Update to ruby/spec@89175b2

74 files changed, 182 insertions, 155 deletions

diff --git a/spec/ruby/core/string/ascii_only_spec.rb b/spec/ruby/core/string/ascii_only_spec.rb
index c7e02fd874..88a0559cfd 100644
--- a/spec/ruby/core/string/ascii_only_spec.rb
+++ b/spec/ruby/core/string/ascii_only_spec.rb
@@ -7,12 +7,12 @@ describe "String#ascii_only?" do
     it "returns true if the encoding is UTF-8" do
       [ ["hello",                         true],
         ["hello".encode('UTF-8'),         true],
-        ["hello".force_encoding('UTF-8'), true],
+        ["hello".dup.force_encoding('UTF-8'), true],
       ].should be_computed_by(:ascii_only?)
     end
 
     it "returns true if the encoding is US-ASCII" do
-      "hello".force_encoding(Encoding::US_ASCII).ascii_only?.should be_true
+      "hello".dup.force_encoding(Encoding::US_ASCII).ascii_only?.should be_true
       "hello".encode(Encoding::US_ASCII).ascii_only?.should be_true
     end
 
@@ -34,13 +34,13 @@ describe "String#ascii_only?" do
       [ ["\u{6666}",                          false],
         ["hello, \u{6666}",                   false],
         ["\u{6666}".encode('UTF-8'),          false],
-        ["\u{6666}".force_encoding('UTF-8'),  false],
+        ["\u{6666}".dup.force_encoding('UTF-8'),  false],
       ].should be_computed_by(:ascii_only?)
     end
 
     it "returns false if the encoding is US-ASCII" do
-      [ ["\u{6666}".force_encoding(Encoding::US_ASCII),         false],
-        ["hello, \u{6666}".force_encoding(Encoding::US_ASCII),  false],
+      [ ["\u{6666}".dup.force_encoding(Encoding::US_ASCII),         false],
+        ["hello, \u{6666}".dup.force_encoding(Encoding::US_ASCII),  false],
       ].should be_computed_by(:ascii_only?)
     end
   end
@@ -51,17 +51,16 @@ describe "String#ascii_only?" do
   end
 
   it "returns false for the empty String with a non-ASCII-compatible encoding" do
-    "".force_encoding('UTF-16LE').ascii_only?.should be_false
+    "".dup.force_encoding('UTF-16LE').ascii_only?.should be_false
     "".encode('UTF-16BE').ascii_only?.should be_false
   end
 
   it "returns false for a non-empty String with non-ASCII-compatible encoding" do
-    "\x78\x00".force_encoding("UTF-16LE").ascii_only?.should be_false
+    "\x78\x00".dup.force_encoding("UTF-16LE").ascii_only?.should be_false
   end
 
   it "returns false when interpolating non ascii strings" do
-    base = "EU currency is"
-    base.force_encoding(Encoding::US_ASCII)
+    base = "EU currency is".dup.force_encoding(Encoding::US_ASCII)
     euro = "\u20AC"
     interp = "#{base} #{euro}"
     euro.ascii_only?.should be_false
@@ -70,14 +69,14 @@ describe "String#ascii_only?" do
   end
 
   it "returns false after appending non ASCII characters to an empty String" do
-    ("" << "λ").ascii_only?.should be_false
+    ("".dup << "λ").ascii_only?.should be_false
   end
 
   it "returns false when concatenating an ASCII and non-ASCII String" do
-    "".concat("λ").ascii_only?.should be_false
+    "".dup.concat("λ").ascii_only?.should be_false
   end
 
   it "returns false when replacing an ASCII String with a non-ASCII String" do
-    "".replace("λ").ascii_only?.should be_false
+    "".dup.replace("λ").ascii_only?.should be_false
   end
 end
diff --git a/spec/ruby/core/string/b_spec.rb b/spec/ruby/core/string/b_spec.rb
index 37c7994700..4b1fafff11 100644
--- a/spec/ruby/core/string/b_spec.rb
+++ b/spec/ruby/core/string/b_spec.rb
@@ -1,4 +1,5 @@
 # -*- encoding: utf-8 -*-
+# frozen_string_literal: false
 require_relative '../../spec_helper'
 
 describe "String#b" do
diff --git a/spec/ruby/core/string/byteindex_spec.rb b/spec/ruby/core/string/byteindex_spec.rb
index 7be0c7ec1e..47c7be1029 100644
--- a/spec/ruby/core/string/byteindex_spec.rb
+++ b/spec/ruby/core/string/byteindex_spec.rb
@@ -156,11 +156,11 @@ describe "String#byteindex with String" do
     end
 
     it "handles a substring in a superset encoding" do
-      'abc'.force_encoding(Encoding::US_ASCII).byteindex('é').should == nil
+      'abc'.dup.force_encoding(Encoding::US_ASCII).byteindex('é').should == nil
     end
 
     it "handles a substring in a subset encoding" do
-      'été'.byteindex('t'.force_encoding(Encoding::US_ASCII)).should == 2
+      'été'.byteindex('t'.dup.force_encoding(Encoding::US_ASCII)).should == 2
     end
   end
 end
diff --git a/spec/ruby/core/string/byterindex_spec.rb b/spec/ruby/core/string/byterindex_spec.rb
index 717708c97d..150f709b90 100644
--- a/spec/ruby/core/string/byterindex_spec.rb
+++ b/spec/ruby/core/string/byterindex_spec.rb
@@ -191,11 +191,11 @@ describe "String#byterindex with String" do
     end
 
     it "handles a substring in a superset encoding" do
-      'abc'.force_encoding(Encoding::US_ASCII).byterindex('é').should == nil
+      'abc'.dup.force_encoding(Encoding::US_ASCII).byterindex('é').should == nil
     end
 
     it "handles a substring in a subset encoding" do
-      'été'.byterindex('t'.force_encoding(Encoding::US_ASCII)).should == 2
+      'été'.byterindex('t'.dup.force_encoding(Encoding::US_ASCII)).should == 2
     end
   end
 end
diff --git a/spec/ruby/core/string/bytes_spec.rb b/spec/ruby/core/string/bytes_spec.rb
index 859b346550..02151eebbc 100644
--- a/spec/ruby/core/string/bytes_spec.rb
+++ b/spec/ruby/core/string/bytes_spec.rb
@@ -50,6 +50,6 @@ describe "String#bytes" do
   end
 
   it "is unaffected by #force_encoding" do
-    @utf8.force_encoding('ASCII').bytes.to_a.should == @utf8.bytes.to_a
+    @utf8.dup.force_encoding('ASCII').bytes.to_a.should == @utf8.bytes.to_a
   end
 end
diff --git a/spec/ruby/core/string/bytesize_spec.rb b/spec/ruby/core/string/bytesize_spec.rb
index a31f3ae671..2bbefc0820 100644
--- a/spec/ruby/core/string/bytesize_spec.rb
+++ b/spec/ruby/core/string/bytesize_spec.rb
@@ -13,21 +13,21 @@ describe "String#bytesize" do
   end
 
   it "works with pseudo-ASCII strings containing single UTF-8 characters" do
-    "\u{6666}".force_encoding('ASCII').bytesize.should == 3
+    "\u{6666}".dup.force_encoding('ASCII').bytesize.should == 3
   end
 
   it "works with strings containing UTF-8 characters" do
-    "c \u{6666}".force_encoding('UTF-8').bytesize.should == 5
+    "c \u{6666}".dup.force_encoding('UTF-8').bytesize.should == 5
     "c \u{6666}".bytesize.should == 5
   end
 
   it "works with pseudo-ASCII strings containing UTF-8 characters" do
-    "c \u{6666}".force_encoding('ASCII').bytesize.should == 5
+    "c \u{6666}".dup.force_encoding('ASCII').bytesize.should == 5
   end
 
   it "returns 0 for the empty string" do
     "".bytesize.should == 0
-    "".force_encoding('ASCII').bytesize.should == 0
-    "".force_encoding('UTF-8').bytesize.should == 0
+    "".dup.force_encoding('ASCII').bytesize.should == 0
+    "".dup.force_encoding('UTF-8').bytesize.should == 0
   end
 end
diff --git a/spec/ruby/core/string/byteslice_spec.rb b/spec/ruby/core/string/byteslice_spec.rb
index 312229523d..5b1027f4a5 100644
--- a/spec/ruby/core/string/byteslice_spec.rb
+++ b/spec/ruby/core/string/byteslice_spec.rb
@@ -19,10 +19,10 @@ end
 
