Merge #1419

1419: Const checker r=CohenArthur a=CohenArthur

This commit serves as a base to ensure that only the allowed subset of
the language is called from const contexts. For example, this limits
function calls to calls to const functions only when in a const context

You can skip reviewing b4864c7e5a59a19fea6203c4b581889d4d6c20a1 as it's the exact same commit cherry-picked from #1415 

Co-authored-by: Arthur Cohen <arthur.cohen@embecosm.com>

11 files changed, 1129 insertions, 11 deletions

diff --git a/gcc/rust/Make-lang.in b/gcc/rust/Make-lang.in
index 1ae9bd7..78c3d0f 100644
--- a/gcc/rust/Make-lang.in
+++ b/gcc/rust/Make-lang.in
@@ -120,6 +120,7 @@ GRS_OBJS = \
     rust/rust-hir-type-check-base.o \
     rust/rust-autoderef.o \
     rust/rust-substitution-mapper.o \
+    rust/rust-const-checker.o \
     rust/rust-lint-marklive.o \
     rust/rust-lint-unused-var.o \
     rust/rust-hir-type-check-path.o \
diff --git a/gcc/rust/checks/errors/rust-const-checker.cc b/gcc/rust/checks/errors/rust-const-checker.cc
new file mode 100644
index 0000000..bb6ae10
--- /dev/null
+++ b/gcc/rust/checks/errors/rust-const-checker.cc
@@ -0,0 +1,803 @@
+// Copyright (C) 2020-2022 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-const-checker.h"
+#include "rust-hir.h"
+#include "rust-hir-expr.h"
+#include "rust-hir-stmt.h"
+#include "rust-hir-item.h"
+
+namespace Rust {
+namespace HIR {
+
+ConstChecker::ConstChecker ()
+  : resolver (*Resolver::Resolver::get ()),
+    mappings (*Analysis::Mappings::get ())
+{}
+
+void
+ConstChecker::go (HIR::Crate &crate)
+{
+  for (auto &item : crate.items)
+    item->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (IdentifierExpr &ident_expr)
+{}
+
+void
+ConstChecker::visit (Lifetime &lifetime)
+{}
+
+void
+ConstChecker::visit (LifetimeParam &lifetime_param)
+{}
+
+void
+ConstChecker::visit (PathInExpression &path)
+{}
+
+void
+ConstChecker::visit (TypePathSegment &segment)
+{}
+
+void
+ConstChecker::visit (TypePathSegmentGeneric &segment)
+{}
+
+void
+ConstChecker::visit (TypePathSegmentFunction &segment)
+{}
+
+void
+ConstChecker::visit (TypePath &path)
+{}
+
+void
+ConstChecker::visit (QualifiedPathInExpression &path)
+{}
+
+void
+ConstChecker::visit (QualifiedPathInType &path)
+{}
+
+void
+ConstChecker::visit (LiteralExpr &expr)
+{}
+
+void
+ConstChecker::visit (BorrowExpr &expr)
+{
+  expr.get_expr ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (DereferenceExpr &expr)
+{
+  expr.get_expr ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (ErrorPropagationExpr &expr)
+{
+  expr.get_expr ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (NegationExpr &expr)
+{
+  expr.get_expr ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (ArithmeticOrLogicalExpr &expr)
+{
+  expr.get_lhs ()->accept_vis (*this);
+  expr.get_rhs ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (ComparisonExpr &expr)
+{
+  expr.get_lhs ()->accept_vis (*this);
+  expr.get_rhs ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (LazyBooleanExpr &expr)
+{
+  expr.get_lhs ()->accept_vis (*this);
+  expr.get_rhs ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (TypeCastExpr &expr)
+{
+  expr.get_expr ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (AssignmentExpr &expr)
+{
+  expr.get_lhs ()->accept_vis (*this);
+  expr.get_rhs ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (CompoundAssignmentExpr &expr)
+{
+  expr.get_left_expr ()->accept_vis (*this);
+  expr.get_right_expr ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (GroupedExpr &expr)
