derive(Hash): Add implementation.

gcc/rust/ChangeLog:

	* Make-lang.in: Compile it.
	* expand/rust-derive.cc (DeriveVisitor::derive): Call it.
	* expand/rust-derive-hash.cc: New file.
	* expand/rust-derive-hash.h: New file.

gcc/testsuite/ChangeLog:

	* rust/compile/derive-hash1.rs: New test.
	* rust/compile/nr2/exclude: Add testcase to exclusion list.

3 files changed, 367 insertions, 9 deletions

diff --git a/gcc/rust/expand/rust-derive-hash.cc b/gcc/rust/expand/rust-derive-hash.cc
new file mode 100644
index 0000000..0c9b0f7
--- /dev/null
+++ b/gcc/rust/expand/rust-derive-hash.cc
@@ -0,0 +1,293 @@
+// 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-hash.h"
+#include "rust-ast.h"
+#include "rust-expr.h"
+#include "rust-item.h"
+#include "rust-path.h"
+#include "rust-pattern.h"
+#include "rust-stmt.h"
+#include "rust-system.h"
+
+namespace Rust {
+namespace AST {
+
+DeriveHash::DeriveHash (location_t loc) : DeriveVisitor (loc) {}
+
+std::unique_ptr<AST::Item>
+DeriveHash::go (Item &item)
+{
+  item.accept_vis (*this);
+
+  return std::move (expanded);
+}
+
+std::unique_ptr<Expr>
+DeriveHash::hash_call (std::unique_ptr<Expr> &&value)
+{
+  auto hash
+    = builder.path_in_expression ({"core", "hash", "Hash", "hash"}, true);
+
+  return builder.call (ptrify (hash),
+		       vec (std::move (value),
+			    builder.identifier (DeriveHash::state)));
+}
+
+std::unique_ptr<AssociatedItem>
+DeriveHash::hash_fn (std::unique_ptr<BlockExpr> &&block)
+{
+  auto hash_calls = std::vector<std::unique_ptr<Stmt>> ();
+
+  auto state_type = std::unique_ptr<TypeNoBounds> (
+    new TypePath (builder.type_path (DeriveHash::state_type)));
+  auto state_param =
+
+    builder.function_param (builder.identifier_pattern (DeriveHash::state),
+			    builder.reference_type (std::move (state_type),
+						    true));
+
+  auto params = vec (builder.self_ref_param (), std::move (state_param));
+  auto bounds = vec (
+    builder.trait_bound (builder.type_path ({"core", "hash", "Hasher"}, true)));
+  auto generics = vec (
+    builder.generic_type_param (DeriveHash::state_type, std::move (bounds)));
+
+  return builder.function ("hash", std::move (params), nullptr,
+			   std::move (block), std::move (generics));
+}
+
+std::unique_ptr<Item>
+DeriveHash::hash_impl (
+  std::unique_ptr<AssociatedItem> &&hash_fn, std::string name,
+  const std::vector<std::unique_ptr<GenericParam>> &type_generics)
+{
+  auto hash_path = builder.type_path ({"core", "hash", "Hash"}, true);
+
+  auto trait_items = vec (std::move (hash_fn));
+
+  auto generics = setup_impl_generics (name, type_generics,
+				       builder.trait_bound (hash_path));
+
+  return builder.trait_impl (hash_path, std::move (generics.self_type),
+			     std::move (trait_items),
+			     std::move (generics.impl));
+}
+
+void
+DeriveHash::visit_struct (StructStruct &item)
+{
+  auto hash_calls = std::vector<std::unique_ptr<Stmt>> ();
+
+  for (auto &field : item.get_fields ())
+    {
+      auto value = builder.ref (
+	builder.field_access (builder.identifier ("self"),
+			      field.get_field_name ().as_string ()));
+
+      auto stmt = builder.statementify (hash_call (std::move (value)));
+
+      hash_calls.emplace_back (std::move (stmt));
+    }
+
+  auto block = builder.block (std::move (hash_calls));
+
+  expanded = hash_impl (hash_fn (std::move (block)),
+			item.get_identifier ().as_string (),
+			item.get_generic_params ());
+}
+
+void
+DeriveHash::visit_tuple (TupleStruct &item)
+{
+  auto hash_calls = std::vector<std::unique_ptr<Stmt>> ();
+
+  for (size_t idx = 0; idx < item.get_fields ().size (); idx++)
+    {
+      auto value = builder.ref (builder.tuple_idx ("self", idx));
+
+      auto stmt = builder.statementify (hash_call (std::move (value)));
+
+      hash_calls.emplace_back (std::move (stmt));
+    }
+
+  auto block = builder.block (std::move (hash_calls));
+
+  expanded = hash_impl (hash_fn (std::move (block)),
+			item.get_identifier ().as_string (),
+			item.get_generic_params ());
+}
+
+MatchCase
+DeriveHash::match_enum_tuple (PathInExpression variant_path,
+			      const EnumItemTuple &variant)
+{
+  auto self_patterns = std::vector<std::unique_ptr<Pattern>> ();
+  auto hash_calls = std::vector<std::unique_ptr<Stmt>> ();
+
+  for (size_t i = 0; i < variant.get_tuple_fields ().size (); i++)
+    {
+      auto pattern = "__self_" + std::to_string (i);
+
+      auto call = hash_call (builder.ref (builder.identifier (pattern)));
+
+      self_patterns.emplace_back (builder.identifier_pattern (pattern));
+      hash_calls.emplace_back (builder.statementify (std::move (call)));
+    }
+
+  auto patterns_elts = std::unique_ptr<TupleStructItems> (
+    new TupleStructItemsNoRange (std::move (self_patterns)));
+  auto pattern = std::unique_ptr<Pattern> (
+    new ReferencePattern (std::unique_ptr<Pattern> (new TupleStructPattern (
+			    variant_path, std::move (patterns_elts))),
+			  false, false, loc));
+
+  auto block = builder.block (std::move (hash_calls));
+
+  return builder.match_case (std::move (pattern), std::move (block));
+}
+
+MatchCase
+DeriveHash::match_enum_struct (PathInExpression variant_path,
+			       const EnumItemStruct &variant)
+{
+  auto field_patterns = std::vector<std::unique_ptr<StructPatternField>> ();
+  auto hash_calls = std::vector<std::unique_ptr<Stmt>> ();
+
+  for (const auto &field : variant.get_struct_fields ())
+    {
+      auto call = hash_call (builder.ref (
+	builder.identifier (field.get_field_name ().as_string ())));