 describe "String#byteslice on on non ASCII strings" do
   it "returns byteslice of unicode strings" do
-    "\u3042".byteslice(1).should == "\x81".force_encoding("UTF-8")
-    "\u3042".byteslice(1, 2).should == "\x81\x82".force_encoding("UTF-8")
-    "\u3042".byteslice(1..2).should == "\x81\x82".force_encoding("UTF-8")
-    "\u3042".byteslice(-1).should == "\x82".force_encoding("UTF-8")
+    "\u3042".byteslice(1).should == "\x81".dup.force_encoding("UTF-8")
+    "\u3042".byteslice(1, 2).should == "\x81\x82".dup.force_encoding("UTF-8")
+    "\u3042".byteslice(1..2).should == "\x81\x82".dup.force_encoding("UTF-8")
+    "\u3042".byteslice(-1).should == "\x82".dup.force_encoding("UTF-8")
   end
 
   it "returns a String in the same encoding as self" do
diff --git a/spec/ruby/core/string/bytesplice_spec.rb b/spec/ruby/core/string/bytesplice_spec.rb
index f13024a79b..967edcba29 100644
--- a/spec/ruby/core/string/bytesplice_spec.rb
+++ b/spec/ruby/core/string/bytesplice_spec.rb
@@ -1,4 +1,5 @@
 # -*- encoding: utf-8 -*-
+# frozen_string_literal: false
 require_relative '../../spec_helper'
 
 describe "String#bytesplice" do
diff --git a/spec/ruby/core/string/capitalize_spec.rb b/spec/ruby/core/string/capitalize_spec.rb
index b79e9cfdbd..5e59b656c5 100644
--- a/spec/ruby/core/string/capitalize_spec.rb
+++ b/spec/ruby/core/string/capitalize_spec.rb
@@ -90,7 +90,7 @@ end
 
 describe "String#capitalize!" do
   it "capitalizes self in place" do
-    a = "hello"
+    a = +"hello"
     a.capitalize!.should equal(a)
     a.should == "Hello"
   end
@@ -103,13 +103,13 @@ describe "String#capitalize!" do
 
   describe "full Unicode case mapping" do
     it "modifies self in place for all of Unicode with no option" do
-      a = "äöÜ"
+      a = +"äöÜ"
       a.capitalize!
       a.should == "Äöü"
     end
 
     it "only capitalizes the first resulting character when upcasing a character produces a multi-character sequence" do
-      a = "ß"
+      a = +"ß"
       a.capitalize!
       a.should == "Ss"
     end
@@ -121,7 +121,7 @@ describe "String#capitalize!" do
     end
 
     it "updates string metadata" do
-      capitalized = "ßeT"
+      capitalized = +"ßeT"
       capitalized.capitalize!
 
       capitalized.should == "Sset"
@@ -133,7 +133,7 @@ describe "String#capitalize!" do
 
   describe "modifies self in place for ASCII-only case mapping" do
     it "does not capitalize non-ASCII characters" do
-      a = "ßet"
+      a = +"ßet"
       a.capitalize!(:ascii)
       a.should == "ßet"
     end
@@ -147,13 +147,13 @@ describe "String#capitalize!" do
 
   describe "modifies self in place for full Unicode case mapping adapted for Turkic languages" do
     it "capitalizes ASCII characters according to Turkic semantics" do
-      a = "iSa"
+      a = +"iSa"
       a.capitalize!(:turkic)
       a.should == "İsa"
     end
 
     it "allows Lithuanian as an extra option" do
-      a = "iSa"
+      a = +"iSa"
       a.capitalize!(:turkic, :lithuanian)
       a.should == "İsa"
     end
@@ -165,13 +165,13 @@ describe "String#capitalize!" do
 
   describe "modifies self in place for full Unicode case mapping adapted for Lithuanian" do
     it "currently works the same as full Unicode case mapping" do
-      a = "iß"
+      a = +"iß"
       a.capitalize!(:lithuanian)
       a.should == "Iß"
     end
 
     it "allows Turkic as an extra option (and applies Turkic semantics)" do
-      a = "iß"
+      a = +"iß"
       a.capitalize!(:lithuanian, :turkic)
       a.should == "İß"
     end
@@ -190,12 +190,12 @@ describe "String#capitalize!" do
   end
 
   it "returns nil when no changes are made" do
-    a = "Hello"
+    a = +"Hello"
     a.capitalize!.should == nil
     a.should == "Hello"
 
-    "".capitalize!.should == nil
-    "H".capitalize!.should == nil
+    (+"").capitalize!.should == nil
+    (+"H").capitalize!.should == nil
   end
 
   it "raises a FrozenError when self is frozen" do
diff --git a/spec/ruby/core/string/center_spec.rb b/spec/ruby/core/string/center_spec.rb
index a59dd2a91b..1667b59327 100644
--- a/spec/ruby/core/string/center_spec.rb
+++ b/spec/ruby/core/string/center_spec.rb
@@ -92,7 +92,7 @@ describe "String#center with length, padding" do
 
   describe "with width" do
     it "returns a String in the same encoding as the original" do
-      str = "abc".force_encoding Encoding::IBM437
+      str = "abc".dup.force_encoding Encoding::IBM437
       result = str.center 6
       result.should == " abc  "
       result.encoding.should equal(Encoding::IBM437)
@@ -101,7 +101,7 @@ describe "String#center with length, padding" do
 
   describe "with width, pattern" do
     it "returns a String in the compatible encoding" do
-      str = "abc".force_encoding Encoding::IBM437
+      str = "abc".dup.force_encoding Encoding::IBM437
       result = str.center 6, "あ"
       result.should == "あabcああ"
       result.encoding.should equal(Encoding::UTF_8)
diff --git a/spec/ruby/core/string/chomp_spec.rb b/spec/ruby/core/string/chomp_spec.rb
index ec0490220b..d27c84c6f6 100644
--- a/spec/ruby/core/string/chomp_spec.rb
+++ b/spec/ruby/core/string/chomp_spec.rb
@@ -1,4 +1,5 @@
 # -*- encoding: utf-8 -*-
+# frozen_string_literal: false
 require_relative '../../spec_helper'
 require_relative 'fixtures/classes'
 
diff --git a/spec/ruby/core/string/chop_spec.rb b/spec/ruby/core/string/chop_spec.rb
index 75f25b39cd..99c2c82190 100644
--- a/spec/ruby/core/string/chop_spec.rb
+++ b/spec/ruby/core/string/chop_spec.rb
@@ -1,4 +1,5 @@
 # -*- encoding: utf-8 -*-
+# frozen_string_literal: false
 require_relative '../../spec_helper'
 require_relative 'fixtures/classes'
 
diff --git a/spec/ruby/core/string/clear_spec.rb b/spec/ruby/core/string/clear_spec.rb
index e1d68e03bd..152986fd0f 100644
--- a/spec/ruby/core/string/clear_spec.rb
+++ b/spec/ruby/core/string/clear_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: false
 require_relative '../../spec_helper'
 
 describe "String#clear" do
diff --git a/spec/ruby/core/string/codepoints_spec.rb b/spec/ruby/core/string/codepoints_spec.rb
index 0b6cde82f7..b276d0baa8 100644
--- a/spec/ruby/core/string/codepoints_spec.rb
+++ b/spec/ruby/core/string/codepoints_spec.rb
@@ -11,7 +11,7 @@ describe "String#codepoints" do
   end
 
   it "raises an ArgumentError when no block is given if self has an invalid encoding" do
-    s = "\xDF".force_encoding(Encoding::UTF_8)
+    s = "\xDF".dup.force_encoding(Encoding::UTF_8)
     s.valid_encoding?.should be_false
     -> { s.codepoints }.should raise_error(ArgumentError)
   end
diff --git a/spec/ruby/core/string/comparison_spec.rb b/spec/ruby/core/string/comparison_spec.rb
index 91cfdca25a..9db0cff5ee 100644
--- a/spec/ruby/core/string/comparison_spec.rb
+++ b/spec/ruby/core/string/comparison_spec.rb
@@ -61,12 +61,12 @@ describe "String#<=> with String" do
   end
 
   it "ignores encoding difference" do
-    ("ÄÖÛ".force_encoding("utf-8") <=> "ÄÖÜ".force_encoding("iso-8859-1")).should == -1
-    ("ÄÖÜ".force_encoding("utf-8") <=> "ÄÖÛ".force_encoding("iso-8859-1")).should == 1
+    ("ÄÖÛ".dup.force_encoding("utf-8") <=> "ÄÖÜ".dup.force_encoding("iso-8859-1")).should == -1
+    ("ÄÖÜ".dup.force_encoding("utf-8") <=> "ÄÖÛ".dup.force_encoding("iso-8859-1")).should == 1
   end
 
   it "returns 0 with identical ASCII-compatible bytes of different encodings" do
-    ("abc".force_encoding("utf-8") <=> "abc".force_encoding("iso-8859-1")).should == 0
+    ("abc".dup.force_encoding("utf-8") <=> "abc".dup.force_encoding("iso-8859-1")).should == 0
   end
 
   it "compares the indices of the encodings when the strings have identical non-ASCII-compatible bytes" do
@@ -77,7 +77,7 @@ describe "String#<=> with String" do
   end
 
