#1142 but with an extra commit to ease reviewing the diff

.github/workflows/ci.yml

@@ -10,6 +10,12 @@ jobs:
       matrix:
         os: [ ubuntu, macos, windows ]
         ruby: [ 3.0.6, 3.1.4, 3.2.2, 3.3.0 ]
+        rubyopt: [""]
+        include:
+          - os: ubuntu
+            ruby: 3.3.0
+            rubyopt: "--enable-frozen-string-literal"
+
     runs-on: ${{ matrix.os }}-latest
     steps:
     - name: git config autocrlf
@@ -28,6 +34,7 @@ jobs:
       if: matrix.os == 'ubuntu'
       env:
         CHECK_LEAKS: true
+        RUBYOPT: "${{ matrix.rubyopt }}"
       run: ../mspec/bin/mspec -j --timeout 30
 
     - name: Run specs (macOS)

core/argf/readpartial_spec.rb

@@ -29,7 +29,7 @@
 
   it "clears output buffer even if EOFError is raised because @argf is at end" do
     begin
-      output = "to be cleared"
+      output = +"to be cleared"
 
       argf [@file1_name] do
         @argf.read

core/argf/shared/getc.rb

@@ -9,7 +9,7 @@
 
   it "reads each char of files" do
     argf [@file1, @file2] do
-      chars = ""
+      chars = +""
       @chars.size.times { chars << @argf.send(@method) }
       chars.should == @chars
     end

core/argf/shared/read.rb

@@ -15,15 +15,15 @@
 
   it "treats second argument as an output buffer" do
     argf [@file1_name] do
-      buffer = ""
+      buffer = +""
       @argf.send(@method, @file1.size, buffer)
       buffer.should == @file1
     end
   end
 
   it "clears output buffer before appending to it" do
     argf [@file1_name] do
-      buffer = "to be cleared"
+      buffer = +"to be cleared"
       @argf.send(@method, @file1.size, buffer)
       buffer.should == @file1
     end

core/array/fill_spec.rb

@@ -21,7 +21,7 @@
 
   it "does not replicate the filler" do
     ary = [1, 2, 3, 4]
-    str = "x"
+    str = +"x"
     ary.fill(str).should == [str, str, str, str]
     str << "y"
     ary.should == [str, str, str, str]

core/array/pack/buffer_spec.rb

@@ -13,13 +13,13 @@
   it "adds result at the end of buffer content" do
     n = [ 65, 66, 67 ] # result without buffer is "ABC"
 
-    buffer = ""
+    buffer = +""
     n.pack("ccc", buffer: buffer).should == "ABC"
 
-    buffer = "123"
+    buffer = +"123"
     n.pack("ccc", buffer: buffer).should == "123ABC"
 
-    buffer = "12345"
+    buffer = +"12345"
     n.pack("ccc", buffer: buffer).should == "12345ABC"
   end
 
@@ -31,19 +31,19 @@
   context "offset (@) is specified" do
     it 'keeps buffer content if it is longer than offset' do
       n = [ 65, 66, 67 ]
-      buffer = "123456"
+      buffer = +"123456"
       n.pack("@3ccc", buffer: buffer).should == "123ABC"
     end
 
     it "fills the gap with \\0 if buffer content is shorter than offset" do
       n = [ 65, 66, 67 ]
-      buffer = "123"
+      buffer = +"123"
       n.pack("@6ccc", buffer: buffer).should == "123\0\0\0ABC"
     end
 
     it 'does not keep buffer content if it is longer than offset + result' do
       n = [ 65, 66, 67 ]
-      buffer = "1234567890"
+      buffer = +"1234567890"
       n.pack("@3ccc", buffer: buffer).should == "123ABC"
     end
   end

core/array/shared/inspect.rb

@@ -19,7 +19,7 @@
   end
 
   it "does not call #to_s on a String returned from #inspect" do
-    str = "abc"
+    str = +"abc"
     str.should_not_receive(:to_s)
 
