`start_with` and `end_with` matcher failure output does not include a diff on multi-line strings

Subject of the issue

RSpec generally has great failure output, but when using start_with or end_with on a multi-line string, the output is quite hard to read. RSpec is totally capable of better output here, though; simply by changing expect(multline_str).to start_with(expected) to expect(multiline.lines.first(n).join).to eq(expected) the output becomes much more readable because it includes a diff.

Similarly, expect(multiline_str).to end_with(expected) is quite hard to read, but expect(multiline_str.lines.last(n).join).to eq(expected) is much easier to read because it includes a diff.

Can start_with and end_with be improved to automatically diff the first or last n lines if they are dealing with multiline strings?

Your environment

• Ruby version: 3.2.2
• rspec-expectations version: 3.12.0

Steps to reproduce

Put this in tmp/start_and_end_with_spec.rb:

module MyApp
  module SomeModule
    RSpec.describe "RSpec matchers" do
      describe "start_with" do
        it "fails on a multi-line string in a way that's hard to read" do
          expect(::File.read(__FILE__)).to start_with(<<~EOS)
            module MyApp
              module SomeModule
                RSpec.describe "RSpec matchers" do
                  describe "start_with", :a_change do
                    it "fails on a multi-line string in a way that's hard to read" do
                      expect(::File.read(__FILE__)).to start_with(<<~EOS)
          EOS
        end
      end

      describe "end_with" do
        it "fails on a multi-line string in a way that's hard to read" do
          expect(::File.read(__FILE__)).to end_with(<<~EOS)
                    end
                  end
                end # a change
              end
            end
          EOS
        end
      end

      describe "eq", :improved do
        it "can be used in place of `start_with` for more readable failure output" do
          expect(::File.read(__FILE__).lines.first(6).join).to eq(<<~EOS)
            module MyApp
              module SomeModule
                RSpec.describe "RSpec matchers" do
                  describe "start_with", :a_change do
                    it "fails on a multi-line string in a way that's hard to read" do
                      expect(::File.read(__FILE__)).to start_with(<<~EOS)
          EOS
        end

        it "can be used in place of `end_with` for more readable failure output" do
          expect(::File.read(__FILE__).lines.last(5).join).to eq(<<~EOS)
                    end
                  end
                end # a change
              end
            end
          EOS
        end
      end
    end
  end
end

Run it to reproduce. Run rspec tmp/start_and_end_with_spec.rb --tag "~improved" to see the current output for start_with and end_with and rspec tmp/start_and_end_with_spec.rb --tag improved for the improved output I'd like to see from these matchers instead.

Expected behavior

rspec tmp/start_and_end_with_spec.rb --tag improve produces output close to what I'd expect:

Run options: include {:improved=>true}
FF

Failures:

  1) RSpec matchers eq can be used in place of `start_with` for more readable failure output
     Failure/Error:
                 expect(::File.read(__FILE__).lines.first(6).join).to eq(<<~EOS)
                   module MyApp
                     module SomeModule
                       RSpec.describe "RSpec matchers" do
                         describe "start_with", :a_change do
                           it "fails on a multi-line string in a way that's hard to read" do
                             expect(::File.read(__FILE__)).to start_with(<<~EOS)
                 EOS

       expected: "module MyApp\n  module SomeModule\n    RSpec.describe \"RSpec matchers\" do\n      describe \"start_... in a way that's hard to read\" do\n          expect(::File.read(__FILE__)).to start_with(<<~EOS)\n"
            got: "module MyApp\n  module SomeModule\n    RSpec.describe \"RSpec matchers\" do\n      describe \"start_... in a way that's hard to read\" do\n          expect(::File.read(__FILE__)).to start_with(<<~EOS)\n"

       (compared using ==)