   it "returns 0 when comparing 2 empty strings but one is not ASCII-compatible" do
-    ("" <=> "".force_encoding('iso-2022-jp')).should == 0
+    ("" <=> "".dup.force_encoding('iso-2022-jp')).should == 0
   end
 end
 
diff --git a/spec/ruby/core/string/concat_spec.rb b/spec/ruby/core/string/concat_spec.rb
index 6f487eaa3a..cbd7df54e2 100644
--- a/spec/ruby/core/string/concat_spec.rb
+++ b/spec/ruby/core/string/concat_spec.rb
@@ -8,19 +8,19 @@ describe "String#concat" do
   it_behaves_like :string_concat_type_coercion, :concat
 
   it "takes multiple arguments" do
-    str = "hello "
+    str = +"hello "
     str.concat "wo", "", "rld"
     str.should == "hello world"
   end
 
   it "concatenates the initial value when given arguments contain 2 self" do
-    str = "hello"
+    str = +"hello"
     str.concat str, str
     str.should == "hellohellohello"
   end
 
   it "returns self when given no arguments" do
-    str = "hello"
+    str = +"hello"
     str.concat.should equal(str)
     str.should == "hello"
   end
diff --git a/spec/ruby/core/string/delete_prefix_spec.rb b/spec/ruby/core/string/delete_prefix_spec.rb
index 4214fdecce..ee7f044905 100644
--- a/spec/ruby/core/string/delete_prefix_spec.rb
+++ b/spec/ruby/core/string/delete_prefix_spec.rb
@@ -1,4 +1,5 @@
 # -*- encoding: utf-8 -*-
+# frozen_string_literal: false
 require_relative '../../spec_helper'
 require_relative 'fixtures/classes'
 
diff --git a/spec/ruby/core/string/delete_spec.rb b/spec/ruby/core/string/delete_spec.rb
index 3b9aa4fb75..6d359776e4 100644
--- a/spec/ruby/core/string/delete_spec.rb
+++ b/spec/ruby/core/string/delete_spec.rb
@@ -1,4 +1,5 @@
 # -*- encoding: utf-8 -*-
+# frozen_string_literal: false
 require_relative '../../spec_helper'
 require_relative 'fixtures/classes'
 
diff --git a/spec/ruby/core/string/delete_suffix_spec.rb b/spec/ruby/core/string/delete_suffix_spec.rb
index 9381f4cee7..1842d75aa5 100644
--- a/spec/ruby/core/string/delete_suffix_spec.rb
+++ b/spec/ruby/core/string/delete_suffix_spec.rb
@@ -1,4 +1,5 @@
 # -*- encoding: utf-8 -*-
+# frozen_string_literal: false
 require_relative '../../spec_helper'
 require_relative 'fixtures/classes'
 
diff --git a/spec/ruby/core/string/downcase_spec.rb b/spec/ruby/core/string/downcase_spec.rb
index 7ee9d6df1d..2d260f23f1 100644
--- a/spec/ruby/core/string/downcase_spec.rb
+++ b/spec/ruby/core/string/downcase_spec.rb
@@ -1,4 +1,5 @@
 # -*- encoding: utf-8 -*-
+# frozen_string_literal: false
 require_relative '../../spec_helper'
 require_relative 'fixtures/classes'
 
diff --git a/spec/ruby/core/string/dup_spec.rb b/spec/ruby/core/string/dup_spec.rb
index 73f71b8ffc..073802d84b 100644
--- a/spec/ruby/core/string/dup_spec.rb
+++ b/spec/ruby/core/string/dup_spec.rb
@@ -51,7 +51,7 @@ describe "String#dup" do
   end
 
   it "does not modify the original setbyte-mutated string when changing dupped string" do
-    orig = "a"
+    orig = +"a"
     orig.setbyte 0, "b".ord
     copy = orig.dup
     orig.setbyte 0, "c".ord
diff --git a/spec/ruby/core/string/each_byte_spec.rb b/spec/ruby/core/string/each_byte_spec.rb
index e04dca807f..7b3db265ac 100644
--- a/spec/ruby/core/string/each_byte_spec.rb
+++ b/spec/ruby/core/string/each_byte_spec.rb
@@ -9,26 +9,26 @@ describe "String#each_byte" do
   end
 
   it "keeps iterating from the old position (to new string end) when self changes" do
-    r = ""
-    s = "hello world"
+    r = +""
+    s = +"hello world"
     s.each_byte do |c|
       r << c
       s.insert(0, "<>") if r.size < 3
     end
     r.should == "h><>hello world"
 
-    r = ""
-    s = "hello world"
+    r = +""
+    s = +"hello world"
     s.each_byte { |c| s.slice!(-1); r << c }
     r.should == "hello "
 
-    r = ""
-    s = "hello world"
+    r = +""
+    s = +"hello world"
     s.each_byte { |c| s.slice!(0); r << c }
     r.should == "hlowrd"
 
-    r = ""
-    s = "hello world"
+    r = +""
+    s = +"hello world"
     s.each_byte { |c| s.slice!(0..-1); r << c }
     r.should == "h"
   end
diff --git a/spec/ruby/core/string/element_set_spec.rb b/spec/ruby/core/string/element_set_spec.rb
index fa041fa31d..e7599f832c 100644
--- a/spec/ruby/core/string/element_set_spec.rb
+++ b/spec/ruby/core/string/element_set_spec.rb
@@ -1,4 +1,5 @@
 # -*- encoding: utf-8 -*-
+# frozen_string_literal: false
 require_relative '../../spec_helper'
 require_relative 'fixtures/classes'
 
diff --git a/spec/ruby/core/string/encode_spec.rb b/spec/ruby/core/string/encode_spec.rb
index 35ed27bb40..97dd753b62 100644
--- a/spec/ruby/core/string/encode_spec.rb
+++ b/spec/ruby/core/string/encode_spec.rb
@@ -34,8 +34,8 @@ describe "String#encode" do
 
     it "encodes an ascii substring of a binary string to UTF-8" do
       x82 = [0x82].pack('C')
-      str =  "#{x82}foo".force_encoding("binary")[1..-1].encode("utf-8")
-      str.should == "foo".force_encoding("utf-8")
+      str =  "#{x82}foo".dup.force_encoding("binary")[1..-1].encode("utf-8")
+      str.should == "foo".dup.force_encoding("utf-8")
       str.encoding.should equal(Encoding::UTF_8)
     end
   end
@@ -49,7 +49,7 @@ describe "String#encode" do
     end
 
     it "round trips a String" do
-      str = "abc def".force_encoding Encoding::US_ASCII
+      str = "abc def".dup.force_encoding Encoding::US_ASCII
       str.encode("utf-32be").encode("ascii").should == "abc def"
     end
   end
@@ -122,8 +122,7 @@ describe "String#encode" do
 
   describe "when passed to, from" do
     it "returns a copy in the destination encoding when both encodings are the same" do
-      str = "あ"
-      str.force_encoding("binary")
+      str = "あ".dup.force_encoding("binary")
       encoded = str.encode("utf-8", "utf-8")
 
       encoded.should_not equal(str)
@@ -155,8 +154,7 @@ describe "String#encode" do
     end
 
     it "returns a copy in the destination encoding when both encodings are the same" do
-      str = "あ"
-      str.force_encoding("binary")
+      str = "あ".dup.force_encoding("binary")
       encoded = str.encode("utf-8", "utf-8", invalid: :replace)
 
       encoded.should_not equal(str)
@@ -191,13 +189,13 @@ describe "String#encode!" do
   describe "when passed no options" do
     it "returns self when Encoding.default_internal is nil" do
       Encoding.default_internal = nil
-      str = "あ"
+      str = +"あ"
       str.encode!.should equal(str)
     end
 
     it "returns self for a ASCII-only String when Encoding.default_internal is nil" do
       Encoding.default_internal = nil
-      str = "abc"
+      str = +"abc"
       str.encode!.should equal(str)
     end
   end
@@ -205,14 +203,14 @@ describe "String#encode!" do
   describe "when passed options" do
     it "returns self for ASCII-only String when Encoding.default_internal is nil" do
       Encoding.default_internal = nil
-      str = "abc"
+      str = +"abc"
       str.encode!(invalid: :replace).should equal(str)
     end
   end
 
   describe "when passed to encoding" do
     it "returns self" do
-      str = "abc"
+      str = +"abc"
       result = str.encode!(Encoding::BINARY)
       result.encoding.should equal(Encoding::BINARY)
       result.should equal(str)
@@ -221,7 +219,7 @@ describe "String#encode!" do
 
   describe "when passed to, from" do
     it "returns self" do
-      str = "ああ"
+      str = +"ああ"
       result = str.encode!("euc-jp", "utf-8")
       result.encoding.should equal(Encoding::EUC_JP)
       result.should equal(str)
diff --git a/spec/ruby/core/string/encoding_spec.rb b/spec/ruby/core/string/encoding_spec.rb
index 574a1e2f92..f6e8fd3470 100644
--- a/spec/ruby/core/string/encoding_spec.rb
+++ b/spec/ruby/core/string/encoding_spec.rb
@@ -14,11 +14,11 @@ describe "String#encoding" do
   end
 
   it "returns the given encoding if #force_encoding has been called" do
-    "a".force_encoding(Encoding::SHIFT_JIS).encoding.should == Encoding::SHIFT_JIS
+    "a".dup.force_encoding(Encoding::SHIFT_JIS).encoding.should == Encoding::SHIFT_JIS
   end
 
   it "returns the given encoding if #encode!has been called" do
-    "a".encode!(Encoding::SHIFT_JIS).encoding.should == Encoding::SHIFT_JIS
+    "a".dup.encode!(Encoding::SHIFT_JIS).encoding.should == Encoding::SHIFT_JIS
   end
 end
 