     [str].send(@method).should == '["abc"]'
@@ -98,8 +98,8 @@
     end
 
     it "does not raise if inspected result is not default external encoding" do
-      utf_16be = mock("utf_16be")
-      utf_16be.should_receive(:inspect).and_return(%<"utf_16be \u3042">.encode!(Encoding::UTF_16BE))
+      utf_16be = mock(+"utf_16be")
+      utf_16be.should_receive(:inspect).and_return(%<"utf_16be \u3042">.encode(Encoding::UTF_16BE))
 
       [utf_16be].send(@method).should == '["utf_16be \u3042"]'
     end

core/complex/inspect_spec.rb

@@ -17,7 +17,8 @@
 
   it "calls #inspect on real and imaginary" do
     real = NumericSpecs::Subclass.new
-    real.should_receive(:inspect).and_return("1")
+    # + because of https://bugs.ruby-lang.org/issues/20337
+    real.should_receive(:inspect).and_return(+"1")
     imaginary = NumericSpecs::Subclass.new
     imaginary.should_receive(:inspect).and_return("2")
     imaginary.should_receive(:<).any_number_of_times.and_return(false)
@@ -26,7 +27,8 @@
 
   it "adds an `*' before the `i' if the last character of the imaginary part is not numeric" do
     real = NumericSpecs::Subclass.new
-    real.should_receive(:inspect).and_return("(1)")
+    # + because of https://bugs.ruby-lang.org/issues/20337
+    real.should_receive(:inspect).and_return(+"(1)")
     imaginary = NumericSpecs::Subclass.new
     imaginary.should_receive(:inspect).and_return("(2)")
     imaginary.should_receive(:<).any_number_of_times.and_return(false)

core/complex/to_s_spec.rb

@@ -45,7 +45,8 @@
 
   it "treats real and imaginary parts as strings" do
     real = NumericSpecs::Subclass.new
-    real.should_receive(:to_s).and_return("1")
+    # + because of https://bugs.ruby-lang.org/issues/20337
+    real.should_receive(:to_s).and_return(+"1")
     imaginary = NumericSpecs::Subclass.new
     imaginary.should_receive(:to_s).and_return("2")
     imaginary.should_receive(:<).any_number_of_times.and_return(false)

core/encoding/converter/convert_spec.rb

@@ -1,4 +1,5 @@
 # -*- encoding: binary -*-
+# frozen_string_literal: true
 require_relative '../../../spec_helper'
 
 describe "Encoding::Converter#convert" do

core/encoding/converter/last_error_spec.rb

@@ -15,39 +15,39 @@
 
   it "returns nil when #primitive_convert last returned :destination_buffer_full" do
     ec = Encoding::Converter.new("utf-8", "iso-2022-jp")
-    ec.primitive_convert("\u{9999}", "", 0, 0, partial_input: false) \
+    ec.primitive_convert(+"\u{9999}", +"", 0, 0, partial_input: false) \
       .should == :destination_buffer_full
     ec.last_error.should be_nil
   end
 
   it "returns nil when #primitive_convert last returned :finished" do
     ec = Encoding::Converter.new("utf-8", "iso-8859-1")
-    ec.primitive_convert("glark".force_encoding('utf-8'),"").should == :finished
+    ec.primitive_convert("glark".force_encoding('utf-8'), +"").should == :finished
     ec.last_error.should be_nil
   end
 
   it "returns nil if the last conversion succeeded but the penultimate failed" do
     ec = Encoding::Converter.new("utf-8", "iso-8859-1")
-    ec.primitive_convert("\xf1abcd","").should == :invalid_byte_sequence
-    ec.primitive_convert("glark".force_encoding('utf-8'),"").should == :finished
+    ec.primitive_convert(+"\xf1abcd", +"").should == :invalid_byte_sequence
+    ec.primitive_convert("glark".force_encoding('utf-8'), +"").should == :finished
     ec.last_error.should be_nil
   end
 
   it "returns an Encoding::InvalidByteSequenceError when #primitive_convert last returned :invalid_byte_sequence" do
     ec = Encoding::Converter.new("utf-8", "iso-8859-1")
-    ec.primitive_convert("\xf1abcd","").should == :invalid_byte_sequence
+    ec.primitive_convert(+"\xf1abcd", +"").should == :invalid_byte_sequence
     ec.last_error.should be_an_instance_of(Encoding::InvalidByteSequenceError)
   end
 
