diff --git a/CHANGELOG.md b/CHANGELOG.md
index 425e5d131c8..6621b6cb810 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -50,6 +50,7 @@
 * [#8276](https://github.com/rubocop-hq/rubocop/issues/8276): Cop `Metrics/CyclomaticComplexity` not longer counts `&.` when repeated on the same variable. ([@marcandre][])
 * [#8204](https://github.com/rubocop-hq/rubocop/pull/8204): **(Breaking)** Cop `Metrics/PerceivedComplexity` now counts `else` in `case` statements, `&.`, `||=`, `&&=` and blocks known to iterate. Default bumped from 7 to 8. Consider using `rubocop -a --disable-uncorrectable` to ease transition. ([@marcandre][])
 * [#8416](https://github.com/rubocop-hq/rubocop/issues/8416): Cop `Lint/InterpolationCheck` marked as unsafe. ([@marcandre][])
+* [#8442](https://github.com/rubocop-hq/rubocop/pull/8442): Remove `RuboCop::Cop::ParserDiagnostic` mixin module. ([@koic][])
 
 ## 0.88.0 (2020-07-13)
 
diff --git a/lib/rubocop.rb b/lib/rubocop.rb
index eebfd2e36c1..aa05076f8f3 100644
--- a/lib/rubocop.rb
+++ b/lib/rubocop.rb
@@ -95,7 +95,6 @@
 require_relative 'rubocop/cop/mixin/on_normal_if_unless'
 require_relative 'rubocop/cop/mixin/ordered_gem_node'
 require_relative 'rubocop/cop/mixin/parentheses'
-require_relative 'rubocop/cop/mixin/parser_diagnostic'
 require_relative 'rubocop/cop/mixin/percent_array'
 require_relative 'rubocop/cop/mixin/percent_literal'
 require_relative 'rubocop/cop/mixin/preceding_following_alignment'
diff --git a/lib/rubocop/cop/lint/ambiguous_operator.rb b/lib/rubocop/cop/lint/ambiguous_operator.rb
index 50e2c55b112..2df1285a861 100644
--- a/lib/rubocop/cop/lint/ambiguous_operator.rb
+++ b/lib/rubocop/cop/lint/ambiguous_operator.rb
@@ -21,7 +21,6 @@ module Lint
       #   # With parentheses, there's no ambiguity.
       #   do_something(*some_array)
       class AmbiguousOperator < Base
-        include ParserDiagnostic
         extend AutoCorrector
 
         AMBIGUITIES = {
@@ -39,16 +38,23 @@ class AmbiguousOperator < Base
                      'a whitespace to the right of the `%<operator>s` if it ' \
                      'should be a %<possible>s.'
 
-        private
+        def on_new_investigation
+          processed_source.diagnostics.each do |diagnostic|
+            next unless diagnostic.reason == :ambiguous_prefix
 
-        def autocorrect(corrector, node)
-          add_parentheses(node, corrector)
-        end
+            offense_node = find_offense_node_by(diagnostic)
+            message = message(diagnostic)
 
-        def relevant_diagnostic?(diagnostic)
-          diagnostic.reason == :ambiguous_prefix
+            add_offense(
+              diagnostic.location, message: message, severity: diagnostic.level
+            ) do |corrector|
+              add_parentheses(offense_node, corrector)
+            end
+          end
         end
 
+        private
+
         def find_offense_node_by(diagnostic)
           ast = processed_source.ast
           ast.each_node(:splat, :block_pass, :kwsplat) do |node|
@@ -67,7 +73,7 @@ def find_offense_node_by(diagnostic)
           end
         end
 
-        def alternative_message(diagnostic)
+        def message(diagnostic)
           operator = diagnostic.location.source
           hash = AMBIGUITIES[operator]
           format(MSG_FORMAT, hash)
diff --git a/lib/rubocop/cop/lint/ambiguous_regexp_literal.rb b/lib/rubocop/cop/lint/ambiguous_regexp_literal.rb
index e9786768455..87a70ff284a 100644
--- a/lib/rubocop/cop/lint/ambiguous_regexp_literal.rb
+++ b/lib/rubocop/cop/lint/ambiguous_regexp_literal.rb
@@ -22,23 +22,26 @@ module Lint
       #   # With parentheses, there's no ambiguity.
       #   do_something(/pattern/i)
       class AmbiguousRegexpLiteral < Base
-        include ParserDiagnostic
         extend AutoCorrector
 
         MSG = 'Ambiguous regexp literal. Parenthesize the method arguments ' \
               "if it's surely a regexp literal, or add a whitespace to the " \
               'right of the `/` if it should be a division.'
 