@@ -108,13 +108,13 @@ describe "String#encoding for Strings with \\u escapes" do
   end
 
   it "returns the given encoding if #force_encoding has been called" do
-    "\u{20}".force_encoding(Encoding::SHIFT_JIS).encoding.should == Encoding::SHIFT_JIS
-    "\u{2020}".force_encoding(Encoding::SHIFT_JIS).encoding.should == Encoding::SHIFT_JIS
+    "\u{20}".dup.force_encoding(Encoding::SHIFT_JIS).encoding.should == Encoding::SHIFT_JIS
+    "\u{2020}".dup.force_encoding(Encoding::SHIFT_JIS).encoding.should == Encoding::SHIFT_JIS
   end
 
   it "returns the given encoding if #encode!has been called" do
-    "\u{20}".encode!(Encoding::SHIFT_JIS).encoding.should == Encoding::SHIFT_JIS
-    "\u{2020}".encode!(Encoding::SHIFT_JIS).encoding.should == Encoding::SHIFT_JIS
+    "\u{20}".dup.encode!(Encoding::SHIFT_JIS).encoding.should == Encoding::SHIFT_JIS
+    "\u{2020}".dup.encode!(Encoding::SHIFT_JIS).encoding.should == Encoding::SHIFT_JIS
   end
 end
 
@@ -173,16 +173,12 @@ describe "String#encoding for Strings with \\x escapes" do
   end
 
   it "returns the given encoding if #force_encoding has been called" do
-    x50 = "\x50"
-    x50.force_encoding(Encoding::SHIFT_JIS).encoding.should == Encoding::SHIFT_JIS
-    xD4 = [212].pack('C')
-    xD4.force_encoding(Encoding::ISO_8859_9).encoding.should == Encoding::ISO_8859_9
+    "\x50".dup.force_encoding(Encoding::SHIFT_JIS).encoding.should == Encoding::SHIFT_JIS
+    [212].pack('C').force_encoding(Encoding::ISO_8859_9).encoding.should == Encoding::ISO_8859_9
   end
 
   it "returns the given encoding if #encode!has been called" do
-    x50 = "\x50"
-    x50.encode!(Encoding::SHIFT_JIS).encoding.should == Encoding::SHIFT_JIS
-    x00 = "x\00"
-    x00.encode!(Encoding::UTF_8).encoding.should == Encoding::UTF_8
+    "\x50".dup.encode!(Encoding::SHIFT_JIS).encoding.should == Encoding::SHIFT_JIS
+    "x\00".dup.encode!(Encoding::UTF_8).encoding.should == Encoding::UTF_8
   end
 end
diff --git a/spec/ruby/core/string/force_encoding_spec.rb b/spec/ruby/core/string/force_encoding_spec.rb
index f37aaf9eb4..2259dcf3cf 100644
--- a/spec/ruby/core/string/force_encoding_spec.rb
+++ b/spec/ruby/core/string/force_encoding_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: false
 require_relative '../../spec_helper'
 
 describe "String#force_encoding" do
diff --git a/spec/ruby/core/string/freeze_spec.rb b/spec/ruby/core/string/freeze_spec.rb
index 04d1e9513c..2e8e70386d 100644
--- a/spec/ruby/core/string/freeze_spec.rb
+++ b/spec/ruby/core/string/freeze_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: false
 require_relative '../../spec_helper'
 
 describe "String#freeze" do
diff --git a/spec/ruby/core/string/gsub_spec.rb b/spec/ruby/core/string/gsub_spec.rb
index 9e3b50322c..0d9f32eca2 100644
--- a/spec/ruby/core/string/gsub_spec.rb
+++ b/spec/ruby/core/string/gsub_spec.rb
@@ -1,4 +1,5 @@
 # -*- encoding: utf-8 -*-
+# frozen_string_literal: false
 require_relative '../../spec_helper'
 require_relative 'fixtures/classes'
 
diff --git a/spec/ruby/core/string/include_spec.rb b/spec/ruby/core/string/include_spec.rb
index 23e1e134ec..9781140a55 100644
--- a/spec/ruby/core/string/include_spec.rb
+++ b/spec/ruby/core/string/include_spec.rb
@@ -15,16 +15,16 @@ describe "String#include? with String" do
 
   it "returns true if both strings are empty" do
     "".should.include?("")
-    "".force_encoding("EUC-JP").should.include?("")
-    "".should.include?("".force_encoding("EUC-JP"))
-    "".force_encoding("EUC-JP").should.include?("".force_encoding("EUC-JP"))
+    "".dup.force_encoding("EUC-JP").should.include?("")
+    "".should.include?("".dup.force_encoding("EUC-JP"))
+    "".dup.force_encoding("EUC-JP").should.include?("".dup.force_encoding("EUC-JP"))
   end
 
   it "returns true if the RHS is empty" do
     "a".should.include?("")
-    "a".force_encoding("EUC-JP").should.include?("")
-    "a".should.include?("".force_encoding("EUC-JP"))
-    "a".force_encoding("EUC-JP").should.include?("".force_encoding("EUC-JP"))
+    "a".dup.force_encoding("EUC-JP").should.include?("")
+    "a".should.include?("".dup.force_encoding("EUC-JP"))
+    "a".dup.force_encoding("EUC-JP").should.include?("".dup.force_encoding("EUC-JP"))
   end
 
   it "tries to convert other to string using to_str" do
diff --git a/spec/ruby/core/string/index_spec.rb b/spec/ruby/core/string/index_spec.rb
index b500cf6ca7..be79708045 100644
--- a/spec/ruby/core/string/index_spec.rb
+++ b/spec/ruby/core/string/index_spec.rb
@@ -161,16 +161,16 @@ describe "String#index with String" do
   end
 
   it "handles a substring in a superset encoding" do
-    'abc'.force_encoding(Encoding::US_ASCII).index('é').should == nil
+    'abc'.dup.force_encoding(Encoding::US_ASCII).index('é').should == nil
   end
 
   it "handles a substring in a subset encoding" do
-    'été'.index('t'.force_encoding(Encoding::US_ASCII)).should == 1
+    'été'.index('t'.dup.force_encoding(Encoding::US_ASCII)).should == 1
   end
 
   it "raises an Encoding::CompatibilityError if the encodings are incompatible" do
-    str = 'abc'.force_encoding("ISO-2022-JP")
-    pattern = 'b'.force_encoding("EUC-JP")
+    str = 'abc'.dup.force_encoding("ISO-2022-JP")
+    pattern = 'b'.dup.force_encoding("EUC-JP")
 
     -> { str.index(pattern) }.should raise_error(Encoding::CompatibilityError, "incompatible character encodings: ISO-2022-JP and EUC-JP")
   end
diff --git a/spec/ruby/core/string/insert_spec.rb b/spec/ruby/core/string/insert_spec.rb
index 0c87df3a95..483f3c9367 100644
--- a/spec/ruby/core/string/insert_spec.rb
+++ b/spec/ruby/core/string/insert_spec.rb
@@ -1,5 +1,5 @@
 # -*- encoding: utf-8 -*-
-
+# frozen_string_literal: false
 require_relative '../../spec_helper'
 require_relative 'fixtures/classes'
 
diff --git a/spec/ruby/core/string/inspect_spec.rb b/spec/ruby/core/string/inspect_spec.rb
index 8bf3d3161f..15db06c7f5 100644
--- a/spec/ruby/core/string/inspect_spec.rb
+++ b/spec/ruby/core/string/inspect_spec.rb
@@ -327,7 +327,7 @@ describe "String#inspect" do
   end
 
   it "works for broken US-ASCII strings" do
-    s = "©".force_encoding("US-ASCII")
+    s = "©".dup.force_encoding("US-ASCII")
     s.inspect.should == '"\xC2\xA9"'
   end
 
diff --git a/spec/ruby/core/string/ljust_spec.rb b/spec/ruby/core/string/ljust_spec.rb
index 9208ec5897..47324c59d2 100644
--- a/spec/ruby/core/string/ljust_spec.rb
+++ b/spec/ruby/core/string/ljust_spec.rb
@@ -75,7 +75,7 @@ describe "String#ljust with length, padding" do
 