+
+      field_patterns.emplace_back (
+	std::unique_ptr<StructPatternField> (new StructPatternFieldIdent (
+	  field.get_field_name (), false /* is_ref? true? */, false, {}, loc)));
+
+      hash_calls.emplace_back (builder.statementify (std::move (call)));
+    }
+
+  auto pattern_elts = StructPatternElements (std::move (field_patterns));
+  auto pattern = std::unique_ptr<Pattern> (
+    new ReferencePattern (std::unique_ptr<Pattern> (new StructPattern (
+			    variant_path, loc, pattern_elts)),
+			  false, false, loc));
+
+  auto block = builder.block (std::move (hash_calls));
+  return builder.match_case (std::move (pattern), std::move (block));
+}
+
+void
+DeriveHash::visit_enum (Enum &item)
+{
+  // Enums are a bit different: We start by hashing the discriminant value of
+  // the enum instance, and then hash all of the data contained in each of the
+  // enum's variants. For data-less variants, we don't have any data to hash, so
+  // hashing the discriminant value is enough. To access the rest of the
+  // variants' data, we create a match and destructure each internal field and
+  // hash it.
+  //
+  // So for example with the following enum:
+  //
+  // ```rust
+  // enum Foo {
+  //     A,
+  //     B(i32),
+  //     C { a: i32 },
+  // }
+  // ```
+  //
+  // we create the following implementation:
+  //
+  // ```rust
+  // fn hash<H: Hasher>(&self, state: &mut H) {
+  //     let discriminant = intrinsics::discriminant_value(&self);
+  //     Hash::hash(&discriminant, state);
+  //
+  //     match self {
+  //         B(self_0) => { Hash::hash(self_0, state); },
+  //         C { a } => { Hash::hash(a, state); },
+  //         _ => {},
+  //     }
+  // }
+  // ```
+  //
+  // Note the extra wildcard pattern to satisfy the exhaust checker.
+
+  auto cases = std::vector<MatchCase> ();
+  auto type_name = item.get_identifier ().as_string ();
+
+  auto intrinsic = ptrify (
+    builder.path_in_expression ({"core", "intrinsics", "discriminant_value"},
+				true));
+
+  auto let_discr
+    = builder.let (builder.identifier_pattern (DeriveHash::discr), nullptr,
+		   builder.call (std::move (intrinsic),
+				 builder.identifier ("self")));
+
+  auto discr_hash = builder.statementify (
+    hash_call (builder.ref (builder.identifier (DeriveHash::discr))));
+
+  for (auto &variant : item.get_variants ())
+    {
+      auto variant_path
+	= builder.variant_path (type_name,
+				variant->get_identifier ().as_string ());
+
+      switch (variant->get_enum_item_kind ())
+	{
+	case EnumItem::Kind::Identifier:
+	case EnumItem::Kind::Discriminant:
+	  // nothing to do in these cases, as we just need to hash the
+	  // discriminant value
+	  continue;
+	case EnumItem::Kind::Tuple:
+	  cases.emplace_back (
+	    match_enum_tuple (variant_path,
+			      static_cast<EnumItemTuple &> (*variant)));
+	  break;
+	case EnumItem::Kind::Struct:
+	  cases.emplace_back (
+	    match_enum_struct (variant_path,
+			       static_cast<EnumItemStruct &> (*variant)));
+	  break;
+	}
+    }
+
+  // The extra empty wildcard case
+  cases.emplace_back (
+    builder.match_case (builder.wildcard (), builder.block ()));
+
+  auto match = builder.match (builder.identifier ("self"), std::move (cases));
+
+  auto block
+    = builder.block (vec (std::move (let_discr), std::move (discr_hash)),
+		     std::move (match));
+
+  expanded = hash_impl (hash_fn (std::move (block)), type_name,
+			item.get_generic_params ());
+}
+
+void
+DeriveHash::visit_union (Union &item)
+{
+  rust_error_at (item.get_locus (), "derive(Hash) cannot be used on unions");
+}
+
+} // namespace AST
+} // namespace Rust
diff --git a/gcc/rust/expand/rust-derive-hash.h b/gcc/rust/expand/rust-derive-hash.h
new file mode 100644
index 0000000..02b0bee
--- /dev/null
+++ b/gcc/rust/expand/rust-derive-hash.h
@@ -0,0 +1,61 @@
+// 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_HASH_H
+#define RUST_DERIVE_HASH_H
+
+#include "rust-derive.h"
+
+namespace Rust {
+namespace AST {
+
+class DeriveHash : DeriveVisitor
+{
+public:
+  DeriveHash (location_t loc);
+
+  std::unique_ptr<AST::Item> go (Item &item);
+
+private:
+  std::unique_ptr<Item> expanded;
+
+  constexpr static const char *state = "#state";
+  constexpr static const char *state_type = "#__H";
+  constexpr static const char *discr = "#discr";
+
+  std::unique_ptr<Expr> hash_call (std::unique_ptr<Expr> &&value);
+  std::unique_ptr<AssociatedItem> hash_fn (std::unique_ptr<BlockExpr> &&block);
+  std::unique_ptr<Item>
+  hash_impl (std::unique_ptr<AssociatedItem> &&hash_fn, std::string name,
+	     const std::vector<std::unique_ptr<GenericParam>> &type_generics);
+
+  MatchCase match_enum_tuple (PathInExpression variant_path,
+			      const EnumItemTuple &variant);
+  MatchCase match_enum_struct (PathInExpression variant_path,
+			       const EnumItemStruct &variant);
+
+  virtual void visit_struct (StructStruct &item);
+  virtual void visit_tuple (TupleStruct &item);
+  virtual void visit_enum (Enum &item);
+  virtual void visit_union (Union &item);
+};
+
+} // namespace AST
+} // namespace Rust
+
+#endif // ! RUST_DERIVE_HASH_H
diff --git a/gcc/rust/expand/rust-derive.cc b/gcc/rust/expand/rust-derive.cc
index 8223281..fc8875d 100644
--- a/gcc/rust/expand/rust-derive.cc
+++ b/gcc/rust/expand/rust-derive.cc
@@ -23,6 +23,7 @@
 #include "rust-derive-default.h"
 #include "rust-derive-eq.h"
 #include "rust-derive-partial-eq.h"
+#include "rust-derive-hash.h"
 