       Diff:
       @@ -1,7 +1,7 @@
        module MyApp
          module SomeModule
            RSpec.describe "RSpec matchers" do
       -      describe "start_with", :a_change do
       +      describe "start_with" do
                it "fails on a multi-line string in a way that's hard to read" do
                  expect(::File.read(__FILE__)).to start_with(<<~EOS)

     # ./tmp/start_and_end_with_spec.rb:31:in `block (3 levels) in <module:SomeModule>'

  2) RSpec matchers eq can be used in place of `end_with` for more readable failure output
     Failure/Error:
                 expect(::File.read(__FILE__).lines.last(5).join).to eq(<<~EOS)
                           end
                         end
                       end # a change
                     end
                   end
                 EOS

       expected: "        end\n      end\n    end # a change\n  end\nend\n"
            got: "        end\n      end\n    end\n  end\nend\n"

       (compared using ==)

       Diff:
       @@ -1,6 +1,6 @@
                end
              end
       -    end # a change
       +    end
          end
        end

     # ./tmp/start_and_end_with_spec.rb:42:in `block (3 levels) in <module:SomeModule>'

Finished in 0.01014 seconds (files took 0.0588 seconds to load)
2 examples, 2 failures

Failed examples:

rspec ./tmp/start_and_end_with_spec.rb:30 # RSpec matchers eq can be used in place of `start_with` for more readable failure output
rspec ./tmp/start_and_end_with_spec.rb:41 # RSpec matchers eq can be used in place of `end_with` for more readable failure output

Actual behavior

rspec tmp/start_and_end_with_spec.rb --tag "~improve" produces the output I find hard to read:

Run options: exclude {:improved=>true}
FF

Failures:

  1) RSpec matchers start_with fails on a multi-line string in a way that's hard to read
     Failure/Error:
                 expect(::File.read(__FILE__)).to start_with(<<~EOS)
                   module MyApp
                     module SomeModule
                       RSpec.describe "RSpec matchers" do
                         describe "start_with", :a_change do
                           it "fails on a multi-line string in a way that's hard to read" do
                             expect(::File.read(__FILE__)).to start_with(<<~EOS)
                 EOS

       expected "module MyApp\n  module SomeModule\n    RSpec.describe \"RSpec matchers\" do\n      describe \"start_...e\n              end\n            end\n          EOS\n        end\n      end\n    end\n  end\nend\n" to start with "module MyApp\n  module SomeModule\n    RSpec.describe \"RSpec matchers\" do\n      describe \"start_... in a way that's hard to read\" do\n          expect(::File.read(__FILE__)).to start_with(<<~EOS)\n"
     # ./tmp/start_and_end_with_spec.rb:6:in `block (3 levels) in <module:SomeModule>'

  2) RSpec matchers end_with fails on a multi-line string in a way that's hard to read
     Failure/Error:
                 expect(::File.read(__FILE__)).to end_with(<<~EOS)
                           end
                         end
                       end # a change
                     end
                   end
                 EOS

       expected "module MyApp\n  module SomeModule\n    RSpec.describe \"RSpec matchers\" do\n      describe \"start_...e\n              end\n            end\n          EOS\n        end\n      end\n    end\n  end\nend\n" to end with "        end\n      end\n    end # a change\n  end\nend\n"
     # ./tmp/start_and_end_with_spec.rb:19:in `block (3 levels) in <module:SomeModule>'

Finished in 0.01098 seconds (files took 0.05936 seconds to load)
2 examples, 2 failures

Failed examples:

rspec ./tmp/start_and_end_with_spec.rb:5 # RSpec matchers start_with fails on a multi-line string in a way that's hard to read
rspec ./tmp/start_and_end_with_spec.rb:18 # RSpec matchers end_with fails on a multi-line string in a way that's hard to read

Start with the reproducer in tmp/start_and_end_with_spec.rb and run the two commands provided for the current and improved output. Trace the start_with and end_with matcher failure paths, then add coverage showing multiline failures include a useful diff while preserving existing behavior. Done means the matcher failures resemble the expected diff output.