   describe "with width" do
     it "returns a String in the same encoding as the original" do
-      str = "abc".force_encoding Encoding::IBM437
+      str = "abc".dup.force_encoding Encoding::IBM437
       result = str.ljust 5
       result.should == "abc  "
       result.encoding.should equal(Encoding::IBM437)
@@ -84,7 +84,7 @@ describe "String#ljust with length, padding" do
 
   describe "with width, pattern" do
     it "returns a String in the compatible encoding" do
-      str = "abc".force_encoding Encoding::IBM437
+      str = "abc".dup.force_encoding Encoding::IBM437
       result = str.ljust 5, "あ"
       result.should == "abcああ"
       result.encoding.should equal(Encoding::UTF_8)
diff --git a/spec/ruby/core/string/lstrip_spec.rb b/spec/ruby/core/string/lstrip_spec.rb
index 85685deb0a..99bab6f349 100644
--- a/spec/ruby/core/string/lstrip_spec.rb
+++ b/spec/ruby/core/string/lstrip_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: false
 require_relative '../../spec_helper'
 require_relative 'fixtures/classes'
 require_relative 'shared/strip'
diff --git a/spec/ruby/core/string/ord_spec.rb b/spec/ruby/core/string/ord_spec.rb
index 4cf26990fe..35af3b5458 100644
--- a/spec/ruby/core/string/ord_spec.rb
+++ b/spec/ruby/core/string/ord_spec.rb
@@ -27,7 +27,7 @@ describe "String#ord" do
   end
 
   it "raises ArgumentError if the character is broken" do
-    s = "©".force_encoding("US-ASCII")
+    s = "©".dup.force_encoding("US-ASCII")
     -> { s.ord }.should raise_error(ArgumentError, "invalid byte sequence in US-ASCII")
   end
 end
diff --git a/spec/ruby/core/string/partition_spec.rb b/spec/ruby/core/string/partition_spec.rb
index 9cb3672881..d5370dcc73 100644
--- a/spec/ruby/core/string/partition_spec.rb
+++ b/spec/ruby/core/string/partition_spec.rb
@@ -40,7 +40,7 @@ describe "String#partition with String" do
   end
 
   it "handles a pattern in a superset encoding" do
-    string = "hello".force_encoding(Encoding::US_ASCII)
+    string = "hello".dup.force_encoding(Encoding::US_ASCII)
 
     result = string.partition("é")
 
@@ -51,7 +51,7 @@ describe "String#partition with String" do
   end
 
   it "handles a pattern in a subset encoding" do
-    pattern = "o".force_encoding(Encoding::US_ASCII)
+    pattern = "o".dup.force_encoding(Encoding::US_ASCII)
 
     result = "héllo world".partition(pattern)
 
diff --git a/spec/ruby/core/string/prepend_spec.rb b/spec/ruby/core/string/prepend_spec.rb
index a0393d4760..5248ea8056 100644
--- a/spec/ruby/core/string/prepend_spec.rb
+++ b/spec/ruby/core/string/prepend_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: false
 require_relative '../../spec_helper'
 require_relative 'fixtures/classes'
 
diff --git a/spec/ruby/core/string/reverse_spec.rb b/spec/ruby/core/string/reverse_spec.rb
index e67122c05c..aa6abe6036 100644
--- a/spec/ruby/core/string/reverse_spec.rb
+++ b/spec/ruby/core/string/reverse_spec.rb
@@ -1,4 +1,5 @@
 # encoding: utf-8
+# frozen_string_literal: false
 
 require_relative '../../spec_helper'
 require_relative 'fixtures/classes'
diff --git a/spec/ruby/core/string/rindex_spec.rb b/spec/ruby/core/string/rindex_spec.rb
index f6271b270d..88ce733583 100644
--- a/spec/ruby/core/string/rindex_spec.rb
+++ b/spec/ruby/core/string/rindex_spec.rb
@@ -197,16 +197,16 @@ describe "String#rindex with String" do
   end
 
   it "handles a substring in a superset encoding" do
-    'abc'.force_encoding(Encoding::US_ASCII).rindex('é').should == nil
+    'abc'.dup.force_encoding(Encoding::US_ASCII).rindex('é').should == nil
   end
 
   it "handles a substring in a subset encoding" do
-    'été'.rindex('t'.force_encoding(Encoding::US_ASCII)).should == 1
+    'été'.rindex('t'.dup.force_encoding(Encoding::US_ASCII)).should == 1
   end
 
   it "raises an Encoding::CompatibilityError if the encodings are incompatible" do
-    str = 'abc'.force_encoding("ISO-2022-JP")
-    pattern = 'b'.force_encoding("EUC-JP")
+    str = 'abc'.dup.force_encoding("ISO-2022-JP")
+    pattern = 'b'.dup.force_encoding("EUC-JP")
 
     -> { str.rindex(pattern) }.should raise_error(Encoding::CompatibilityError, "incompatible character encodings: ISO-2022-JP and EUC-JP")
   end
diff --git a/spec/ruby/core/string/rjust_spec.rb b/spec/ruby/core/string/rjust_spec.rb
index fcbaf3b938..4ad3e54aea 100644
--- a/spec/ruby/core/string/rjust_spec.rb
+++ b/spec/ruby/core/string/rjust_spec.rb
@@ -75,7 +75,7 @@ describe "String#rjust with length, padding" do
 
   describe "with width" do
     it "returns a String in the same encoding as the original" do
-      str = "abc".force_encoding Encoding::IBM437
+      str = "abc".dup.force_encoding Encoding::IBM437
       result = str.rjust 5
       result.should == "  abc"
       result.encoding.should equal(Encoding::IBM437)
@@ -84,7 +84,7 @@ describe "String#rjust with length, padding" do
 
   describe "with width, pattern" do
     it "returns a String in the compatible encoding" do
-      str = "abc".force_encoding Encoding::IBM437
+      str = "abc".dup.force_encoding Encoding::IBM437
       result = str.rjust 5, "あ"
       result.should == "ああabc"
       result.encoding.should equal(Encoding::UTF_8)
diff --git a/spec/ruby/core/string/rpartition_spec.rb b/spec/ruby/core/string/rpartition_spec.rb
index 21e87f530a..cef0384c73 100644
--- a/spec/ruby/core/string/rpartition_spec.rb
+++ b/spec/ruby/core/string/rpartition_spec.rb
@@ -48,7 +48,7 @@ describe "String#rpartition with String" do
   end
 
   it "handles a pattern in a superset encoding" do
-    string = "hello".force_encoding(Encoding::US_ASCII)
+    string = "hello".dup.force_encoding(Encoding::US_ASCII)
 
     result = string.rpartition("é")
 
@@ -59,7 +59,7 @@ describe "String#rpartition with String" do
   end
 
   it "handles a pattern in a subset encoding" do
-    pattern = "o".force_encoding(Encoding::US_ASCII)
+    pattern = "o".dup.force_encoding(Encoding::US_ASCII)
 
     result = "héllo world".rpartition(pattern)
 
diff --git a/spec/ruby/core/string/rstrip_spec.rb b/spec/ruby/core/string/rstrip_spec.rb
index e4cf93315e..6d46eb590e 100644
--- a/spec/ruby/core/string/rstrip_spec.rb
+++ b/spec/ruby/core/string/rstrip_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: false
 require_relative '../../spec_helper'
 require_relative 'fixtures/classes'
 require_relative 'shared/strip'
diff --git a/spec/ruby/core/string/scrub_spec.rb b/spec/ruby/core/string/scrub_spec.rb
index bcee4db463..b9ef0f1a16 100644
--- a/spec/ruby/core/string/scrub_spec.rb
+++ b/spec/ruby/core/string/scrub_spec.rb
@@ -1,4 +1,5 @@
 # -*- encoding: utf-8 -*-
+# frozen_string_literal: false
 require_relative '../../spec_helper'
 require_relative 'fixtures/classes'
 
diff --git a/spec/ruby/core/string/setbyte_spec.rb b/spec/ruby/core/string/setbyte_spec.rb
index 77bff64038..85403ca62c 100644
--- a/spec/ruby/core/string/setbyte_spec.rb
+++ b/spec/ruby/core/string/setbyte_spec.rb
@@ -1,4 +1,5 @@
 # -*- encoding: utf-8 -*-
+# frozen_string_literal: false
 require_relative '../../spec_helper'
 
 describe "String#setbyte" do
diff --git a/spec/ruby/core/string/shared/chars.rb b/spec/ruby/core/string/shared/chars.rb
index e9fdf89fd6..c730643cf4 100644
--- a/spec/ruby/core/string/shared/chars.rb
+++ b/spec/ruby/core/string/shared/chars.rb
@@ -21,12 +21,12 @@ describe :string_chars, shared: true do
   end
 
