derive(Debug): Add stub implementation.

gcc/rust/ChangeLog:

	* expand/rust-derive-debug.cc: New file.
	* expand/rust-derive-debug.h: New file.
	* Make-lang.in: Compile them.
	* expand/rust-derive.cc (DeriveVisitor::derive): Call into DeriveDebug.

gcc/testsuite/ChangeLog:

	* rust/compile/derive-debug1.rs: New test.
	* rust/compile/nr2/exclude: Exclude it.

3 files changed, 191 insertions, 0 deletions

diff --git a/gcc/rust/expand/rust-derive-debug.cc b/gcc/rust/expand/rust-derive-debug.cc
new file mode 100644
index 0000000..910f27c
--- /dev/null
+++ b/gcc/rust/expand/rust-derive-debug.cc
@@ -0,0 +1,130 @@
+// 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-derive-debug.h"
+#include "rust-ast.h"
+#include "rust-hir-map.h"
+#include "rust-system.h"
+
+namespace Rust {
+namespace AST {
+
+DeriveDebug::DeriveDebug (location_t loc)
+  : DeriveVisitor (loc), expanded (nullptr)
+{}
+
+std::unique_ptr<Item>
+DeriveDebug::go (Item &item)
+{
+  item.accept_vis (*this);
+
+  rust_assert (expanded);
+
+  return std::move (expanded);
+}
+
+/* Pointer-ify something */
+template <typename T>
+static std::unique_ptr<T>
+ptrify (T value)
+{
+  return std::unique_ptr<T> (new T (value));
+}
+
+std::unique_ptr<AssociatedItem>
+DeriveDebug::stub_debug_fn ()
+{
+  auto unit_expr = builder.tuple ();
+  auto ok_expr
+    = ptrify (builder.path_in_expression (LangItem::Kind::RESULT_OK));
+
+  auto stub_return = builder.call (std::move (ok_expr), std::move (unit_expr));
+
+  // we can't use builder.block() here as it returns a unique_ptr<Expr> and
+  // Function's constructor expects a unique_ptr<BlockExpr>
+  auto block = std::unique_ptr<BlockExpr> (
+    new BlockExpr ({}, std::move (stub_return), {}, {},
+		   AST::LoopLabel::error (), loc, loc));
+
+  auto self = builder.self_ref_param ();
+
+  auto return_type
+    = ptrify (builder.type_path ({"core", "fmt", "Result"}, true));
+
+  auto mut_fmt_type_inner
+    = ptrify (builder.type_path ({"core", "fmt", "Formatter"}, true));
+
+  auto mut_fmt_type
+    = builder.reference_type (std::move (mut_fmt_type_inner), true);
+
+  auto fmt = builder.function_param (builder.identifier_pattern ("_fmt"),
+				     std::move (mut_fmt_type));
+
+  auto params = vec (std::move (self), std::move (fmt));
+
+  auto function = builder.function ({"fmt"}, std::move (params),
+				    std::move (return_type), std::move (block));
+
+  return ptrify (function);
+}
+
+std::unique_ptr<Item>
+DeriveDebug::stub_derive_impl (
+  std::string name,
+  const std::vector<std::unique_ptr<GenericParam>> &type_generics)
+{
+  auto trait_items = vec (stub_debug_fn ());
+
+  auto debug = builder.type_path ({"core", "fmt", "Debug"}, true);
+  auto generics
+    = setup_impl_generics (name, type_generics, builder.trait_bound (debug));
+
+  return builder.trait_impl (debug, std::move (generics.self_type),
+			     std::move (trait_items),
+			     std::move (generics.impl));
+}
+
+void
+DeriveDebug::visit_struct (StructStruct &struct_item)
+{
+  expanded = stub_derive_impl (struct_item.get_identifier ().as_string (),
+			       struct_item.get_generic_params ());
+}
+
+void
+DeriveDebug::visit_tuple (TupleStruct &tuple_item)
+{
+  expanded = stub_derive_impl (tuple_item.get_identifier ().as_string (),
+			       tuple_item.get_generic_params ());
+}
+
+void
+DeriveDebug::visit_enum (Enum &enum_item)
+{
+  expanded = stub_derive_impl (enum_item.get_identifier ().as_string (),
+			       enum_item.get_generic_params ());
+}
+
+void
+DeriveDebug::visit_union (Union &enum_item)
+{
+  rust_error_at (loc, "derive(Debug) cannot be derived for unions");
+}
+
+} // namespace AST
+} // namespace Rust
diff --git a/gcc/rust/expand/rust-derive-debug.h b/gcc/rust/expand/rust-derive-debug.h
new file mode 100644
index 0000000..14af89e
--- /dev/null
+++ b/gcc/rust/expand/rust-derive-debug.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_DERIVE_DEBUG_H
+#define RUST_DERIVE_DEBUG_H
+
+#include "rust-derive.h"
+#include "rust-ast.h"
+
+namespace Rust {
+namespace AST {
+
+// This derive is currently incomplete and only generate a stub implementation
+// which does not do any debug formatting
+class DeriveDebug : DeriveVisitor
+{
+public:
+  DeriveDebug (location_t loc);
+
+  std::unique_ptr<Item> go (Item &);
+
+private:
+  std::unique_ptr<Item> expanded;
+
+  std::unique_ptr<AssociatedItem> stub_debug_fn ();
+
+  std::unique_ptr<Item> stub_derive_impl (
+    std::string name,
+    const std::vector<std::unique_ptr<GenericParam>> &type_generics);
+
+  virtual void visit_struct (StructStruct &struct_item) override;
+  virtual void visit_tuple (TupleStruct &tuple_item) override;
+  virtual void visit_enum (Enum &enum_item) override;
+  virtual void visit_union (Union &enum_item) override;
+};
+
+} // namespace AST
+} // namespace Rust
+
+#endif // ! RUST_DERIVE_DEBUG_H
diff --git a/gcc/rust/expand/rust-derive.cc b/gcc/rust/expand/rust-derive.cc
index c988412..e52fce4 100644
--- a/gcc/rust/expand/rust-derive.cc
+++ b/gcc/rust/expand/rust-derive.cc
@@ -19,6 +19,7 @@
 #include "rust-derive.h"
 #include "rust-derive-clone.h"
 #include "rust-derive-copy.h"
+#include "rust-derive-debug.h"
 
 namespace Rust {
 namespace AST {
@@ -38,6 +39,11 @@ DeriveVisitor::derive (Item &item, const Attribute &attr,
     case BuiltinMacro::Copy:
       return DeriveCopy (attr.get_locus ()).go (item);
     case BuiltinMacro::Debug:
+      rust_warning_at (
+	attr.get_locus (), 0,
+	"derive(Debug) is not fully implemented yet and has no effect - only a "
+	"stub implementation will be generated");
+      return DeriveDebug (attr.get_locus ()).go (item);
     case BuiltinMacro::Default:
     case BuiltinMacro::Eq:
     case BuiltinMacro::PartialEq: