Present the actual error message from raise_error

Display the actual error message of the raised error when a raise_error
asserton fails.  The failure message will now include the error message
on a separate line followed by the backtrace.  If there is no error
message, only the backtrace will be shown.

lib/rspec/matchers/built_in/raise_error.rb

@@ -238,10 +238,10 @@ def given_error
           return " but nothing was raised" unless @actual_error
 
           backtrace = format_backtrace(@actual_error.backtrace)
-          [
-            ", got #{description_of(@actual_error)} with backtrace:",
-            *backtrace
-          ].join("\n  # ")
+
+          ", got #{description_of(@actual_error)}" +
+            error_message_text +
+            text_with_indent(" with backtrace:", backtrace)
         end
 
         def expecting_specific_exception?
@@ -257,6 +257,25 @@ def warning
           warning = "Actual error raised was #{description_of(@actual_error)}. "
           warning if @actual_error
         end
+
+        private
+
+        def error_message_text
+          return '' if @actual_error.message.nil? || @actual_error.message.empty?
+          return '' if @actual_error.message == @actual_error.class.to_s
+
+          text_with_indent(" with message:", @actual_error.message.split(/\n/)) +
+            "\nand"
+        end
+
+        INDENT = ["\n  # ".freeze].freeze
+
+        def text_with_indent(prefix, text_lines)
+          text_lines = Array(text_lines)
+          message_text = [prefix]
+          message_text += ([INDENT] * text_lines.size).zip(text_lines)
+          message_text.join
+        end
       end
       # rubocop:enable RescueException
       # rubocop:enable ClassLength

spec/rspec/matchers/built_in/raise_error_spec.rb

@@ -105,11 +105,35 @@ def to_s
   end
 
   it "fails if a different error instance is thrown from the one that is expected" do
-    s = StandardError.new("Error 1")
+    error = StandardError.new("Error 1")
     to_raise = StandardError.new("Error 2")
     expect do
-      expect { raise to_raise }.to raise_error(s)
-    end.to fail_with(Regexp.new("expected #{s.inspect}, got #{to_raise.inspect} with backtrace"))
+      expect { raise to_raise }.to raise_error(error)
+    end.to fail_with(Regexp.new("expected #{error.inspect}, got #{to_raise.inspect} with message:\n  # Error 2\nand with backtrace:"))
+  end
+
+  it "fails without a message if a different error instance is thrown from the one that is expected" do
+    error = StandardError.new("Error 1")
+    to_raise = StandardError.new
+    expect do
+      expect { raise to_raise }.to raise_error(error)
+    end.to fail_with(Regexp.new("expected #{error.inspect}, got #{to_raise.inspect} with backtrace:"))
+  end
+
+  it "fails without a message if a generic runtime error is thrown instead of the one that is expected" do
+    error = RuntimeError.new("Error 1")
+    to_raise = RuntimeError
+    expect do
+      expect { raise }.to raise_error(error)
+    end.to fail_with(Regexp.new("expected #{error.inspect}, got #{to_raise.inspect} with backtrace:"))
+  end
+
+  it "fails without a message if an error with an empty message is thrown instead of the one that is expected" do
+    error = StandardError.new("Error 1")
+    to_raise = RuntimeError.new("")
+    expect do
+      expect { raise to_raise }.to raise_error(error)
+    end.to fail_with(Regexp.new("expected #{error.inspect}, got #{to_raise.inspect} with backtrace:"))
   end
 
   it "passes if an error class is expected and an instance of that class is thrown" do
@@ -242,6 +266,12 @@ def to_s
     end.to fail_with(/expected Exception with \"blah\", got #<RuntimeError: blarg>/)
   end
 
+  it "fails if RuntimeError error is raised without a matching message" do
+    expect do
+      expect { raise "blarg\nblarg\nblam" }.to raise_error(/blah/)
+    end.to fail_with(/expected \/blah\/, got #<RuntimeError: blarg\nblarg\nblam>/)
+  end
+
   it "fails if any other error is raised with the wrong message" do
     expect do
       expect { raise NameError.new('blarg') }.to raise_error('blah')
@@ -283,6 +313,12 @@ def to_s
     end.to fail_with(/expected Exception with \"blah\", got #<RuntimeError: blarg>/)
   end
 
+  it "fails if RuntimeError error is raised without a matching message" do
+    expect do
+      expect { raise "blarg\nblarg" }.to raise_error.with_message(/blah/)
+    end.to fail_with(/expected Exception with message matching \/blah\/, got #<RuntimeError: blarg\nblarg> with message:\n  # blarg\n  # blarg/)
+  end
+
   it "fails if any other error is raised with the wrong message" do
     expect do
       expect { raise NameError.new('blarg') }.to raise_error.with_message('blah')