   it "returns characters in the same encoding as self" do
-    "&%".force_encoding('Shift_JIS').send(@method).to_a.all? {|c| c.encoding.name.should == 'Shift_JIS'}
+    "&%".dup.force_encoding('Shift_JIS').send(@method).to_a.all? {|c| c.encoding.name.should == 'Shift_JIS'}
     "&%".encode('BINARY').send(@method).to_a.all? {|c| c.encoding.should == Encoding::BINARY }
   end
 
   it "works with multibyte characters" do
-    s = "\u{8987}".force_encoding("UTF-8")
+    s = "\u{8987}".dup.force_encoding("UTF-8")
     s.bytesize.should == 3
     s.send(@method).to_a.should == [s]
   end
@@ -39,14 +39,14 @@ describe :string_chars, shared: true do
   end
 
   it "returns a different character if the String is transcoded" do
-    s = "\u{20AC}".force_encoding('UTF-8')
-    s.encode('UTF-8').send(@method).to_a.should == ["\u{20AC}".force_encoding('UTF-8')]
+    s = "\u{20AC}".dup.force_encoding('UTF-8')
+    s.encode('UTF-8').send(@method).to_a.should == ["\u{20AC}".dup.force_encoding('UTF-8')]
     s.encode('iso-8859-15').send(@method).to_a.should == [[0xA4].pack('C').force_encoding('iso-8859-15')]
-    s.encode('iso-8859-15').encode('UTF-8').send(@method).to_a.should == ["\u{20AC}".force_encoding('UTF-8')]
+    s.encode('iso-8859-15').encode('UTF-8').send(@method).to_a.should == ["\u{20AC}".dup.force_encoding('UTF-8')]
   end
 
   it "uses the String's encoding to determine what characters it contains" do
-    s = "\u{24B62}"
+    s = +"\u{24B62}"
 
     s.force_encoding('UTF-8').send(@method).to_a.should == [
       s.force_encoding('UTF-8')
diff --git a/spec/ruby/core/string/shared/codepoints.rb b/spec/ruby/core/string/shared/codepoints.rb
index 0b2e078e0a..f71263054a 100644
--- a/spec/ruby/core/string/shared/codepoints.rb
+++ b/spec/ruby/core/string/shared/codepoints.rb
@@ -7,7 +7,7 @@ describe :string_codepoints, shared: true do
   end
 
   it "raises an ArgumentError when self has an invalid encoding and a method is called on the returned Enumerator" do
-    s = "\xDF".force_encoding(Encoding::UTF_8)
+    s = "\xDF".dup.force_encoding(Encoding::UTF_8)
     s.valid_encoding?.should be_false
     -> { s.send(@method).to_a }.should raise_error(ArgumentError)
   end
@@ -21,7 +21,7 @@ describe :string_codepoints, shared: true do
   end
 
   it "raises an ArgumentError if self's encoding is invalid and a block is given" do
-    s = "\xDF".force_encoding(Encoding::UTF_8)
+    s = "\xDF".dup.force_encoding(Encoding::UTF_8)
     s.valid_encoding?.should be_false
     -> { s.send(@method) { } }.should raise_error(ArgumentError)
   end
@@ -49,7 +49,7 @@ describe :string_codepoints, shared: true do
 
   it "round-trips to the original String using Integer#chr" do
     s = "\u{13}\u{7711}\u{1010}"
-    s2 = ""
+    s2 = +""
     s.send(@method) {|n| s2 << n.chr(Encoding::UTF_8)}
     s.should == s2
   end
diff --git a/spec/ruby/core/string/shared/concat.rb b/spec/ruby/core/string/shared/concat.rb
index ee5ef2a98f..dded9a69e7 100644
--- a/spec/ruby/core/string/shared/concat.rb
+++ b/spec/ruby/core/string/shared/concat.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: false
 describe :string_concat, shared: true do
   it "concatenates the given argument to self and returns self" do
     str = 'hello '
diff --git a/spec/ruby/core/string/shared/dedup.rb b/spec/ruby/core/string/shared/dedup.rb
index 893fd1e360..97b5df6ed1 100644
--- a/spec/ruby/core/string/shared/dedup.rb
+++ b/spec/ruby/core/string/shared/dedup.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: false
 describe :string_dedup, shared: true do
   it 'returns self if the String is frozen' do
     input  = 'foo'.freeze
diff --git a/spec/ruby/core/string/shared/each_codepoint_without_block.rb b/spec/ruby/core/string/shared/each_codepoint_without_block.rb
index 92b7f76032..31b4c02c9c 100644
--- a/spec/ruby/core/string/shared/each_codepoint_without_block.rb
+++ b/spec/ruby/core/string/shared/each_codepoint_without_block.rb
@@ -6,7 +6,7 @@ describe :string_each_codepoint_without_block, shared: true do
     end
 
     it "returns an Enumerator even when self has an invalid encoding" do
-      s = "\xDF".force_encoding(Encoding::UTF_8)
+      s = "\xDF".dup.force_encoding(Encoding::UTF_8)
       s.valid_encoding?.should be_false
       s.send(@method).should be_an_instance_of(Enumerator)
     end
@@ -23,7 +23,7 @@ describe :string_each_codepoint_without_block, shared: true do
         end
 
         it "should return the size of the string even when the string has an invalid encoding" do
-          s = "\xDF".force_encoding(Encoding::UTF_8)
+          s = "\xDF".dup.force_encoding(Encoding::UTF_8)
           s.valid_encoding?.should be_false
           s.send(@method).size.should == 1
         end
diff --git a/spec/ruby/core/string/shared/each_line.rb b/spec/ruby/core/string/shared/each_line.rb
index a14b4d7779..231a6d9d4f 100644
--- a/spec/ruby/core/string/shared/each_line.rb
+++ b/spec/ruby/core/string/shared/each_line.rb
@@ -106,7 +106,7 @@ describe :string_each_line, shared: true do
   end
 
   it "does not care if the string is modified while substituting" do
-    str = "hello\nworld."
+    str = +"hello\nworld."
     out = []
     str.send(@method){|x| out << x; str[-1] = '!' }.should == "hello\nworld!"
     out.should == ["hello\n", "world."]
diff --git a/spec/ruby/core/string/shared/encode.rb b/spec/ruby/core/string/shared/encode.rb
index a73de5b943..3776e0d709 100644
--- a/spec/ruby/core/string/shared/encode.rb
+++ b/spec/ruby/core/string/shared/encode.rb
@@ -1,4 +1,5 @@
 # -*- encoding: utf-8 -*-
+# frozen_string_literal: false
 describe :string_encode, shared: true do
   describe "when passed no options" do
     it "transcodes to Encoding.default_internal when set" do
diff --git a/spec/ruby/core/string/shared/eql.rb b/spec/ruby/core/string/shared/eql.rb
index 6f268c929c..845b0a3e15 100644
--- a/spec/ruby/core/string/shared/eql.rb
+++ b/spec/ruby/core/string/shared/eql.rb
@@ -13,15 +13,15 @@ describe :string_eql_value, shared: true do
   end
 
   it "ignores encoding difference of compatible string" do
-    "hello".force_encoding("utf-8").send(@method, "hello".force_encoding("iso-8859-1")).should be_true
+    "hello".dup.force_encoding("utf-8").send(@method, "hello".dup.force_encoding("iso-8859-1")).should be_true
   end
 
   it "considers encoding difference of incompatible string" do
-    "\xff".force_encoding("utf-8").send(@method, "\xff".force_encoding("iso-8859-1")).should be_false
+    "\xff".dup.force_encoding("utf-8").send(@method, "\xff".dup.force_encoding("iso-8859-1")).should be_false
   end
 
   it "considers encoding compatibility" do
-    "abcd".force_encoding("utf-8").send(@method, "abcd".force_encoding("utf-32le")).should be_false
+    "abcd".dup.force_encoding("utf-8").send(@method, "abcd".dup.force_encoding("utf-32le")).should be_false
   end
 
   it "ignores subclass differences" do
@@ -33,6 +33,6 @@ describe :string_eql_value, shared: true do
   end
 
   it "returns true when comparing 2 empty strings but one is not ASCII-compatible" do
-    "".send(@method, "".force_encoding('iso-2022-jp')).should == true
+    "".send(@method, "".dup.force_encoding('iso-2022-jp')).should == true
   end
 end
diff --git a/spec/ruby/core/string/shared/length.rb b/spec/ruby/core/string/shared/length.rb
index 94e5ec135b..ae572ba755 100644
--- a/spec/ruby/core/string/shared/length.rb
+++ b/spec/ruby/core/string/shared/length.rb
@@ -18,7 +18,7 @@ describe :string_length, shared: true do
   end
 
   it "returns the length of the new self after encoding is changed" do
-    str = 'こにちわ'
+    str = +'こにちわ'
     str.send(@method)
 
     str.force_encoding('BINARY').send(@method).should == 12
@@ -44,12 +44,12 @@ describe :string_length, shared: true do
   end
 