   it "returns an Encoding::UndefinedConversionError when #primitive_convert last returned :undefined_conversion" do
     ec = Encoding::Converter.new("utf-8", "iso-8859-1")
-    ec.primitive_convert("\u{9876}","").should == :undefined_conversion
+    ec.primitive_convert(+"\u{9876}", +"").should == :undefined_conversion
     ec.last_error.should be_an_instance_of(Encoding::UndefinedConversionError)
   end
 
   it "returns an Encoding::InvalidByteSequenceError when #primitive_convert last returned :incomplete_input" do
     ec = Encoding::Converter.new("EUC-JP", "ISO-8859-1")
-    ec.primitive_convert("\xa4", "", nil, 10).should == :incomplete_input
+    ec.primitive_convert(+"\xa4", +"", nil, 10).should == :incomplete_input
     ec.last_error.should be_an_instance_of(Encoding::InvalidByteSequenceError)
   end
 

core/encoding/converter/primitive_convert_spec.rb

@@ -1,4 +1,5 @@
 # -*- encoding: binary -*-
+# frozen_string_literal: false
 require_relative '../../../spec_helper'
 
 describe "Encoding::Converter#primitive_convert" do

core/encoding/converter/primitive_errinfo_spec.rb

@@ -1,4 +1,5 @@
 # -*- encoding: binary -*-
+# frozen_string_literal: false
 require_relative '../../../spec_helper'
 
 describe "Encoding::Converter#primitive_errinfo" do

core/encoding/converter/putback_spec.rb

@@ -4,7 +4,7 @@
 describe "Encoding::Converter#putback" do
   before :each do
     @ec = Encoding::Converter.new("EUC-JP", "ISO-8859-1")
-    @ret = @ec.primitive_convert(@src="abc\xa1def", @dst="", nil, 10)
+    @ret = @ec.primitive_convert(@src=+"abc\xa1def", @dst=+"", nil, 10)
   end
 
   it "returns a String" do
@@ -36,8 +36,8 @@
 
   it "returns the problematic bytes for UTF-16LE" do
     ec = Encoding::Converter.new("utf-16le", "iso-8859-1")
-    src = "\x00\xd8\x61\x00"
-    dst = ""
+    src = +"\x00\xd8\x61\x00"
+    dst = +""
     ec.primitive_convert(src, dst).should == :invalid_byte_sequence
     ec.primitive_errinfo.should == [:invalid_byte_sequence, "UTF-16LE", "UTF-8", "\x00\xD8", "a\x00"]
     ec.putback.should == "a\x00".force_encoding("utf-16le")
@@ -46,8 +46,8 @@
 
   it "accepts an integer argument corresponding to the number of bytes to be put back" do
     ec = Encoding::Converter.new("utf-16le", "iso-8859-1")
-    src = "\x00\xd8\x61\x00"
-    dst = ""
+    src = +"\x00\xd8\x61\x00"
+    dst = +""
     ec.primitive_convert(src, dst).should == :invalid_byte_sequence
     ec.primitive_errinfo.should == [:invalid_byte_sequence, "UTF-16LE", "UTF-8", "\x00\xD8", "a\x00"]
     ec.putback(2).should == "a\x00".force_encoding("utf-16le")

core/encoding/converter/replacement_spec.rb

@@ -46,15 +46,15 @@
   it "raises an UndefinedConversionError is the argument cannot be converted into the destination encoding" do
     ec = Encoding::Converter.new("sjis", "ascii")
     utf8_q = "\u{986}".force_encoding('utf-8')
-    ec.primitive_convert(utf8_q.dup, "").should == :undefined_conversion
+    ec.primitive_convert(utf8_q.dup, +"").should == :undefined_conversion
     -> { ec.replacement = utf8_q }.should \
       raise_error(Encoding::UndefinedConversionError)
   end
 