+{
+  expr.get_expr_in_parens ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (ArrayElemsValues &elems)
+{
+  for (auto &elem : elems.get_values ())
+    elem->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (ArrayElemsCopied &elems)
+{
+  elems.get_elem_to_copy ()->accept_vis (*this);
+
+  const_context.enter (elems.get_mappings ().get_hirid ());
+
+  elems.get_num_copies_expr ()->accept_vis (*this);
+
+  const_context.exit ();
+}
+
+void
+ConstChecker::visit (ArrayExpr &expr)
+{
+  expr.get_internal_elements ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (ArrayIndexExpr &expr)
+{
+  expr.get_array_expr ()->accept_vis (*this);
+  expr.get_index_expr ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (TupleExpr &expr)
+{
+  for (auto &elem : expr.get_tuple_elems ())
+    elem->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (TupleIndexExpr &expr)
+{
+  expr.get_tuple_expr ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (StructExprStruct &expr)
+{}
+
+void
+ConstChecker::visit (StructExprFieldIdentifier &field)
+{}
+
+void
+ConstChecker::visit (StructExprFieldIdentifierValue &field)
+{
+  field.get_value ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (StructExprFieldIndexValue &field)
+{
+  field.get_value ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (StructExprStructFields &expr)
+{
+  for (auto &field : expr.get_fields ())
+    field->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (StructExprStructBase &expr)
+{}
+
+void
+ConstChecker::check_function_call (HirId fn_id, Location locus)
+{
+  if (!const_context.is_in_context ())
+    return;
+
+  auto maybe_fn = mappings.lookup_hir_item (fn_id);
+  if (!maybe_fn || maybe_fn->get_item_kind () != Item::ItemKind::Function)
+    return;
+
+  auto fn = static_cast<Function *> (maybe_fn);
+  if (!fn->get_qualifiers ().is_const ())
+    rust_error_at (locus, "only functions marked as %<const%> are allowed to "
+			  "be called from constant contexts");
+}
+
+void
+ConstChecker::visit (CallExpr &expr)
+{
+  auto fn = expr.get_fnexpr ();
+  if (!fn)
+    return;
+
+  NodeId ast_node_id = fn->get_mappings ().get_nodeid ();
+  NodeId ref_node_id;
+  HirId definition_id;
+
+  // We don't care about types here
+  if (!resolver.lookup_resolved_name (ast_node_id, &ref_node_id))
+    return;
+
+  rust_assert (mappings.lookup_node_to_hir (ref_node_id, &definition_id));
+
+  check_function_call (definition_id, expr.get_locus ());
+
+  for (auto &arg : expr.get_arguments ())
+    arg->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (MethodCallExpr &expr)
+{
+  expr.get_receiver ()->accept_vis (*this);
+
+  for (auto &arg : expr.get_arguments ())
+    arg->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (FieldAccessExpr &expr)
+{
+  expr.get_receiver_expr ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (ClosureExprInner &expr)
+{}
+
+void
+ConstChecker::visit (BlockExpr &expr)
+{
+  for (auto &stmt : expr.get_statements ())
+    stmt->accept_vis (*this);
+
+  if (expr.has_expr ())
+    expr.get_final_expr ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (ClosureExprInnerTyped &expr)
+{}
+
+void
+ConstChecker::visit (ContinueExpr &expr)
+{}
+
+void
+ConstChecker::visit (BreakExpr &expr)
+{
+  if (expr.has_break_expr ())
+    expr.get_expr ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (RangeFromToExpr &expr)
+{
+  expr.get_from_expr ()->accept_vis (*this);
