Magic comment "immutable: string" makes "literal".freeze the default for that file

parse.y

@@ -267,6 +267,7 @@ struct parser_params {
     int parser_ruby_sourceline;	/* current line no. */
     VALUE parser_ruby_sourcefile_string;
     rb_encoding *enc;
+    int immutable_strings;
 
     int parser_yydebug;
 
@@ -3816,6 +3817,11 @@ strings		: string
 			else {
 			    node = evstr2dstr(node);
 			}
+
+			if (parser->immutable_strings) {
+			    node = NEW_CALL(node, idFreeze, 0); 
+			}
+
 			$$ = node;
 		    /*%
 			$$ = $1;
@@ -3838,7 +3844,9 @@ string		: tCHAR
 string1		: tSTRING_BEG string_contents tSTRING_END
 		    {
 		    /*%%%*/
+
 			$$ = $2;
+
 		    /*%
 			$$ = dispatch1(string_literal, $2);
 		    %*/
@@ -3865,7 +3873,12 @@ xstring		: tXSTRING_BEG xstring_contents tSTRING_END
 				break;
 			    }
 			}
-			$$ = node;
+
+			if (parser->immutable_strings) {
+			    $$ = NEW_CALL(node, idFreeze, 0);
+ 			} else {
+			    $$ = node;
+ 			}
 		    /*%
 			$$ = dispatch1(xstring_literal, $2);
 		    %*/
@@ -6653,6 +6666,12 @@ parser_set_encode(struct parser_params *parser, const char *name)
 #endif
 }
 
+static void
+parser_set_immutable(struct parser_params *parser, const char *name, const char *val)
+{
+     if (strcmp(val, "string") == 0) { parser->immutable_strings = 1; }
+}
+
 static int
 comment_at_top(struct parser_params *parser)
 {
@@ -6710,6 +6729,7 @@ static const struct magic_comment magic_comments[] = {
     {"coding", magic_comment_encoding, parser_encode_length},
     {"encoding", magic_comment_encoding, parser_encode_length},
     {"warn_indent", parser_set_token_info},
+    {"immutable", parser_set_immutable}
 };
 #endif
 
@@ -10862,6 +10882,7 @@ parser_initialize(struct parser_params *parser)
     parser->heap = NULL;
 #endif
     parser->enc = rb_utf8_encoding();
+    parser->immutable_strings = 0;
 }
 
 #ifdef RIPPER

test/ruby/test_string_literal.rb

@@ -0,0 +1,72 @@
+# -*- immutable: string -*-
+
+require 'test/unit'
+require_relative 'test_string_literal_mutable.rb'
+
+
+STRINGS = [
+  'hello',
+  "hello",
+  %{Hello},
+  %Q{Hello},
+  %q{Hello},
+  <<-EOS
+    Hello World
+  EOS
+]
+
+class TestStringLiteral < Test::Unit::TestCase
+  def mutate(str)
+    str.slice!(1, 2)
+  end
+
+  def interpolated_string(message)
+    "Interpolated: #{message}"
+  end
+
+  def some_string
+    "A nice frozen string!"
+  end
+
+  def test_strings_are_immutable_in_this_file
+    STRINGS.each do |s|
+      exception = assert_raise RuntimeError, s do
+        mutate(s)
+      end
+      assert_match /can't modify frozen String/, exception.message
+    end
+  end
+
+  def test_strings_in_other_file_are_mutable
+    mutate(TestStringLiteralMutable::CONSTANT)
+    assert_equal "SING", TestStringLiteralMutable::CONSTANT
+  end
+
+  def test_literal_strings_should_have_the_same_object_id
+    s1 = some_string
+    s2 = some_string
+    assert_equal s1.object_id, s2.object_id
+  end
+
+  def test_different_literal_strings_with_the_same_value_in_the_same_file_should_have_the_same_object_id
+    s1 = some_string
+    s2 = "A nice frozen string!"
+    assert_equal s2.object_id, s2.object_id
+  end
+
+  def test_string_interpolation
+    str = interpolated_string("blah blah")
+    exception = assert_raise RuntimeError do
+      mutate(str)
+    end
+    assert_match /can't modify frozen String/, exception.message
+  end
+
+  def test_interpolated_strings_should_have_a_different_object_id
+    s1 = interpolated_string('x')
+    s2 = interpolated_string('x')
+    assert_equal "Interpolated: x", s1
+    assert_equal "Interpolated: x", s2
+    assert_not_equal s1.object_id, s2.object_id
+  end
+end

test/ruby/test_string_literal_mutable.rb

@@ -0,0 +1,3 @@
+module TestStringLiteralMutable
+  CONSTANT = "STRING"
+end