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.

4 files changed, 133 insertions, 1 deletions

diff --git a/gcc/rust/Make-lang.in b/gcc/rust/Make-lang.in
index c024f24..5ae50d2 100644
--- a/gcc/rust/Make-lang.in
+++ b/gcc/rust/Make-lang.in
@@ -115,6 +115,7 @@ GRS_OBJS = \
     rust/rust-macro-builtins-format-args.o \
     rust/rust-macro-builtins-location.o \
     rust/rust-macro-builtins-include.o \
+    rust/rust-token-tree-desugar.o \
 	rust/rust-fmt.o \
     rust/rust-hir.o \
     rust/rust-hir-map.o \
diff --git a/gcc/rust/expand/rust-macro-expand.cc b/gcc/rust/expand/rust-macro-expand.cc
index c22db1c..e7407a4 100644
--- a/gcc/rust/expand/rust-macro-expand.cc
+++ b/gcc/rust/expand/rust-macro-expand.cc
@@ -28,6 +28,7 @@
 #include "rust-cfg-strip.h"
 #include "rust-early-name-resolver.h"
 #include "rust-proc-macro.h"
+#include "rust-token-tree-desugar.h"
 
 namespace Rust {
 
@@ -78,7 +79,10 @@ MacroExpander::expand_decl_macro (location_t invoc_locus,
    * trees.
    */
 
-  AST::DelimTokenTree &invoc_token_tree = invoc.get_delim_tok_tree ();
+  AST::DelimTokenTree &invoc_token_tree_sugar = invoc.get_delim_tok_tree ();
+
+  // We must first desugar doc comments into proper attributes
+  auto invoc_token_tree = AST::TokenTreeDesugar ().go (invoc_token_tree_sugar);
 
   // find matching arm
   AST::MacroRule *matched_rule = nullptr;
diff --git a/gcc/rust/expand/rust-token-tree-desugar.cc b/gcc/rust/expand/rust-token-tree-desugar.cc
new file mode 100644
index 0000000..3b47180
--- /dev/null
+++ b/gcc/rust/expand/rust-token-tree-desugar.cc
@@ -0,0 +1,72 @@
+// 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/>.
+
+#include "rust-token-tree-desugar.h"
+#include "rust-ast.h"
+#include "rust-token.h"
+
+namespace Rust {
+namespace AST {
+
+DelimTokenTree
+TokenTreeDesugar::go (DelimTokenTree &tts)
+{
+  tts.accept_vis (*this);
+
+  return DelimTokenTree (tts.get_delim_type (), std::move (desugared),
+			 tts.get_locus ());
+}
+
+void
+TokenTreeDesugar::append (TokenPtr &&new_token)
+{
+  desugared.emplace_back (std::make_unique<Token> (std::move (new_token)));
+}
+
+void
+TokenTreeDesugar::append (std::unique_ptr<TokenTree> &&new_token)
+{
+  desugared.emplace_back (std::move (new_token));
+}
+
+void
+TokenTreeDesugar::visit (Token &tts)
+{
+  if (tts.get_id () == TokenId::OUTER_DOC_COMMENT
+      || tts.get_id () == TokenId::INNER_DOC_COMMENT)
+    {
+      append (Rust::Token::make (TokenId::HASH, tts.get_locus ()));
+
+      if (tts.get_id () == TokenId::INNER_DOC_COMMENT)
+	append (Rust::Token::make (EXCLAM, tts.get_locus ()));
+
+      append (Rust::Token::make (TokenId::LEFT_SQUARE, tts.get_locus ()));
+      append (Rust::Token::make_identifier (tts.get_locus (), "doc"));
+      append (Rust::Token::make (TokenId::EQUAL, tts.get_locus ()));
+      append (Rust::Token::make_string (tts.get_locus (),
+					std::string (tts.get_str ())));
+      append (Rust::Token::make (TokenId::RIGHT_SQUARE, tts.get_locus ()));
+    }
+  else
+    {
+      append (tts.clone_token ());
+    }
+}
+
+}; // namespace AST
+}; // namespace Rust
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