   it "adds 1 (and not 2) for a incomplete surrogate in UTF-16" do
-    "\x00\xd8".force_encoding("UTF-16LE").send(@method).should == 1
-    "\xd8\x00".force_encoding("UTF-16BE").send(@method).should == 1
+    "\x00\xd8".dup.force_encoding("UTF-16LE").send(@method).should == 1
+    "\xd8\x00".dup.force_encoding("UTF-16BE").send(@method).should == 1
   end
 
   it "adds 1 for a broken sequence in UTF-32" do
-    "\x04\x03\x02\x01".force_encoding("UTF-32LE").send(@method).should == 1
-    "\x01\x02\x03\x04".force_encoding("UTF-32BE").send(@method).should == 1
+    "\x04\x03\x02\x01".dup.force_encoding("UTF-32LE").send(@method).should == 1
+    "\x01\x02\x03\x04".dup.force_encoding("UTF-32BE").send(@method).should == 1
   end
 end
diff --git a/spec/ruby/core/string/shared/replace.rb b/spec/ruby/core/string/shared/replace.rb
index a5108d9e7c..24dac0eb27 100644
--- a/spec/ruby/core/string/shared/replace.rb
+++ b/spec/ruby/core/string/shared/replace.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: false
 describe :string_replace, shared: true do
   it "returns self" do
     a = "a"
diff --git a/spec/ruby/core/string/shared/slice.rb b/spec/ruby/core/string/shared/slice.rb
index 3ef4bc50d7..2f69b9ddce 100644
--- a/spec/ruby/core/string/shared/slice.rb
+++ b/spec/ruby/core/string/shared/slice.rb
@@ -84,8 +84,8 @@ describe :string_slice_index_length, shared: true do
     s = "hello there"
     s.send(@method, 1, 9).encoding.should == s.encoding
 
-    a = "hello".force_encoding("binary")
-    b = " there".force_encoding("ISO-8859-1")
+    a = "hello".dup.force_encoding("binary")
+    b = " there".dup.force_encoding("ISO-8859-1")
     c = (a + b).force_encoding(Encoding::US_ASCII)
 
     c.send(@method, 0, 5).encoding.should == Encoding::US_ASCII
diff --git a/spec/ruby/core/string/shared/succ.rb b/spec/ruby/core/string/shared/succ.rb
index 24a729ce26..b69a394875 100644
--- a/spec/ruby/core/string/shared/succ.rb
+++ b/spec/ruby/core/string/shared/succ.rb
@@ -73,6 +73,7 @@ end
 describe :string_succ_bang, shared: true do
   it "is equivalent to succ, but modifies self in place (still returns self)" do
     ["", "abcd", "THX1138"].each do |s|
+      s = +s
       r = s.dup.send(@method)
       s.send(@method).should equal(s)
       s.should == r
diff --git a/spec/ruby/core/string/shared/to_sym.rb b/spec/ruby/core/string/shared/to_sym.rb
index 52d8314211..833eae100e 100644
--- a/spec/ruby/core/string/shared/to_sym.rb
+++ b/spec/ruby/core/string/shared/to_sym.rb
@@ -56,9 +56,9 @@ describe :string_to_sym, shared: true do
   it "ignores existing symbols with different encoding" do
     source = "fée"
 
-    iso_symbol = source.force_encoding(Encoding::ISO_8859_1).send(@method)
+    iso_symbol = source.dup.force_encoding(Encoding::ISO_8859_1).send(@method)
     iso_symbol.encoding.should == Encoding::ISO_8859_1
-    binary_symbol = source.force_encoding(Encoding::BINARY).send(@method)
+    binary_symbol = source.dup.force_encoding(Encoding::BINARY).send(@method)
     binary_symbol.encoding.should == Encoding::BINARY
   end
 
diff --git a/spec/ruby/core/string/slice_spec.rb b/spec/ruby/core/string/slice_spec.rb
index 87c5a7ac37..5aba2d3be0 100644
--- a/spec/ruby/core/string/slice_spec.rb
+++ b/spec/ruby/core/string/slice_spec.rb
@@ -1,5 +1,5 @@
 # -*- encoding: utf-8 -*-
-
+# frozen_string_literal: false
 require_relative '../../spec_helper'
 require_relative 'fixtures/classes'
 require_relative 'shared/slice'
diff --git a/spec/ruby/core/string/split_spec.rb b/spec/ruby/core/string/split_spec.rb
index c5cca651c2..3c6d1864d1 100644
--- a/spec/ruby/core/string/split_spec.rb
+++ b/spec/ruby/core/string/split_spec.rb
@@ -4,7 +4,7 @@ require_relative 'fixtures/classes'
 
 describe "String#split with String" do
   it "throws an ArgumentError if the string  is not a valid" do
-    s = "\xDF".force_encoding(Encoding::UTF_8)
+    s = "\xDF".dup.force_encoding(Encoding::UTF_8)
 
     -> { s.split }.should raise_error(ArgumentError)
     -> { s.split(':') }.should raise_error(ArgumentError)
@@ -12,7 +12,7 @@ describe "String#split with String" do
 
   it "throws an ArgumentError if the pattern is not a valid string" do
     str = 'проверка'
-    broken_str = "\xDF".force_encoding(Encoding::UTF_8)
+    broken_str = "\xDF".dup.force_encoding(Encoding::UTF_8)
 
     -> { str.split(broken_str) }.should raise_error(ArgumentError)
   end
@@ -229,7 +229,7 @@ end
 
 describe "String#split with Regexp" do
   it "throws an ArgumentError if the string  is not a valid" do
-    s = "\xDF".force_encoding(Encoding::UTF_8)
+    s = "\xDF".dup.force_encoding(Encoding::UTF_8)
 
     -> { s.split(/./) }.should raise_error(ArgumentError)
   end
@@ -409,7 +409,7 @@ describe "String#split with Regexp" do
   end
 
   it "returns an ArgumentError if an invalid UTF-8 string is supplied" do
-    broken_str = 'проверка' # in russian, means "test"
+    broken_str = +'проверка' # in russian, means "test"
     broken_str.force_encoding('binary')
     broken_str.chop!
     broken_str.force_encoding('utf-8')
diff --git a/spec/ruby/core/string/squeeze_spec.rb b/spec/ruby/core/string/squeeze_spec.rb
index 4796a170f2..4ea238e6b5 100644
--- a/spec/ruby/core/string/squeeze_spec.rb
+++ b/spec/ruby/core/string/squeeze_spec.rb
@@ -1,4 +1,5 @@
 # -*- encoding: binary -*-
+# frozen_string_literal: false
 require_relative '../../spec_helper'
 require_relative 'fixtures/classes'
 
diff --git a/spec/ruby/core/string/strip_spec.rb b/spec/ruby/core/string/strip_spec.rb
index 5e90fe35d0..edb6ea3b44 100644
--- a/spec/ruby/core/string/strip_spec.rb
+++ b/spec/ruby/core/string/strip_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: false
 require_relative '../../spec_helper'
 require_relative 'fixtures/classes'
 require_relative 'shared/strip'
diff --git a/spec/ruby/core/string/sub_spec.rb b/spec/ruby/core/string/sub_spec.rb
index 51920486f5..4f9f87a433 100644
--- a/spec/ruby/core/string/sub_spec.rb
+++ b/spec/ruby/core/string/sub_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: false
 require_relative '../../spec_helper'
 require_relative 'fixtures/classes'
 
diff --git a/spec/ruby/core/string/swapcase_spec.rb b/spec/ruby/core/string/swapcase_spec.rb
index d740fb86c6..7f4c68366d 100644
--- a/spec/ruby/core/string/swapcase_spec.rb
+++ b/spec/ruby/core/string/swapcase_spec.rb
@@ -1,4 +1,5 @@
 # -*- encoding: utf-8 -*-
+# frozen_string_literal: false
 require_relative '../../spec_helper'
 require_relative 'fixtures/classes'
 
diff --git a/spec/ruby/core/string/tr_s_spec.rb b/spec/ruby/core/string/tr_s_spec.rb
index 3c31473044..dd72da440c 100644
--- a/spec/ruby/core/string/tr_s_spec.rb
+++ b/spec/ruby/core/string/tr_s_spec.rb
@@ -1,4 +1,5 @@
 # -*- encoding: utf-8 -*-
+# frozen_string_literal: false
 require_relative '../../spec_helper'
 require_relative 'fixtures/classes'
 
diff --git a/spec/ruby/core/string/tr_spec.rb b/spec/ruby/core/string/tr_spec.rb
index d60480dc7e..75841a974f 100644
--- a/spec/ruby/core/string/tr_spec.rb
+++ b/spec/ruby/core/string/tr_spec.rb
@@ -1,4 +1,5 @@
 # -*- encoding: utf-8 -*-
+# frozen_string_literal: false
 require_relative '../../spec_helper'
 require_relative 'fixtures/classes'
 
diff --git a/spec/ruby/core/string/unicode_normalize_spec.rb b/spec/ruby/core/string/unicode_normalize_spec.rb
index 6de7533fc7..2e7d22394a 100644
--- a/spec/ruby/core/string/unicode_normalize_spec.rb
+++ b/spec/ruby/core/string/unicode_normalize_spec.rb
@@ -1,4 +1,5 @@
 # -*- encoding: utf-8 -*-
+# frozen_string_literal: false
 require_relative '../../spec_helper'
 