+  expr.get_to_expr ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (RangeFromExpr &expr)
+{
+  expr.get_from_expr ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (RangeToExpr &expr)
+{
+  expr.get_to_expr ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (RangeFullExpr &expr)
+{}
+
+void
+ConstChecker::visit (RangeFromToInclExpr &expr)
+{
+  expr.get_from_expr ()->accept_vis (*this);
+  expr.get_to_expr ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (RangeToInclExpr &expr)
+{
+  // FIXME: Visit to_expr
+}
+
+void
+ConstChecker::visit (ReturnExpr &expr)
+{
+  if (expr.has_return_expr ())
+    expr.get_expr ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (UnsafeBlockExpr &expr)
+{
+  expr.get_block_expr ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (LoopExpr &expr)
+{
+  expr.get_loop_block ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (WhileLoopExpr &expr)
+{
+  expr.get_predicate_expr ()->accept_vis (*this);
+  expr.get_loop_block ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (WhileLetLoopExpr &expr)
+{
+  expr.get_cond ()->accept_vis (*this);
+  expr.get_loop_block ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (ForLoopExpr &expr)
+{
+  expr.get_iterator_expr ()->accept_vis (*this);
+  expr.get_loop_block ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (IfExpr &expr)
+{
+  expr.get_if_condition ()->accept_vis (*this);
+  expr.get_if_block ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (IfExprConseqElse &expr)
+{
+  expr.get_if_condition ()->accept_vis (*this);
+  expr.get_if_block ()->accept_vis (*this);
+  expr.get_else_block ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (IfExprConseqIf &expr)
+{
+  expr.get_if_condition ()->accept_vis (*this);
+  expr.get_if_block ()->accept_vis (*this);
+  expr.get_conseq_if_expr ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (IfExprConseqIfLet &expr)
+{
+  expr.get_if_condition ()->accept_vis (*this);
+  expr.get_if_block ()->accept_vis (*this);
+
+  // TODO: Visit conseq if let expression
+}
+
+void
+ConstChecker::visit (IfLetExpr &expr)
+{
+  expr.get_scrutinee_expr ()->accept_vis (*this);
+  expr.get_if_block ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (IfLetExprConseqElse &expr)
+{
+  expr.get_scrutinee_expr ()->accept_vis (*this);
+  expr.get_if_block ()->accept_vis (*this);
+
+  // TODO: Visit else expression
+}
+
+void
+ConstChecker::visit (IfLetExprConseqIf &expr)
+{
+  expr.get_scrutinee_expr ()->accept_vis (*this);
+  expr.get_if_block ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (IfLetExprConseqIfLet &expr)
+{
+  expr.get_scrutinee_expr ()->accept_vis (*this);
+  expr.get_if_block ()->accept_vis (*this);
+
+  // TODO: Visit conseq if let expression
+}
+
+void
+ConstChecker::visit (MatchExpr &expr)
+{
+  expr.get_scrutinee_expr ()->accept_vis (*this);
+
+  for (auto &match_arm : expr.get_match_cases ())
+    match_arm.get_expr ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (AwaitExpr &expr)
+{
+  // TODO: Visit expression
+}
+
+void
+ConstChecker::visit (AsyncBlockExpr &expr)
+{
+  // TODO: Visit block expression
+}
+
+void
+ConstChecker::visit (TypeParam &param)
+{}
+
+void
+ConstChecker::visit (ConstGenericParam &param)
+{}
+
+void
+ConstChecker::visit (LifetimeWhereClauseItem &item)
+{}
+
+void
+ConstChecker::visit (TypeBoundWhereClauseItem &item)
+{}
+
+void
+ConstChecker::visit (Module &module)
+{
+  for (auto &item : module.get_items ())
+    item->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (ExternCrate &crate)
