Implement unused assignments lints on HIR

gcc/rust/Make-lang.in

@@ -200,6 +200,9 @@ GRS_OBJS = \
     rust/rust-const-checker.o \
     rust/rust-lint-marklive.o \
     rust/rust-lint-unused-var.o \
+    rust/rust-unused-checker.o \
+    rust/rust-unused-collector.o \
+    rust/rust-unused-context.o \
     rust/rust-readonly-check.o \
     rust/rust-hir-type-check-path.o \
     rust/rust-unsafe-checker.o \
@@ -432,6 +435,7 @@ RUST_INCLUDES = -I $(srcdir)/rust \
 	-I $(srcdir)/rust/typecheck \
 	-I $(srcdir)/rust/checks/lints \
 	-I $(srcdir)/rust/checks/errors \
+    -I $(srcdir)/rust/checks/lints/unused \
 	-I $(srcdir)/rust/checks/errors/privacy \
 	-I $(srcdir)/rust/checks/errors/borrowck \
 	-I $(srcdir)/rust/checks/errors/feature \
@@ -502,6 +506,11 @@ rust/%.o: rust/checks/lints/%.cc
 	$(COMPILE) $(RUST_CXXFLAGS) $(RUST_INCLUDES) $<
 	$(POSTCOMPILE)
 
+# build unused checking pass files in rust folder
+rust/%.o: rust/checks/lints/unused/%.cc
+	$(COMPILE) $(RUST_CXXFLAGS) $(RUST_INCLUDES) $<
+	$(POSTCOMPILE)
+
 # build rust/checks/errors files in rust folder
 rust/%.o: rust/checks/errors/%.cc
 	$(COMPILE) $(RUST_CXXFLAGS) $(RUST_INCLUDES) $<

gcc/rust/checks/lints/unused/rust-unused-checker.cc

@@ -0,0 +1,99 @@
+// 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-unused-checker.h"
+#include "rust-hir-expr.h"
+#include "rust-hir-item.h"
+
+#include "options.h"
+
+namespace Rust {
+namespace Analysis {
+UnusedChecker::UnusedChecker ()
+  : nr_context (
+    Resolver2_0::ImmutableNameResolutionContext::get ().resolver ()),
+    mappings (Analysis::Mappings::get ()), unused_context (UnusedContext ())
+{}
+void
+UnusedChecker::go (HIR::Crate &crate)
+{
+  UnusedCollector collector (unused_context);
+  collector.go (crate);
+  for (auto &item : crate.get_items ())
+    item->accept_vis (*this);
+}
+void
+UnusedChecker::visit (HIR::ConstantItem &item)
+{
+  std::string var_name = item.get_identifier ().as_string ();
+  bool starts_with_under_score = var_name.compare (0, 1, "_") == 0;
+  auto id = item.get_mappings ().get_hirid ();
+  if (!unused_context.is_variable_used (id) && !starts_with_under_score)
+    rust_warning_at (item.get_locus (), OPT_Wunused_variable,
+		     "unused variable %qs",
+		     item.get_identifier ().as_string ().c_str ());
+}
+
+void
+UnusedChecker::visit (HIR::StaticItem &item)
+{
+  std::string var_name = item.get_identifier ().as_string ();
+  bool starts_with_under_score = var_name.compare (0, 1, "_") == 0;
+  auto id = item.get_mappings ().get_hirid ();
+  if (!unused_context.is_variable_used (id) && !starts_with_under_score)
+    rust_warning_at (item.get_locus (), OPT_Wunused_variable,
+		     "unused variable %qs",
+		     item.get_identifier ().as_string ().c_str ());
+}
+
+void
+UnusedChecker::visit (HIR::TraitItemFunc &item)
+{
+  // TODO: check trait item functions if they are not derived.
+}
+void
+UnusedChecker::visit (HIR::IdentifierPattern &pattern)
+{
+  std::string var_name = pattern.get_identifier ().as_string ();
+  bool starts_with_under_score = var_name.compare (0, 1, "_") == 0;
+  auto id = pattern.get_mappings ().get_hirid ();
+  if (!unused_context.is_variable_used (id) && var_name != "self"
+      && !starts_with_under_score)
+    rust_warning_at (pattern.get_locus (), OPT_Wunused_variable,
+		     "unused variable %qs",
+		     pattern.get_identifier ().as_string ().c_str ());
+}
+void
+
+UnusedChecker::visit (HIR::AssignmentExpr &expr)
+
+{
+  const auto &lhs = expr.get_lhs ();
+  auto s = lhs.as_string ();
+  std::string var_name = s.substr (0, s.find (':'));
+  bool starts_with_under_score = var_name.compare (0, 1, "_") == 0;
+  NodeId ast_node_id = lhs.get_mappings ().get_nodeid ();
+  NodeId def_id = nr_context.lookup (ast_node_id).value ();
+  HirId id = mappings.lookup_node_to_hir (def_id).value ();
+  if (unused_context.is_variable_assigned (id, lhs.get_mappings ().get_hirid ())
+      && !starts_with_under_score)
+    rust_warning_at (lhs.get_locus (), OPT_Wunused_variable,
+		     "unused assignment %qs", var_name.c_str ());
+}
+} // namespace Analysis
+} // namespace Rust

