Skip LineLength phase on --auto-gen-only-exclude

Phase 1 of config auto-generation runs the `Layout/LineLength` cop on
all files to discover the `Max` line length, so it can be used by cops
which depend on it.

The `--auto-gen-only-exclude` config skips generating any `Max` configs,
meaning phase 1 is redundant, as we won't generate the `Max` for use by
phase 2.

Therefore, we can skip phase 1 entirely.

changelog/change_skip_line_length_phase_on_auto_gen_only_exclude.md

@@ -0,0 +1 @@
+* [#12730](https://github.com/rubocop/rubocop/pull/12730): Skip `LineLength` phase on `--auto-gen-only-exclude`. ([@sambostock][])

lib/rubocop/cli/command/auto_generate_config.rb

@@ -17,7 +17,10 @@ class AutoGenerateConfig < Base
 
         PHASE_1_OVERRIDDEN = '(skipped because the default Layout/LineLength:Max is overridden)'
         PHASE_1_DISABLED = '(skipped because Layout/LineLength is disabled)'
-        PHASE_1_SKIPPED = '(skipped because a list of cops is passed to the `--only` flag)'
+        PHASE_1_SKIPPED_ONLY_COPS =
+          '(skipped because a list of cops is passed to the `--only` flag)'
+        PHASE_1_SKIPPED_ONLY_EXCLUDE =
+          '(skipped because only excludes will be generated due to `--auto-gen-only-exclude` flag)'
 
         def run
           add_formatter
@@ -29,12 +32,14 @@ def run
         private
 
         def maybe_run_line_length_cop
-          if !line_length_enabled?(@config_store.for_pwd)
+          if only_exclude?
+            skip_line_length_cop(PHASE_1_SKIPPED_ONLY_EXCLUDE)
+          elsif !line_length_enabled?(@config_store.for_pwd)
             skip_line_length_cop(PHASE_1_DISABLED)
           elsif !same_max_line_length?(@config_store.for_pwd, ConfigLoader.default_configuration)
             skip_line_length_cop(PHASE_1_OVERRIDDEN)
           elsif options_has_only_flag?
-            skip_line_length_cop(PHASE_1_SKIPPED)
+            skip_line_length_cop(PHASE_1_SKIPPED_ONLY_COPS)
           else
             run_line_length_cop
           end
@@ -65,6 +70,10 @@ def options_has_only_flag?
           @options[:only]
         end
 
+        def only_exclude?
+          @options[:auto_gen_only_exclude]
+        end
+
         # Do an initial run with only Layout/LineLength so that cops that
         # depend on Layout/LineLength:Max get the correct value for that
         # parameter.

spec/rubocop/cli/auto_gen_config_spec.rb

@@ -1309,8 +1309,13 @@ def function(arg1, arg2, arg3, arg4, arg5, arg6, arg7)
       # absolute Exclude paths will point into this example's work directory.
       RuboCop::ConfigLoader.default_configuration = nil
 
+      $stdout = StringIO.new
       expect(cli.run(['--auto-gen-config', '--auto-gen-only-exclude',
                       '--exclude-limit', '1'])).to eq(0)
+      expect($stdout.string.include?(<<~STRING)).to be(true)
+        Phase 1 of 2: run Layout/LineLength cop (skipped because only excludes will be generated due to `--auto-gen-only-exclude` flag)
+        Phase 2 of 2: run all cops
+      STRING
       actual = File.read('.rubocop_todo.yml').split($RS)
 
       # With --exclude-limit 1 we get MinDigits generated for NumericLiterals
@@ -1319,6 +1324,9 @@ def function(arg1, arg2, arg3, arg4, arg5, arg6, arg7)
       # the same cop in a single file. Exclude properties are generated for
       # them.
       expect(actual.grep(/^[^#]/).join($RS)).to eq(<<~YAML.chomp)
+        Layout/LineLength:
+          Exclude:
+            - 'example1.rb'
         Lint/UnusedMethodArgument:
           Exclude:
             - 'example2.rb'
@@ -1329,9 +1337,6 @@ def function(arg1, arg2, arg3, arg4, arg5, arg6, arg7)
           Enabled: false
         Style/NumericLiterals:
           MinDigits: 7
-        Layout/LineLength:
-          Exclude:
-            - 'example1.rb'
       YAML
     end