Rename --rule argument into --only, add documentation.

README.md

@@ -201,6 +201,18 @@ class SomeClass
 }
 ```
 
+### Filter Rectors
+
+If you have a configuration file for Rector including many sets and Rectors, you might want at times to run only a single Rector from them. The `--only` argument allows that, for example : 
+
+```bash
+vendor/bin/rector process --set solid --only "Rector\SOLID\Rector\Class_\FinalizeClassesWithoutChildrenRector" src/
+```
+
+Will only run `Rector\SOLID\Rector\Class_\FinalizeClassesWithoutChildrenRector`.
+
+Please note that the backslash in the Rector's fully-qualified class name needs to be properly escaped (by surrounding the string in double quotes).
+
 ### Provide PHP Version
 
 By default Rector uses the language features matching your system version of PHP. You can configure it for a different PHP version:

src/Application/RectorApplication.php

@@ -144,9 +144,9 @@ public function runOnFileInfos(array $fileInfos): void
         }
 
         // active only one rule
-        if ($this->configuration->getRule() !== null) {
-            $rule = $this->configuration->getRule();
-            $this->enabledRectorsProvider->addEnabledRector($rule);
+        if ($this->configuration->getOnlyRector() !== null) {
+            $onlyRector = $this->configuration->getOnlyRector();
+            $this->enabledRectorsProvider->addEnabledRector($onlyRector);
         }
 
         // 2. change nodes with Rectors

src/Configuration/Configuration.php

@@ -37,7 +37,7 @@ final class Configuration
     /**
      * @var string|null
      */
-    private $rule;
+    private $onlyRector;
 
     /**
      * @var bool
@@ -59,7 +59,10 @@ public function resolveFromInput(InputInterface $input): void
         $this->mustMatchGitDiff = (bool) $input->getOption(Option::MATCH_GIT_DIFF);
         $this->showProgressBar = $this->canShowProgressBar($input);
 
-        $this->setRule($input->getOption(Option::OPTION_RULE));
+        /** @var string|null $onlyRector */
+        $onlyRector = $input->getOption(Option::OPTION_ONLY);
+
+        $this->setOnlyRector($onlyRector);
     }
 
     public function setFirstResolverConfig(?string $firstResolvedConfig): void
@@ -102,11 +105,6 @@ public function showProgressBar(): bool
         return $this->showProgressBar;
     }
 
-    public function getRule(): ?string
-    {
-        return $this->rule;
-    }
-
     public function areAnyPhpRectorsLoaded(): bool
     {
         if (PHPUnitEnvironment::isPHPUnitRun()) {
@@ -126,23 +124,28 @@ public function mustMatchGitDiff(): bool
         return $this->mustMatchGitDiff;
     }
 
-    private function canShowProgressBar(InputInterface $input): bool
+    public function getOnlyRector(): ?string
     {
-        $noProgressBar = (bool) $input->getOption(Option::OPTION_NO_PROGRESS_BAR);
-
-        return ! $noProgressBar && $input->getOption(Option::OPTION_OUTPUT_FORMAT) !== JsonOutputFormatter::NAME;
+        return $this->onlyRector;
     }
 
-    private function setRule(?string $rule): void
+    private function setOnlyRector(?string $rector): void
     {
-        if ($rule) {
-            $this->ensureIsValidRectorClass($rule);
-            $this->rule = $rule;
+        if ($rector) {
+            $this->ensureIsValidRectorClass($rector);
+            $this->onlyRector = $rector;
         } else {
-            $this->rule = null;
+            $this->onlyRector = null;
         }
     }
 
+    private function canShowProgressBar(InputInterface $input): bool
+    {
+        $noProgressBar = (bool) $input->getOption(Option::OPTION_NO_PROGRESS_BAR);
+
+        return ! $noProgressBar && $input->getOption(Option::OPTION_OUTPUT_FORMAT) !== JsonOutputFormatter::NAME;
+    }
+
     private function ensureIsValidRectorClass(string $rector): void
     {
         // simple check

src/Configuration/Option.php

@@ -44,7 +44,7 @@ final class Option
     /**
      * @var string
      */
-    public const OPTION_RULE = 'rule';
+    public const OPTION_ONLY = 'only';
 
     /**
      * @var string

src/Console/Command/ProcessCommand.php

@@ -155,7 +155,12 @@ protected function configure(): void
             'Execute only on file(s) matching the git diff.'
         );
 
-        $this->addOption(Option::OPTION_RULE, 'r', InputOption::VALUE_REQUIRED, 'Run only this single rule.');
+        $this->addOption(
+            Option::OPTION_ONLY,
+            'r',
+            InputOption::VALUE_REQUIRED,
+            'Run only one single Rector from the loaded Rectors (in services, sets, etc).'
+        );
 
         $availableOutputFormatters = $this->outputFormatterCollector->getNames();
         $this->addOption(