gcc/rust/checks/lints/unused/rust-unused-checker.h

@@ -0,0 +1,47 @@
+// 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-hir-expr.h"
+#include "rust-hir-item.h"
+#include "rust-hir-pattern.h"
+#include "rust-hir-visitor.h"
+#include "rust-unused-collector.h"
+#include "rust-immutable-name-resolution-context.h"
+
+namespace Rust {
+namespace Analysis {
+class UnusedChecker : public HIR::DefaultHIRVisitor
+{
+public:
+  UnusedChecker ();
+  void go (HIR::Crate &crate);
+
+private:
+  const Resolver2_0::NameResolutionContext &nr_context;
+  Analysis::Mappings &mappings;
+  UnusedContext unused_context;
+
+  using HIR::DefaultHIRVisitor::visit;
+  virtual void visit (HIR::TraitItemFunc &decl) override;
+  virtual void visit (HIR::ConstantItem &item) override;
+  virtual void visit (HIR::StaticItem &item) override;
+  virtual void visit (HIR::IdentifierPattern &identifier) override;
+  virtual void visit (HIR::AssignmentExpr &identifier) override;
+};
+} // namespace Analysis
+} // namespace Rust

gcc/rust/checks/lints/unused/rust-unused-collector.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-unused-collector.h"
+#include "rust-hir-expr.h"
+#include "rust-hir-full-decls.h"
+#include "rust-hir-item.h"
+#include "rust-hir-path.h"
+#include "rust-hir-pattern.h"
+#include "rust-immutable-name-resolution-context.h"
+
+namespace Rust {
+namespace Analysis {
+UnusedCollector::UnusedCollector (UnusedContext &context)
+  : nr_context (
+    Resolver2_0::ImmutableNameResolutionContext::get ().resolver ()),
+    mappings (Analysis::Mappings::get ()), unused_context (context)
+{}
+void
+UnusedCollector::go (HIR::Crate &crate)
+{
+  for (auto &item : crate.get_items ())
+    item->accept_vis (*this);
+}
+
+void
+UnusedCollector::visit (HIR::PathInExpression &expr)
+{
+  mark_path_used (expr);
+  walk (expr);
+}
+
+void
+UnusedCollector::visit (HIR::QualifiedPathInExpression &expr)
+{
+  mark_path_used (expr);
+  walk (expr);
+}
+
+void
+UnusedCollector::visit (HIR::StructExprFieldIdentifier &ident)
+{
+  mark_path_used (ident);
+  walk (ident);
+}
+void
+UnusedCollector::visit (HIR::AssignmentExpr &expr)
+{
+  auto def_id = get_def_id (expr.get_lhs ());
+  HirId id = expr.get_lhs ().get_mappings ().get_hirid ();
+  unused_context.add_assign (def_id, id);
+  visit_outer_attrs (expr);
+  expr.get_rhs ().accept_vis (*this);
+}
+
+} // namespace Analysis
+} // namespace Rust

gcc/rust/checks/lints/unused/rust-unused-collector.h

@@ -0,0 +1,62 @@
+// 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-hir-expr.h"
+#include "rust-hir-path.h"
+#include "rust-hir-pattern.h"
+#include "rust-hir-visitor.h"
+#include "rust-mapping-common.h"
+#include "rust-name-resolution-context.h"
+#include "rust-unused-context.h"
+
+namespace Rust {
+namespace Analysis {
+class UnusedCollector : public HIR::DefaultHIRVisitor
+{
+public:
+  UnusedCollector (UnusedContext &context);
+  void go (HIR::Crate &crate);
+
+private:
+  const Resolver2_0::NameResolutionContext &nr_context;
+  Analysis::Mappings &mappings;
+  UnusedContext &unused_context;
+
+  using HIR::DefaultHIRVisitor::visit;
+  virtual void visit (HIR::PathInExpression &expr) override;
+  virtual void visit (HIR::StructExprFieldIdentifier &ident) override;
+  virtual void visit (HIR::QualifiedPathInExpression &expr) override;
+  virtual void visit (HIR::AssignmentExpr &expr) override;
+
+  template <typename T> HirId get_def_id (T &path_expr)
+  {
+    NodeId ast_node_id = path_expr.get_mappings ().get_nodeid ();
+    NodeId id = nr_context.lookup (ast_node_id).value ();
+    HirId def_id = mappings.lookup_node_to_hir (id).value ();
+    return def_id;
+  }
+
+  template <typename T> void mark_path_used (T &path_expr)
+  {
+    auto def_id = get_def_id (path_expr);
+    unused_context.add_variable (def_id);
+    unused_context.remove_assign (def_id);
+  }
+};
+} // namespace Analysis
+} // namespace Rust

gcc/rust/checks/lints/unused/rust-unused-context.cc

@@ -0,0 +1,70 @@
+// 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-unused-context.h"
+
+namespace Rust {
+namespace Analysis {
+
+void
+UnusedContext::add_variable (HirId id)
+
+{
+  used_vars.emplace (id);
+}
+
+bool
+UnusedContext::is_variable_used (HirId id) const
+{
+  return used_vars.find (id) != used_vars.end ();
+}
+
+void
+UnusedContext::add_assign (HirId id_def, HirId id)
+{
+  assigned_vars[id_def].push_back (id);
+}
+
+void
+UnusedContext::remove_assign (HirId id_def)
+{
+  if (assigned_vars.find (id_def) != assigned_vars.end ())
+    assigned_vars[id_def].pop_back ();
+}
+bool
+UnusedContext::is_variable_assigned (HirId id_def, HirId id)
+{
+  auto assigned_vec = assigned_vars[id_def];
+  return std::find (assigned_vec.begin (), assigned_vec.end (), id)
+	 != assigned_vec.end ();
+}
+
+std::string
+UnusedContext::as_string () const
+{
+  std::stringstream ss;
+  ss << "UnusedContext: ";
+  for (const auto &v : used_vars)
+    {
+      ss << "HirId: " << v << "\n";
+    }
+  return ss.str ();
+}
+
+} // namespace Analysis
+} // namespace Rust