-        private
+        def on_new_investigation
+          processed_source.diagnostics.each do |diagnostic|
+            next unless diagnostic.reason == :ambiguous_literal
 
-        def autocorrect(corrector, node)
-          add_parentheses(node, corrector)
-        end
+            offense_node = find_offense_node_by(diagnostic)
 
-        def relevant_diagnostic?(diagnostic)
-          diagnostic.reason == :ambiguous_literal
+            add_offense(diagnostic.location, severity: diagnostic.level) do |corrector|
+              add_parentheses(offense_node, corrector)
+            end
+          end
         end
 
+        private
+
         def find_offense_node_by(diagnostic)
           node = processed_source.ast.each_node(:regexp).find do |regexp_node|
             regexp_node.source_range.begin_pos == diagnostic.location.begin_pos
@@ -46,10 +49,6 @@ def find_offense_node_by(diagnostic)
 
           node.parent
         end
-
-        def alternative_message(_diagnostic)
-          MSG
-        end
       end
     end
   end
diff --git a/lib/rubocop/cop/lint/useless_else_without_rescue.rb b/lib/rubocop/cop/lint/useless_else_without_rescue.rb
index 068bd5bad26..222b351ede9 100644
--- a/lib/rubocop/cop/lint/useless_else_without_rescue.rb
+++ b/lib/rubocop/cop/lint/useless_else_without_rescue.rb
@@ -30,23 +30,14 @@ module Lint
       #     do_something_else
       #   end
       class UselessElseWithoutRescue < Base
-        include ParserDiagnostic
-
         MSG = '`else` without `rescue` is useless.'
 
-        private
-
-        def relevant_diagnostic?(diagnostic)
-          diagnostic.reason == :useless_else
-        end
-
-        def find_offense_node_by(diagnostic)
-          # TODO: When implementing auto-correction, this method should return
-          # an offense node passed as first argument of `add_offense` method.
-        end
+        def on_new_investigation
+          processed_source.diagnostics.each do |diagnostic|
+            next unless diagnostic.reason == :useless_else
 
-        def alternative_message(_diagnostic)
-          MSG
+            add_offense(diagnostic.location, severity: diagnostic.level)
+          end
         end
       end
     end
diff --git a/lib/rubocop/cop/mixin/parser_diagnostic.rb b/lib/rubocop/cop/mixin/parser_diagnostic.rb
deleted file mode 100644
index 430c6eac0fd..00000000000
--- a/lib/rubocop/cop/mixin/parser_diagnostic.rb
+++ /dev/null
@@ -1,42 +0,0 @@
-# frozen_string_literal: true
-
-module RuboCop
-  module Cop
-    # Common functionality for cops which processes Parser's diagnostics.
-    # This mixin requires its user class to define `#relevant_diagnostic?`.
-    #
-    #     def relevant_diagnostic?(diagnostic)
-    #       diagnostic.reason == :my_interested_diagnostic_type
-    #     end
-    #
-    # If you want to use an alternative offense message rather than the one in
-    # Parser's diagnostic, define `#alternative_message`.
-    #
-    #     def alternative_message(diagnostic)
-    #       'My custom message'
-    #     end
-    module ParserDiagnostic
-      def on_new_investigation
-        processed_source.diagnostics.each do |d|
-          next unless relevant_diagnostic?(d)
-
-          message = if respond_to?(:alternative_message, true)
-                      alternative_message(d)
-                    else
-                      d.message.capitalize
-                    end
-          offense_node = find_offense_node_by(d)
-
-          add_offense(d.location, message: message, severity: d.level) do |corrector|
-            autocorrect(corrector, offense_node)
-          end
-        end
-      end
-
-      private
-
-      # If a mixed-in subclass has auto-correction, implement it in the mixed-in subclass.
-      def autocorrect(corrector, node); end
-    end
-  end
-end