+{}
+
+void
+ConstChecker::visit (UseTreeGlob &use_tree)
+{}
+
+void
+ConstChecker::visit (UseTreeList &use_tree)
+{}
+
+void
+ConstChecker::visit (UseTreeRebind &use_tree)
+{}
+
+void
+ConstChecker::visit (UseDeclaration &use_decl)
+{}
+
+void
+ConstChecker::visit (Function &function)
+{
+  auto const_fn = function.get_qualifiers ().is_const ();
+  if (const_fn)
+    const_context.enter (function.get_mappings ().get_hirid ());
+
+  function.get_definition ()->accept_vis (*this);
+
+  if (const_fn)
+    const_context.exit ();
+}
+
+void
+ConstChecker::visit (TypeAlias &type_alias)
+{}
+
+void
+ConstChecker::visit (StructStruct &struct_item)
+{}
+
+void
+ConstChecker::visit (TupleStruct &tuple_struct)
+{}
+
+void
+ConstChecker::visit (EnumItem &item)
+{}
+
+void
+ConstChecker::visit (EnumItemTuple &item)
+{}
+
+void
+ConstChecker::visit (EnumItemStruct &item)
+{}
+
+void
+ConstChecker::visit (EnumItemDiscriminant &item)
+{
+  const_context.enter (item.get_mappings ().get_hirid ());
+
+  item.get_discriminant_expression ()->accept_vis (*this);
+
+  const_context.exit ();
+}
+
+void
+ConstChecker::visit (Enum &enum_item)
+{}
+
+void
+ConstChecker::visit (Union &union_item)
+{}
+
+void
+ConstChecker::visit (ConstantItem &const_item)
+{
+  const_context.enter (const_item.get_mappings ().get_hirid ());
+
+  const_item.get_expr ()->accept_vis (*this);
+
+  const_context.exit ();
+}
+
+void
+ConstChecker::visit (StaticItem &static_item)
+{
+  const_context.enter (static_item.get_mappings ().get_hirid ());
+
+  static_item.get_expr ()->accept_vis (*this);
+
+  const_context.exit ();
+}
+
+void
+ConstChecker::visit (TraitItemFunc &item)
+{
+  if (item.has_block_defined ())
+    item.get_block_expr ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (TraitItemConst &item)
+{
+  if (item.has_expr ())
+    item.get_expr ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (TraitItemType &item)
+{}
+
+void
+ConstChecker::visit (Trait &trait)
+{
+  for (auto &item : trait.get_trait_items ())
+    item->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (ImplBlock &impl)
+{
+  for (auto &item : impl.get_impl_items ())
+    item->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (ExternalStaticItem &item)
+{}
+
+void
+ConstChecker::visit (ExternalFunctionItem &item)
+{}
+
+void
+ConstChecker::visit (ExternBlock &block)
+{
+  // FIXME: Do we need to do this?
+  for (auto &item : block.get_extern_items ())
+    item->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (LiteralPattern &pattern)
+{}
+
+void
+ConstChecker::visit (IdentifierPattern &pattern)
+{}
+
+void
+ConstChecker::visit (WildcardPattern &pattern)
+{}
+
+void
+ConstChecker::visit (RangePatternBoundLiteral &bound)
+{}
+
+void
+ConstChecker::visit (RangePatternBoundPath &bound)
+{}
+
+void
+ConstChecker::visit (RangePatternBoundQualPath &bound)
+{}
+
+void
+ConstChecker::visit (RangePattern &pattern)
+{}
+
+void
+ConstChecker::visit (ReferencePattern &pattern)
+{}
+
+void
+ConstChecker::visit (StructPatternFieldTuplePat &field)
+{}
+
+void
+ConstChecker::visit (StructPatternFieldIdentPat &field)
+{}
+
+void
+ConstChecker::visit (StructPatternFieldIdent &field)
+{}
+
+void
+ConstChecker::visit (StructPattern &pattern)
+{}
+
+void
+ConstChecker::visit (TupleStructItemsNoRange &tuple_items)
+{}
+
+void
+ConstChecker::visit (TupleStructItemsRange &tuple_items)
+{}
+
+void
+ConstChecker::visit (TupleStructPattern &pattern)
+{}
+
+void
+ConstChecker::visit (TuplePatternItemsMultiple &tuple_items)