 # Examples taken from http://www.unicode.org/reports/tr15/#Norm_Forms
diff --git a/spec/ruby/core/string/unicode_normalized_spec.rb b/spec/ruby/core/string/unicode_normalized_spec.rb
index 87f3740459..91cf2086b2 100644
--- a/spec/ruby/core/string/unicode_normalized_spec.rb
+++ b/spec/ruby/core/string/unicode_normalized_spec.rb
@@ -1,4 +1,5 @@
 # -*- encoding: utf-8 -*-
+# frozen_string_literal: false
 require_relative '../../spec_helper'
 
 describe "String#unicode_normalized?" do
diff --git a/spec/ruby/core/string/unpack/a_spec.rb b/spec/ruby/core/string/unpack/a_spec.rb
index 2d83b4c824..4002ece697 100644
--- a/spec/ruby/core/string/unpack/a_spec.rb
+++ b/spec/ruby/core/string/unpack/a_spec.rb
@@ -31,7 +31,7 @@ describe "String#unpack with format 'A'" do
   end
 
   it "decodes into raw (ascii) string values" do
-    str = "str".force_encoding('UTF-8').unpack("A*")[0]
+    str = "str".dup.force_encoding('UTF-8').unpack("A*")[0]
     str.encoding.should == Encoding::BINARY
   end
 
diff --git a/spec/ruby/core/string/unpack/b_spec.rb b/spec/ruby/core/string/unpack/b_spec.rb
index 5c53eff721..23d93a8aea 100644
--- a/spec/ruby/core/string/unpack/b_spec.rb
+++ b/spec/ruby/core/string/unpack/b_spec.rb
@@ -107,7 +107,7 @@ describe "String#unpack with format 'B'" do
   end
 
   it "decodes into US-ASCII string values" do
-    str = "s".force_encoding('UTF-8').unpack("B*")[0]
+    str = "s".dup.force_encoding('UTF-8').unpack("B*")[0]
     str.encoding.name.should == 'US-ASCII'
   end
 end
@@ -215,7 +215,7 @@ describe "String#unpack with format 'b'" do
   end
 
   it "decodes into US-ASCII string values" do
-    str = "s".force_encoding('UTF-8').unpack("b*")[0]
+    str = "s".dup.force_encoding('UTF-8').unpack("b*")[0]
     str.encoding.name.should == 'US-ASCII'
   end
 end
diff --git a/spec/ruby/core/string/unpack/u_spec.rb b/spec/ruby/core/string/unpack/u_spec.rb
index 7845e6d5f2..456abee784 100644
--- a/spec/ruby/core/string/unpack/u_spec.rb
+++ b/spec/ruby/core/string/unpack/u_spec.rb
@@ -33,7 +33,7 @@ describe "String#unpack with format 'u'" do
     str = "".unpack("u")[0]
     str.encoding.should == Encoding::BINARY
 
-    str = "1".force_encoding('UTF-8').unpack("u")[0]
+    str = "1".dup.force_encoding('UTF-8').unpack("u")[0]
     str.encoding.should == Encoding::BINARY
   end
 
diff --git a/spec/ruby/core/string/upcase_spec.rb b/spec/ruby/core/string/upcase_spec.rb
index a2e34f5f40..652de5c2ef 100644
--- a/spec/ruby/core/string/upcase_spec.rb
+++ b/spec/ruby/core/string/upcase_spec.rb
@@ -1,4 +1,5 @@
 # -*- encoding: utf-8 -*-
+# frozen_string_literal: false
 require_relative '../../spec_helper'
 require_relative 'fixtures/classes'
 
diff --git a/spec/ruby/core/string/uplus_spec.rb b/spec/ruby/core/string/uplus_spec.rb
index 65b66260dd..c0b0c49ede 100644
--- a/spec/ruby/core/string/uplus_spec.rb
+++ b/spec/ruby/core/string/uplus_spec.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: false
 require_relative '../../spec_helper'
 
 describe 'String#+@' do
diff --git a/spec/ruby/core/string/upto_spec.rb b/spec/ruby/core/string/upto_spec.rb
index 3799e338e0..8bc847d5ac 100644
--- a/spec/ruby/core/string/upto_spec.rb
+++ b/spec/ruby/core/string/upto_spec.rb
@@ -81,8 +81,8 @@ describe "String#upto" do
   end
 
   it "raises Encoding::CompatibilityError when incompatible characters are given" do
-    char1 = 'a'.force_encoding("EUC-JP")
-    char2 = 'b'.force_encoding("ISO-2022-JP")
+    char1 = 'a'.dup.force_encoding("EUC-JP")
+    char2 = 'b'.dup.force_encoding("ISO-2022-JP")
     -> { char1.upto(char2) {} }.should raise_error(Encoding::CompatibilityError, "incompatible character encodings: EUC-JP and ISO-2022-JP")
   end
 
diff --git a/spec/ruby/core/string/valid_encoding_spec.rb b/spec/ruby/core/string/valid_encoding_spec.rb
index bb26062c0f..375035cd94 100644
--- a/spec/ruby/core/string/valid_encoding_spec.rb
+++ b/spec/ruby/core/string/valid_encoding_spec.rb
@@ -7,13 +7,13 @@ describe "String#valid_encoding?" do
   end
 
   it "returns true if self is valid in the current encoding and other encodings" do
-    str = "\x77"
+    str = +"\x77"
     str.force_encoding('utf-8').valid_encoding?.should be_true
     str.force_encoding('binary').valid_encoding?.should be_true
   end
 
   it "returns true for all encodings self is valid in" do
-    str = "\xE6\x9D\x94"
+    str = +"\xE6\x9D\x94"
     str.force_encoding('BINARY').valid_encoding?.should be_true
     str.force_encoding('UTF-8').valid_encoding?.should be_true
     str.force_encoding('US-ASCII').valid_encoding?.should be_false
@@ -43,10 +43,10 @@ describe "String#valid_encoding?" do
     str.force_encoding('KOI8-R').valid_encoding?.should be_true
     str.force_encoding('KOI8-U').valid_encoding?.should be_true
     str.force_encoding('Shift_JIS').valid_encoding?.should be_false
-    "\xD8\x00".force_encoding('UTF-16BE').valid_encoding?.should be_false
-    "\x00\xD8".force_encoding('UTF-16LE').valid_encoding?.should be_false
-    "\x04\x03\x02\x01".force_encoding('UTF-32BE').valid_encoding?.should be_false
-    "\x01\x02\x03\x04".force_encoding('UTF-32LE').valid_encoding?.should be_false
+    "\xD8\x00".dup.force_encoding('UTF-16BE').valid_encoding?.should be_false
+    "\x00\xD8".dup.force_encoding('UTF-16LE').valid_encoding?.should be_false
+    "\x04\x03\x02\x01".dup.force_encoding('UTF-32BE').valid_encoding?.should be_false
+    "\x01\x02\x03\x04".dup.force_encoding('UTF-32LE').valid_encoding?.should be_false
     str.force_encoding('Windows-1251').valid_encoding?.should be_true
     str.force_encoding('IBM437').valid_encoding?.should be_true
     str.force_encoding('IBM737').valid_encoding?.should be_true
@@ -101,24 +101,24 @@ describe "String#valid_encoding?" do
   end
 
   it "returns true for IBM720 encoding self is valid in" do
-    str = "\xE6\x9D\x94"
+    str = +"\xE6\x9D\x94"
     str.force_encoding('IBM720').valid_encoding?.should be_true
     str.force_encoding('CP720').valid_encoding?.should be_true
   end
 
   it "returns false if self is valid in one encoding, but invalid in the one it's tagged with" do
-    str = "\u{8765}"
+    str = +"\u{8765}"
     str.valid_encoding?.should be_true
-    str = str.force_encoding('ascii')
+    str.force_encoding('ascii')
     str.valid_encoding?.should be_false
   end
 
   it "returns false if self contains a character invalid in the associated encoding" do
-    "abc#{[0x80].pack('C')}".force_encoding('ascii').valid_encoding?.should be_false
+    "abc#{[0x80].pack('C')}".dup.force_encoding('ascii').valid_encoding?.should be_false
   end
 
   it "returns false if a valid String had an invalid character appended to it" do
-    str = "a"
+    str = +"a"
     str.valid_encoding?.should be_true
     str << [0xDD].pack('C').force_encoding('utf-8')
     str.valid_encoding?.should be_false