 namespace Rust {
 namespace AST {
@@ -35,29 +36,32 @@ std::vector<std::unique_ptr<Item>>
 DeriveVisitor::derive (Item &item, const Attribute &attr,
 		       BuiltinMacro to_derive)
 {
+  auto loc = attr.get_locus ();
+
   switch (to_derive)
     {
     case BuiltinMacro::Clone:
-      return vec (DeriveClone (attr.get_locus ()).go (item));
+      return vec (DeriveClone (loc).go (item));
     case BuiltinMacro::Copy:
-      return vec (DeriveCopy (attr.get_locus ()).go (item));
+      return vec (DeriveCopy (loc).go (item));
     case BuiltinMacro::Debug:
       rust_warning_at (
-	attr.get_locus (), 0,
+	loc, 0,
 	"derive(Debug) is not fully implemented yet and has no effect - only a "
 	"stub implementation will be generated");
-      return vec (DeriveDebug (attr.get_locus ()).go (item));
+      return vec (DeriveDebug (loc).go (item));
     case BuiltinMacro::Default:
-      return vec (DeriveDefault (attr.get_locus ()).go (item));
+      return vec (DeriveDefault (loc).go (item));
     case BuiltinMacro::Eq:
-      return DeriveEq (attr.get_locus ()).go (item);
+      return DeriveEq (loc).go (item);
     case BuiltinMacro::PartialEq:
-      return DerivePartialEq (attr.get_locus ()).go (item);
+      return DerivePartialEq (loc).go (item);
+    case BuiltinMacro::Hash:
+      return vec (DeriveHash (loc).go (item));
     case BuiltinMacro::Ord:
     case BuiltinMacro::PartialOrd:
-    case BuiltinMacro::Hash:
     default:
-      rust_sorry_at (attr.get_locus (), "unimplemented builtin derive macro");
+      rust_sorry_at (loc, "unimplemented builtin derive macro");
       return {};
     };
 }