Suppress error highlight for some incomplete code (#1173)

It is better to not render `class` with RED-REVERSE color.
Filter specific errors if error location is at the last non-newline non-comment token position.

Author: tompng

lib/irb/color.rb

@@ -168,12 +168,13 @@ def colorize_code(code, complete: true, ignore_error: false, colorable: colorabl
         # IRB::ColorPrinter skips colorizing syntax invalid fragments
         return Reline::Unicode.escape_for_print(code) if ignore_error && !result.success?
 
+        prism_node, prism_tokens = result.value
         errors = result.errors
+
         unless complete
-          errors = errors.reject { |error| error.message =~ /\Aexpected a|unexpected end-of-input|unterminated/ }
+          errors = filter_incomplete_code_errors(errors, prism_tokens)
         end
 
-        prism_node, prism_tokens = result.value
         visitor = ColorizeVisitor.new
         prism_node.accept(visitor)
 
@@ -282,6 +283,29 @@ def visit_symbol_node(node)
 
       private
 
+      FILTERED_ERROR_TYPES = [
+        :class_name, :module_name, # `class`, `class owner_module`
+        :write_target_unexpected, # `a, b`
+        :parameter_wild_loose_comma, # `def f(a,`
+        :argument_no_forwarding_star, # `[*`
+        :argument_no_forwarding_star_star, # `f(**`
+        :argument_no_forwarding_ampersand, # `f(&`
+        :def_endless, # `def f =`
+        :embdoc_term, # `=begin`
+      ]
+
+      # Filter out syntax errors that are likely to be caused by incomplete code, to avoid showing misleading error highlights to users.
+      def filter_incomplete_code_errors(errors, tokens)
+        last_non_comment_space_token, = tokens.reverse_each.find do |t,|
+          t.type != :COMMENT && t.type != :EOF && t.type != :IGNORED_NEWLINE && t.type != :NEWLINE
+        end
+        last_offset = last_non_comment_space_token ? last_non_comment_space_token.location.end_offset : 0
+        errors.reject do |error|
+          error.message.match?(/\Aexpected a|unexpected end-of-input|unterminated/) ||
+          (error.location.end_offset == last_offset && FILTERED_ERROR_TYPES.include?(error.type))
+        end
+      end
+
       def without_circular_ref(obj, seen:, &block)
         return false if seen.key?(obj)
         seen[obj] = true

test/irb/test_color.rb

@@ -168,6 +168,29 @@ def test_colorize_code_complete_false
       {
         "'foo' + 'bar" => "#{RED}#{BOLD}'#{CLEAR}#{RED}foo#{CLEAR}#{RED}#{BOLD}'#{CLEAR} + #{RED}#{BOLD}'#{CLEAR}#{RED}bar#{CLEAR}",
         "('foo" => "(#{RED}#{BOLD}'#{CLEAR}#{RED}foo#{CLEAR}",
+        "if true" => "#{GREEN}if#{CLEAR} #{CYAN}#{BOLD}true#{CLEAR}",
+        "tap do end end tap do" => "tap #{GREEN}do#{CLEAR} #{GREEN}end#{CLEAR} #{RED}#{REVERSE}end#{CLEAR} tap #{GREEN}do#{CLEAR}",
+
+        # Specially handled cases
+        "class" => "#{GREEN}class#{CLEAR}",
+        "class#" => "#{GREEN}class#{CLEAR}#{BLUE}#{BOLD}\##{CLEAR}",
+        "class;" => "#{RED}#{REVERSE}class#{CLEAR};",
+        "module" => "#{GREEN}module#{CLEAR}",
+        "module#" => "#{GREEN}module#{CLEAR}#{BLUE}#{BOLD}\##{CLEAR}",
+        "module;" => "#{RED}#{REVERSE}module#{CLEAR};",
+        "class owner_module" => "#{GREEN}class#{CLEAR} owner_module",
+        "module owner_module" => "#{GREEN}module#{CLEAR} owner_module",
+        "a, b" => "a, b",
+        "a, b#" => "a, b#{BLUE}#{BOLD}\##{CLEAR}",
+        "a, b;" => "#{RED}#{REVERSE}a, b#{CLEAR};",
+        "def f(a,#" => "#{GREEN}def#{CLEAR} #{BLUE}#{BOLD}f#{CLEAR}(a,#{BLUE}#{BOLD}\##{CLEAR}",
+        "def f(a,)" => "#{GREEN}def#{CLEAR} #{BLUE}#{BOLD}f#{CLEAR}(a#{RED}#{REVERSE},#{CLEAR})",
+        "[*" => "[*",
+        "f(*" => "f(*",
+        "f(**" => "f(**",
+        "f(&" => "f(&",
+        "def f =" => "#{GREEN}def#{CLEAR} #{BLUE}#{BOLD}f#{CLEAR} =",
+        "=begin" => "#{BLUE}#{BOLD}=begin#{CLEAR}",
       }.each do |code, result|
         assert_equal_with_term(result, code, complete: false)