Simplify comparable behavior of processing stages

Instead of overriding compareTo we can just use an
external comparator

pmd-core/src/main/java/net/sourceforge/pmd/internal/RulesetStageDependencyHelper.java

@@ -72,7 +72,7 @@ public class RulesetStageDependencyHelper {
     /** Builds a sorted list of the dependencies of the given ruleset. */
     private List<AstProcessingStage<?>> buildDependencyList(RuleSets ruleSets, LanguageVersion languageVersion) {
         List<AstProcessingStage<?>> stages = new ArrayList<>(languageVersion.getLanguageVersionHandler().getProcessingStages());
-        SortedSet<AstProcessingStage<?>> result = new TreeSet<>();
+        SortedSet<AstProcessingStage<?>> result = new TreeSet<>(AstProcessingStage.COMPARATOR);
 
         // this loops runs until either all stages have already been
         // picked or there are no rules left, whichever comes first

pmd-core/src/main/java/net/sourceforge/pmd/lang/ast/AstProcessingStage.java

@@ -6,6 +6,7 @@ package net.sourceforge.pmd.lang.ast;
 
 import static java.util.Collections.emptyList;
 
+import java.util.Comparator;
 import java.util.List;
 
 import net.sourceforge.pmd.RuleSets;
@@ -58,6 +59,11 @@ import net.sourceforge.pmd.lang.LanguageVersionHandler;
 @Experimental
 public interface AstProcessingStage<T extends AstProcessingStage<T>> extends Comparable<T> {
 
+    /**
+     * Compares processing stages of possibly different kinds.
+     */
+    Comparator<AstProcessingStage<?>> COMPARATOR = AstProcessingStage::compare;
+
 
     /**
      * Returns the language this processing stage applies to.
@@ -96,6 +102,20 @@ public interface AstProcessingStage<T extends AstProcessingStage<T>> extends Com
     void processAST(RootNode rootNode, AstAnalysisContext configuration);
 
 
-    @Override
-    int compareTo(AstProcessingStage o);
+    /**
+     * Same contract as {@link Comparable#compareTo(Object)}, but we can't extend
+     * Comparable with that type argument if we implement processing stages within
+     * an enum.
+     *
+     * @param t the object to compare
+     *
+     * @return a negative integer, zero, or a positive integer as this object
+     * is less than, equal to, or greater than the specified object.
+     */
+    @SuppressWarnings("unchecked")
+    default int compare(AstProcessingStage<?> t) {
+        return this.compareTo((T) t);
+    }
+
+
 }

pmd-core/src/test/java/net/sourceforge/pmd/lang/ast/DummyAstStages.java

@@ -43,12 +43,6 @@ public enum DummyAstStages implements AstProcessingStage<DummyAstStages> {
         return name();
     }
 
-    @Override
-    public int compareTo(AstProcessingStage o) {
-        return o instanceof DummyAstStages ? compareTo((DummyAstStages) o)
-                                                : 0;
-    }
-
     @Override
     public void processAST(RootNode rootNode, AstAnalysisContext configuration) {
         ((DummyNode) rootNode).getUserData().put(name() + "_STAGE", "done");

pmd-java/src/main/java/net/sourceforge/pmd/lang/java/internal/JavaProcessingStage.java

@@ -89,12 +89,6 @@ public enum JavaProcessingStage implements AstProcessingStage<JavaProcessingStag
         this.dependencies = Collections.unmodifiableList(Arrays.asList(dependencies));
     }
 
-    @Override
-    public int compareTo(AstProcessingStage o) {
-        return o instanceof JavaProcessingStage ? compareTo((JavaProcessingStage) o)
-                                                : 0;
-    }
-
     @Override
     public List<JavaProcessingStage> getDependencies() {
         return dependencies;

pmd-modelica/src/main/java/net/sourceforge/pmd/lang/modelica/internal/ModelicaProcessingStage.java

@@ -30,11 +30,6 @@ public enum ModelicaProcessingStage implements AstProcessingStage<ModelicaProces
         this.displayName = displayName;
     }
 
-    @Override
-    public int compareTo(AstProcessingStage o) {
-        return o instanceof ModelicaProcessingStage ? compareTo((ModelicaProcessingStage) o)
-                                                    : 0;
-    }
 
     @Override
     public Language getLanguage() {

pmd-plsql/src/main/java/net/sourceforge/pmd/lang/plsql/PlsqlProcessingStage.java

@@ -76,12 +76,6 @@ public enum PlsqlProcessingStage implements AstProcessingStage<PlsqlProcessingSt
         return displayName;
     }
 
-    @Override
-    public int compareTo(AstProcessingStage o) {
-        return o instanceof PlsqlProcessingStage ? compareTo((PlsqlProcessingStage) o)
-                                                 : 0;
-    }
-
 
 }