exec do
+        `test #{:command}`
+      end
+    end
+
+    called.should == true
+  end
 end
 
 describe "%x" do
@@ -12,4 +51,43 @@ describe "%x" do
     ip = 'world'
     %x(echo disc #{ip}).should == "disc world\n"
   end
+
+  it "can be redefined and receive a frozen string as argument" do
+    called = false
+    runner = Object.new
+
+    runner.singleton_class.define_method(:`) do |str|
+      called = true
+
+      str.should == "test command"
+      str.frozen?.should == true
+    end
+
+    runner.instance_exec do
+      %x{test command}
+    end
+
+    called.should == true
+  end
+
+  it "the argument isn't frozen if it contains interpolation" do
+    called = false
+    runner = Object.new
+
+    runner.singleton_class.define_method(:`) do |str|
+      called = true
+
+      str.should == "test command"
+      str.frozen?.should == false
+      str << "mutated"
+    end
+
+    2.times do
+      runner.instance_exec do
+        %x{test #{:command}}
+      end
+    end
+
+    called.should == true
+  end
 end