+{}
+
+void
+ConstChecker::visit (TuplePatternItemsRanged &tuple_items)
+{}
+
+void
+ConstChecker::visit (TuplePattern &pattern)
+{}
+
+void
+ConstChecker::visit (GroupedPattern &pattern)
+{}
+
+void
+ConstChecker::visit (SlicePattern &pattern)
+{}
+
+void
+ConstChecker::visit (EmptyStmt &stmt)
+{}
+
+void
+ConstChecker::visit (LetStmt &stmt)
+{
+  if (stmt.has_init_expr ())
+    stmt.get_init_expr ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (ExprStmtWithoutBlock &stmt)
+{
+  stmt.get_expr ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (ExprStmtWithBlock &stmt)
+{
+  stmt.get_expr ()->accept_vis (*this);
+}
+
+void
+ConstChecker::visit (TraitBound &bound)
+{}
+
+void
+ConstChecker::visit (ImplTraitType &type)
+{}
+
+void
+ConstChecker::visit (TraitObjectType &type)
+{}
+
+void
+ConstChecker::visit (ParenthesisedType &type)
+{}
+
+void
+ConstChecker::visit (ImplTraitTypeOneBound &type)
+{}
+
+void
+ConstChecker::visit (TupleType &type)
+{}
+
+void
+ConstChecker::visit (NeverType &type)
+{}
+
+void
+ConstChecker::visit (RawPointerType &type)
+{}
+
+void
+ConstChecker::visit (ReferenceType &type)
+{}
+
+void
+ConstChecker::visit (ArrayType &type)
+{
+  const_context.enter (type.get_mappings ().get_hirid ());
+
+  type.get_size_expr ()->accept_vis (*this);
+
+  const_context.exit ();
+}
+
+void
+ConstChecker::visit (SliceType &type)
+{}
+
+void
+ConstChecker::visit (InferredType &type)
+{}
+
+void
+ConstChecker::visit (BareFunctionType &type)
+{}
+
+} // namespace HIR
+} // namespace Rust
diff --git a/gcc/rust/checks/errors/rust-const-checker.h b/gcc/rust/checks/errors/rust-const-checker.h
new file mode 100644
index 0000000..a474fc8
--- /dev/null
+++ b/gcc/rust/checks/errors/rust-const-checker.h
@@ -0,0 +1,183 @@
+// Copyright (C) 2020-2022 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_CONST_CHECKER_H
+#define RUST_CONST_CHECKER_H
+
+#include "rust-hir-visitor.h"
+#include "rust-hir-type-check.h"
+#include "rust-stacked-contexts.h"
+#include "rust-name-resolver.h"
+
+namespace Rust {
+namespace HIR {
+class ConstChecker : public HIRFullVisitor
+{
+public:
+  ConstChecker ();
+
+  void go (HIR::Crate &crate);
+
+private:
+  /**
+   * Check that only const functions are called in const contexts
+   */
+  void check_function_call (HirId fn_id, Location locus);
+
+  StackedContexts<HirId> const_context;
+  Resolver::Resolver &resolver;
+  Analysis::Mappings &mappings;
+
+  virtual void visit (IdentifierExpr &ident_expr) override;
+  virtual void visit (Lifetime &lifetime) override;
+  virtual void visit (LifetimeParam &lifetime_param) override;
+  virtual void visit (PathInExpression &path) override;
+  virtual void visit (TypePathSegment &segment) override;
+  virtual void visit (TypePathSegmentGeneric &segment) override;
+  virtual void visit (TypePathSegmentFunction &segment) override;
+  virtual void visit (TypePath &path) override;
+  virtual void visit (QualifiedPathInExpression &path) override;
+  virtual void visit (QualifiedPathInType &path) override;
+  virtual void visit (LiteralExpr &expr) override;
+  virtual void visit (BorrowExpr &expr) override;
+  virtual void visit (DereferenceExpr &expr) override;
+  virtual void visit (ErrorPropagationExpr &expr) override;
+  virtual void visit (NegationExpr &expr) override;
+  virtual void visit (ArithmeticOrLogicalExpr &expr) override;
+  virtual void visit (ComparisonExpr &expr) override;
+  virtual void visit (LazyBooleanExpr &expr) override;
