expansion: Desugar doc comments into attributes before expansion

gcc/rust/ChangeLog:

	* expand/rust-macro-expand.cc (MacroExpander::expand_decl_macro): Call into
	TokenTreeDesugar.
	* expand/rust-token-tree-desugar.cc: New file.
	* expand/rust-token-tree-desugar.h: New file.
	* Make-lang.in: Compile them.

gcc/testsuite/ChangeLog:

	* rust/compile/macros/mbe/macro-issue3709-1.rs: New test.
	* rust/compile/macros/mbe/macro-issue3709-2.rs: New test.

1 files changed, 55 insertions, 0 deletions

diff --git a/gcc/rust/expand/rust-token-tree-desugar.h b/gcc/rust/expand/rust-token-tree-desugar.h
new file mode 100644
index 0000000..ccba53b
--- /dev/null
+++ b/gcc/rust/expand/rust-token-tree-desugar.h
@@ -0,0 +1,55 @@
+// Copyright (C) 2025 Free Software Foundation, Inc.
+
+// This file is part of GCC.
+
+// GCC is free software; you can redistribute it and/or modify it under
+// the terms of the GNU General Public License as published by the Free
+// Software Foundation; either version 3, or (at your option) any later
+// version.
+
+// GCC is distributed in the hope that it will be useful, but WITHOUT ANY
+// WARRANTY; without even the implied warranty of MERCHANTABILITY or
+// FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+// for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with GCC; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+#ifndef RUST_TOKEN_TREE_DESUGAR_H
+#define RUST_TOKEN_TREE_DESUGAR_H
+
+#include "rust-ast-visitor.h"
+#include "rust-system.h"
+#include "rust-ast.h"
+
+namespace Rust {
+namespace AST {
+
+/**
+ * Desugar a given token-tree before parsing it for a macro invocation. At the
+ * moment, the sole purpose of this desugar is to transform doc-comments into
+ * their attribute form (/// comment -> #[doc = "comment"])
+ */
+class TokenTreeDesugar : public DefaultASTVisitor
+{
+public:
+  TokenTreeDesugar () : desugared (std::vector<std::unique_ptr<TokenTree>> ())
+  {}
+
+  DelimTokenTree go (DelimTokenTree &tts);
+
+private:
+  std::vector<std::unique_ptr<TokenTree>> desugared;
+  void append (TokenPtr &&new_token);
+  void append (std::unique_ptr<TokenTree> &&new_token);
+
+  using DefaultASTVisitor::visit;
+
+  virtual void visit (Token &tts) override;
+};
+
+}; // namespace AST
+}; // namespace Rust
+
+#endif //! RUST_TOKEN_TREE_DESUGAR_H