Merge pull request #7920 from jonas054/595_erb_processing_of_configur…

…ation

[Fix #595] Add ERB pre-processing for configuration files

CHANGELOG.md

@@ -7,6 +7,7 @@
 * [#7895](https://github.com/rubocop-hq/rubocop/pull/7895): Include `.simplecov` file by default. ([@robotdana][])
 * [#7916](https://github.com/rubocop-hq/rubocop/pull/7916): Support autocorrection for `Lint/AmbiguousRegexpLiteral`. ([@koic][])
 * [#7917](https://github.com/rubocop-hq/rubocop/pull/7917): Support autocorrection for `Lint/UselessAccessModifier`. ([@koic][])
+* [#595](https://github.com/rubocop-hq/rubocop/issues/595): Add ERB pre-processing for configuration files. ([@jonas054][])
 
 ### Bug fixes
 

lib/rubocop/config_loader.rb

@@ -217,7 +217,10 @@ def resolver
       end
 
       def load_yaml_configuration(absolute_path)
-        yaml_code = read_file(absolute_path)
+        file_contents = read_file(absolute_path)
+        yaml_code = Dir.chdir(File.dirname(absolute_path)) do
+          ERB.new(file_contents).result
+        end
         check_duplication(yaml_code, absolute_path)
         hash = yaml_safe_load(yaml_code, absolute_path) || {}
 
@@ -241,8 +244,7 @@ def check_duplication(yaml_code, absolute_path)
                       "#{smart_path}:#{line1}: " \
                       "`#{value}` is concealed by line #{line2}"
                     else
-                      "#{smart_path}: " \
-                        "`#{value}` is concealed by duplicate"
+                      "#{smart_path}: `#{value}` is concealed by duplicate"
                     end
           warn Rainbow(message).yellow
         end
@@ -263,13 +265,11 @@ def yaml_safe_load(yaml_code, filename)
           SafeYAML.load(yaml_code, filename, whitelisted_tags: %w[!ruby/regexp])
         # Ruby 2.6+
         elsif Gem::Version.new(Psych::VERSION) >= Gem::Version.new('3.1.0')
-          YAML.safe_load(
-            yaml_code,
-            permitted_classes: [Regexp, Symbol],
-            permitted_symbols: [],
-            aliases: true,
-            filename: filename
-          )
+          YAML.safe_load(yaml_code,
+                         permitted_classes: [Regexp, Symbol],
+                         permitted_symbols: [],
+                         aliases: true,
+                         filename: filename)
         else
           YAML.safe_load(yaml_code, [Regexp, Symbol], [], true, filename)
         end

manual/configuration.md

@@ -237,6 +237,21 @@ Style/For:
 In this example the `Exclude` would only include `bar.rb`.
 
 
+## Pre-processing
+
+Configuration files are pre-processed using the ERB templating mechanism. This
+makes it possible to add dynamic content that will be evaluated when the
+configuation file is read. For example, you could let RuboCop ignore all files
+ignored by Git.
+
+```yaml
+AllCops:
+  Exclude:
+  <% `git status --ignored --porcelain`.lines.grep(/^!! /).each do |path| %>
+    - <%= path.sub(/^!! /, '') %>
+  <% end %>
+```
+
 ## Defaults
 
 The file [config/default.yml][1] under the RuboCop home directory contains the

spec/rubocop/config_loader_spec.rb

@@ -1007,6 +1007,36 @@ def cop_enabled?(cop_class)
       )
     end
 
+    it 'does ERB pre-processing of the configuration file' do
+      %w[a.rb b.rb].each { |file| create_file(file, 'puts 1') }
+      create_file(configuration_path, <<~YAML)
+        Style/Encoding:
+          Enabled: <%= 1 == 1 %>
+          Exclude:
+          <% Dir['*.rb'].sort.each do |name| %>
+            - <%= name %>
+          <% end %>
+      YAML
+      configuration = load_file
+      expect(configuration['Style/Encoding'])
+        .to eq('Enabled' => true,
+               'Exclude' => [abs('a.rb'), abs('b.rb')])
+    end
+
+    it 'does ERB pre-processing of a configuration file in a subdirectory' do
+      create_file('dir/c.rb', 'puts 1')
+      create_file('dir/.rubocop.yml', <<~YAML)
+        Style/Encoding:
+          Exclude:
+          <% Dir['*.rb'].each do |name| %>
+            - <%= name %>
+          <% end %>
+      YAML
+      configuration = described_class.load_file('dir/.rubocop.yml')
+      expect(configuration['Style/Encoding'])
+        .to eq('Exclude' => [abs('dir/c.rb')])
+    end
+
     it 'fails with a TypeError when loading a malformed configuration file' do
       create_file(configuration_path, 'This string is not a YAML hash')
       expect { load_file }.to raise_error(