Warn users about errors in loading RC files (#817)

1. Because `IRB.rc_file` always generates an rc file name, even if the
   file doesn't exist, we should check the file exists before trying to
   load it.
2. If any type of errors occur while loading the rc file, we should
   warn the user about it.

lib/irb/init.rb

@@ -389,18 +389,16 @@ def IRB.parse_opts(argv: ::ARGV)
     $LOAD_PATH.unshift(*load_path)
   end
 
-  # running config
+  # Run the config file
   def IRB.run_config
     if @CONF[:RC]
       begin
-        load rc_file
-      rescue LoadError, Errno::ENOENT
-      rescue # StandardError, ScriptError
-        print "load error: #{rc_file}\n"
-        print $!.class, ": ", $!, "\n"
-        for err in $@[0, $@.size - 2]
-          print "\t", err, "\n"
-        end
+        file = rc_file
+        # Because rc_file always returns `HOME/.irbrc` even if no rc file is present, we can't warn users about missing rc files.
+        # Otherwise, it'd be very noisy.
+        load file if File.exist?(file)
+      rescue StandardError, ScriptError => e
+        warn "Error loading RC file '#{file}':\n#{e.full_message(highlight: false)}"
       end
     end
   end
@@ -418,7 +416,7 @@ def IRB.rc_file(ext = IRBRC_EXT)
     end
     case rc_file = @CONF[:RC_NAME_GENERATOR].call(ext)
     when String
-      return rc_file
+      rc_file
     else
       fail IllegalRCNameGenerator
     end

test/irb/test_init.rb

@@ -218,4 +218,44 @@ def with_argv(argv)
       ARGV.replace(orig)
     end
   end
+
+  class InitIntegrationTest < IntegrationTestCase
+    def test_load_error_in_rc_file_is_warned
+      write_rc <<~'IRBRC'
+        require "file_that_does_not_exist"
+      IRBRC
+
+      write_ruby <<~'RUBY'
+        binding.irb
+      RUBY
+
+      output = run_ruby_file do
+        type "'foobar'"
+        type "exit"
+      end
+
+      # IRB session should still be started
+      assert_includes output, "foobar"
+      assert_includes output, 'cannot load such file -- file_that_does_not_exist (LoadError)'
+    end
+
+    def test_normal_errors_in_rc_file_is_warned
+      write_rc <<~'IRBRC'
+        raise "I'm an error"
+      IRBRC
+
+      write_ruby <<~'RUBY'
+        binding.irb
+      RUBY
+
+      output = run_ruby_file do
+        type "'foobar'"
+        type "exit"
+      end
+
+      # IRB session should still be started
+      assert_includes output, "foobar"
+      assert_includes output, 'I\'m an error (RuntimeError)'
+    end
+  end
 end