Fix implicit node coloring (#1215)

`{ lvar:, meth:, CONST: }` was unintentionally colored as "lvar:"(magenta), "meth"(cyna) + ":"(no-color), "CONST:"(magenta).
All three labels should be colored as magenta.

Author: tompng

lib/irb/color.rb

@@ -258,6 +258,11 @@ def visit_alias_method_node(node)
           super
         end
 
+        def visit_implicit_node(_node)
+          # Label of implicit nodes are colored as LABEL in visit_symbol_node.
+          # We need to prevent value part from being colored with another type.
+        end
+
         def visit_call_node(node)
           if node.call_operator_loc.nil? && OPERATORS.include?(node.name)
             # Operators should not be colored as method call

test/irb/test_color.rb

@@ -137,6 +137,7 @@ def test_colorize_code
         'ENV' => "#{BLUE}#{BOLD}#{UNDERLINE}ENV#{CLEAR}",
         'f do end' => "#{CYAN}f#{CLEAR} #{GREEN}do#{CLEAR} #{GREEN}end#{CLEAR}",
         'f true do end' => "#{CYAN}f#{CLEAR} #{CYAN}#{BOLD}true#{CLEAR} #{GREEN}do#{CLEAR} #{GREEN}end#{CLEAR}",
+        '{"foo": 1, bar:, BAZ:}' => "{#{MAGENTA}\"foo\":#{CLEAR} #{BLUE}#{BOLD}1#{CLEAR}, #{MAGENTA}bar:#{CLEAR}, #{MAGENTA}BAZ:#{CLEAR}}",
       }
 
       tests.each do |code, result|