ast: Add EnumItem::Kind

gcc/rust/ChangeLog:

	* ast/rust-item.h: Add EnumItem::Kind for differentiating all variants that may be
	used inside an enum declaration.

gcc/rust/ast/rust-item.h

@@ -1994,6 +1994,41 @@ class EnumItem : public VisItem
   location_t locus;
 
 public:
+  enum class Kind
+  {
+    Identifier,
+    Tuple,
+    Struct,
+
+    // FIXME: In the future, we'll need to remove this possibility as well as
+    // remove the EnumItemDiscriminant class. The feature for arbitrary
+    // discriminants on all kinds of variants has been stabilized, and a
+    // "discriminant" is no longer an enum item variant - it's simply an
+    // optional part of all variants.
+    //
+    // Per the reference:
+    //
+    // EnumItem :
+    //    OuterAttribute* Visibility?
+    //    IDENTIFIER ( EnumItemTuple | EnumItemStruct )? EnumItemDiscriminant?
+    //
+    // EnumItemTuple :
+    //    ( TupleFields? )
+    //
+    // EnumItemStruct :
+    //    { StructFields? }
+    //
+    // EnumItemDiscriminant :
+    //    = Expression
+    //
+    // So we instead need to remove the class, and add an optional expression to
+    // the base EnumItem class
+    //
+    // gccrs#3340
+
+    Discriminant,
+  };
+
   virtual ~EnumItem () {}
 
   EnumItem (Identifier variant_name, Visibility vis,
@@ -2002,6 +2037,8 @@ class EnumItem : public VisItem
       variant_name (std::move (variant_name)), locus (locus)
   {}
 
+  virtual Kind get_enum_item_kind () const { return Kind::Identifier; }
+
   // Unique pointer custom clone function
   std::unique_ptr<EnumItem> clone_enum_item () const
   {
@@ -2043,6 +2080,11 @@ class EnumItemTuple : public EnumItem
       tuple_fields (std::move (tuple_fields))
   {}
 
+  EnumItem::Kind get_enum_item_kind () const override
+  {
+    return EnumItem::Kind::Tuple;
+  }
+
   std::string as_string () const override;
 
   void accept_vis (ASTVisitor &vis) override;
@@ -2080,6 +2122,11 @@ class EnumItemStruct : public EnumItem
       struct_fields (std::move (struct_fields))
   {}
 
+  EnumItem::Kind get_enum_item_kind () const override
+  {
+    return EnumItem::Kind::Struct;
+  }
+
   std::string as_string () const override;
 
   void accept_vis (ASTVisitor &vis) override;
@@ -2133,6 +2180,11 @@ class EnumItemDiscriminant : public EnumItem
   EnumItemDiscriminant (EnumItemDiscriminant &&other) = default;
   EnumItemDiscriminant &operator= (EnumItemDiscriminant &&other) = default;
 
+  EnumItem::Kind get_enum_item_kind () const override
+  {
+    return EnumItem::Kind::Discriminant;
+  }
+
   std::string as_string () const override;
 
   void accept_vis (ASTVisitor &vis) override;