ast: Add base Node class with get_ast_kind() function

This adds a new base class common to all abstract base classes of the
AST: We can use it to store information shared by all nodes, such as the
newly introduced `AST::Kind` which helps in differentiating nodes.
We could also consider using it to store location info, since all AST
nodes probably need it.

gcc/rust/ast/rust-ast.h

@@ -36,6 +36,27 @@ namespace AST {
 class ASTVisitor;
 using AttrVec = std::vector<Attribute>;
 
+// The available kinds of AST Nodes
+enum Kind
+{
+  UNKNOWN,
+  MACRO_RULES_DEFINITION,
+};
+
+// Abstract base class for all AST elements
+class Node
+{
+public:
+  /**
+   * Get the kind of Node this is. This is used to differentiate various AST
+   * elements with very little overhead when extracting the derived type through
+   * static casting is not necessary.
+   */
+  // FIXME: Mark this as `= 0` in the future to make sure every node implements
+  // it
+  virtual Kind get_ast_kind () const { return Kind::UNKNOWN; }
+};
+
 // Delimiter types - used in macros and whatever.
 enum DelimType
 {
@@ -813,7 +834,7 @@ class MetaListNameValueStr;
 
 /* Base statement abstract class. Note that most "statements" are not allowed in
  * top-level module scope - only a subclass of statements called "items" are. */
-class Stmt
+class Stmt : public Node
 {
 public:
   // Unique pointer custom clone function

gcc/rust/ast/rust-macro.h

@@ -441,6 +441,8 @@ public:
     is_builtin_rule = true;
   }
 
+  Kind get_ast_kind () const override { return Kind::MACRO_RULES_DEFINITION; }
+
 protected:
   /* Use covariance to implement clone function as returning this object rather
    * than base */