   it "does not change the replacement character if the argument cannot be converted into the destination encoding" do
     ec = Encoding::Converter.new("sjis", "ascii")
     utf8_q = "\u{986}".force_encoding('utf-8')
-    ec.primitive_convert(utf8_q.dup, "").should == :undefined_conversion
+    ec.primitive_convert(utf8_q.dup, +"").should == :undefined_conversion
     -> { ec.replacement = utf8_q }.should \
       raise_error(Encoding::UndefinedConversionError)
     ec.replacement.should == "?".force_encoding('us-ascii')
@@ -63,8 +63,8 @@
   it "uses the replacement character" do
     ec = Encoding::Converter.new("utf-8", "us-ascii", :invalid => :replace, :undef => :replace)
     ec.replacement = "!"
-    dest = ""
-    status = ec.primitive_convert "中文123", dest
+    dest = +""
+    status = ec.primitive_convert(+"中文123", dest)
 
     status.should == :finished
     dest.should == "!!123"

core/encoding/invalid_byte_sequence_error/incomplete_input_spec.rb

@@ -8,7 +8,7 @@
 
   it "returns true if #primitive_convert returned :incomplete_input for the same data" do
     ec = Encoding::Converter.new("EUC-JP", "ISO-8859-1")
-    ec.primitive_convert("\xA1",'').should == :incomplete_input
+    ec.primitive_convert(+"\xA1", +'').should == :incomplete_input
     begin
       ec.convert("\xA1")
     rescue Encoding::InvalidByteSequenceError => e
@@ -18,7 +18,7 @@
 
   it "returns false if #primitive_convert returned :invalid_byte_sequence for the same data" do
     ec = Encoding::Converter.new("ascii", "utf-8")
-    ec.primitive_convert("\xfffffffff",'').should == :invalid_byte_sequence
+    ec.primitive_convert(+"\xfffffffff", +'').should == :invalid_byte_sequence
     begin
       ec.convert("\xfffffffff")
     rescue Encoding::InvalidByteSequenceError => e

core/file/shared/path.rb

@@ -1,7 +1,7 @@
 describe :file_path, shared: true do
   before :each do
-    @name = "file_to_path"
-    @path = tmp(@name)
+    @path = tmp("file_to_path")
+    @name = File.basename(@path)
     touch @path
   end
 

core/hash/assoc_spec.rb

@@ -22,11 +22,11 @@
   end
 
   it "only returns the first matching key-value pair for identity hashes" do
-    # Avoid literal String keys in Hash#[]= due to https://bugs.ruby-lang.org/issues/12855
+    # Avoid literal String keys since string literals can be frozen and interned e.g. with --enable-frozen-string-literal
     h = {}.compare_by_identity
-    k1 = 'pear'
+    k1 = 'pear'.dup
     h[k1] = :red
-    k2 = 'pear'
+    k2 = 'pear'.dup
     h[k2] = :green
     h.size.should == 2
     h.keys.grep(/pear/).size.should == 2

core/hash/compare_by_identity_spec.rb

@@ -85,19 +85,21 @@ def o.hash; 123; end
     -> { @h.compare_by_identity }.should raise_error(FrozenError)
   end
 
-  # Behaviour confirmed in bug #1871
+  # Behaviour confirmed in https://bugs.ruby-lang.org/issues/1871
   it "persists over #dups" do
-    @idh['foo'] = :bar
-    @idh['foo'] = :glark
+    @idh['foo'.dup] = :bar
+    @idh['foo'.dup] = :glark
     @idh.dup.should == @idh
     @idh.dup.size.should == @idh.size
+    @idh.dup.should.compare_by_identity?
   end
 
   it "persists over #clones" do
-    @idh['foo'] = :bar
-    @idh['foo'] = :glark
+    @idh['foo'.dup] = :bar
+    @idh['foo'.dup] = :glark
     @idh.clone.should == @idh
     @idh.clone.size.should == @idh.size
+    @idh.dup.should.compare_by_identity?
   end
 
   it "does not copy string keys" do
@@ -109,8 +111,11 @@ def o.hash; 123; end
   end
 
   it "gives different identity for string literals" do
+    eval <<~RUBY
+    # frozen_string_literal: false
     @idh['foo'] = 1
     @idh['foo'] = 2
+    RUBY
     @idh.values.should == [1, 2]
     @idh.size.should == 2
   end

core/hash/element_reference_spec.rb

@@ -30,7 +30,7 @@
   end
 
   it "does not create copies of the immediate default value" do
-    str = "foo"
+    str = +"foo"
     h = Hash.new(str)
     a = h[:a]
     b = h[:b]