Escape special symbols and newlines in messages. (#9164)

(cherry picked from commit df0800e)

doc/whatsnew/fragments/7874.bugfix

@@ -0,0 +1,3 @@
+Escape special symbols and newlines in messages.
+
+Closes #7874

pylint/extensions/magic_value.py

@@ -84,9 +84,9 @@ def _check_constants_comparison(self, node: nodes.Compare) -> None:
 
         operand_value = None
         if const_operands[LEFT_OPERAND] and self._is_magic_value(left_operand):
-            operand_value = left_operand.value
+            operand_value = left_operand.as_string()
         elif const_operands[RIGHT_OPERAND] and self._is_magic_value(right_operand):
-            operand_value = right_operand.value
+            operand_value = right_operand.as_string()
         if operand_value is not None:
             self.add_message(
                 "magic-value-comparison",

tests/functional/ext/magic_value_comparison/magic_value_comparison.py

@@ -34,3 +34,5 @@ class Christmas(Enum):
 
 shouldnt_raise = var == '\n'
 shouldnt_raise = var == '\\b'
+
+should_escape = var == "foo\nbar"  # [magic-value-comparison]

tests/functional/ext/magic_value_comparison/magic_value_comparison.txt

@@ -5,3 +5,4 @@ comparison-of-constants:24:17:24:22::"Comparison between constants: '5 > 7' has
 singleton-comparison:29:17:29:28::Comparison 'var == True' should be 'var is True' if checking for the singleton value True, or 'bool(var)' if testing for truthiness:UNDEFINED
 singleton-comparison:30:17:30:29::Comparison 'var == False' should be 'var is False' if checking for the singleton value False, or 'not var' if testing for falsiness:UNDEFINED
 singleton-comparison:31:17:31:28::Comparison 'var == None' should be 'var is None':UNDEFINED
+magic-value-comparison:38:16:38:33::Consider using a named constant or an enum instead of ''foo\nbar''.:HIGH