[CALCITE-4004] Show RelOptRuleOperand description in debugger to facilitate debugging

To facilitate IDE shows the operand description in the debugger, override
Object.toString() in RelOptRuleOperand, return the root operand description,
but highlight current operand's matches class with '*' in the description.

e.g. The following are examples of rule operand description for the operands
that match with LogicalFilter.

- SemiJoinRule:project: Project(Join(*RelNode*, Aggregate))
- ProjectFilterTransposeRule: LogicalProject(*LogicalFilter*)
- FilterProjectTransposeRule: *Filter*(Project)
- ReduceExpressionsRule(Filter): *LogicalFilter*
- PruneEmptyJoin(right): Join(*RelNode*, Values)

Close #1978

Author: hsyuan

core/src/main/java/org/apache/calcite/plan/RelOptRuleOperand.java

@@ -184,6 +184,70 @@ public boolean equals(Object obj) {
         && this.children.equals(that.children);
   }
 
+  /**
+   * <b>FOR DEBUG ONLY.</b>
+   *
+   * <p>To facilitate IDE shows the operand description in the debugger,
+   * returns the root operand description, but highlight current
+   * operand's matched class with '*' in the description.</p>
+   *
+   * <p>e.g. The following are examples of rule operand description for
+   * the operands that match with {@code LogicalFilter}.</p>
+   *
+   * <ul>
+   * <li>SemiJoinRule:project: Project(Join(*RelNode*, Aggregate))</li>
+   * <li>ProjectFilterTransposeRule: LogicalProject(*LogicalFilter*)</li>
+   * <li>FilterProjectTransposeRule: *Filter*(Project)</li>
+   * <li>ReduceExpressionsRule(Filter): *LogicalFilter*</li>
+   * <li>PruneEmptyJoin(right): Join(*RelNode*, Values)</li>
+   * </ul>
+   *
+   * @see #describeIt(RelOptRuleOperand)
+   */
+  @Override public String toString() {
+    RelOptRuleOperand root = this;
+    while (root.parent != null) {
+      root = root.parent;
+    }
+    StringBuilder s = root.describeIt(this);
+    return s.toString();
+  }
+
+  /**
+   * Returns this rule operand description, and highlight the operand's
+   * class name with '*' if {@code that} operand equals current operand.
+   *
+   * @param that The rule operand that needs to be highlighted
+   * @return The string builder that describes this rule operand
+   * @see #toString()
+   */
+  private StringBuilder describeIt(RelOptRuleOperand that) {
+    StringBuilder s = new StringBuilder();
+    if (parent == null) {
+      s.append(rule).append(": ");
+    }
+    if (this == that) {
+      s.append('*');
+    }
+    s.append(clazz.getSimpleName());
+    if (this == that) {
+      s.append('*');
+    }
+    if (children != null && !children.isEmpty()) {
+      s.append('(');
+      boolean first = true;
+      for (RelOptRuleOperand child : children) {
+        if (!first) {
+          s.append(", ");
+        }
+        s.append(child.describeIt(that));
+        first = false;
+      }
+      s.append(')');
+    }
+    return s;
+  }
+
   /**
    * @return relational expression class matched by this operand
    */