+  virtual void visit (TypeCastExpr &expr) override;
+  virtual void visit (AssignmentExpr &expr) override;
+  virtual void visit (CompoundAssignmentExpr &expr) override;
+  virtual void visit (GroupedExpr &expr) override;
+  virtual void visit (ArrayElemsValues &elems) override;
+  virtual void visit (ArrayElemsCopied &elems) override;
+  virtual void visit (ArrayExpr &expr) override;
+  virtual void visit (ArrayIndexExpr &expr) override;
+  virtual void visit (TupleExpr &expr) override;
+  virtual void visit (TupleIndexExpr &expr) override;
+  virtual void visit (StructExprStruct &expr) override;
+  virtual void visit (StructExprFieldIdentifier &field) override;
+  virtual void visit (StructExprFieldIdentifierValue &field) override;
+  virtual void visit (StructExprFieldIndexValue &field) override;
+  virtual void visit (StructExprStructFields &expr) override;
+  virtual void visit (StructExprStructBase &expr) override;
+  virtual void visit (CallExpr &expr) override;
+  virtual void visit (MethodCallExpr &expr) override;
+  virtual void visit (FieldAccessExpr &expr) override;
+  virtual void visit (ClosureExprInner &expr) override;
+  virtual void visit (BlockExpr &expr) override;
+  virtual void visit (ClosureExprInnerTyped &expr) override;
+  virtual void visit (ContinueExpr &expr) override;
+  virtual void visit (BreakExpr &expr) override;
+  virtual void visit (RangeFromToExpr &expr) override;
+  virtual void visit (RangeFromExpr &expr) override;
+  virtual void visit (RangeToExpr &expr) override;
+  virtual void visit (RangeFullExpr &expr) override;
+  virtual void visit (RangeFromToInclExpr &expr) override;
+  virtual void visit (RangeToInclExpr &expr) override;
+  virtual void visit (ReturnExpr &expr) override;
+  virtual void visit (UnsafeBlockExpr &expr) override;
+  virtual void visit (LoopExpr &expr) override;
+  virtual void visit (WhileLoopExpr &expr) override;
+  virtual void visit (WhileLetLoopExpr &expr) override;
+  virtual void visit (ForLoopExpr &expr) override;
+  virtual void visit (IfExpr &expr) override;
+  virtual void visit (IfExprConseqElse &expr) override;
+  virtual void visit (IfExprConseqIf &expr) override;
+  virtual void visit (IfExprConseqIfLet &expr) override;
+  virtual void visit (IfLetExpr &expr) override;
+  virtual void visit (IfLetExprConseqElse &expr) override;
+  virtual void visit (IfLetExprConseqIf &expr) override;
+  virtual void visit (IfLetExprConseqIfLet &expr) override;
+  virtual void visit (MatchExpr &expr) override;
+  virtual void visit (AwaitExpr &expr) override;
+  virtual void visit (AsyncBlockExpr &expr) override;
+  virtual void visit (TypeParam &param) override;
+  virtual void visit (ConstGenericParam &param) override;
+  virtual void visit (LifetimeWhereClauseItem &item) override;
+  virtual void visit (TypeBoundWhereClauseItem &item) override;
+  virtual void visit (Module &module) override;
+  virtual void visit (ExternCrate &crate) override;
+  virtual void visit (UseTreeGlob &use_tree) override;
+  virtual void visit (UseTreeList &use_tree) override;
+  virtual void visit (UseTreeRebind &use_tree) override;
+  virtual void visit (UseDeclaration &use_decl) override;
+  virtual void visit (Function &function) override;
+  virtual void visit (TypeAlias &type_alias) override;
+  virtual void visit (StructStruct &struct_item) override;
+  virtual void visit (TupleStruct &tuple_struct) override;
+  virtual void visit (EnumItem &item) override;
+  virtual void visit (EnumItemTuple &item) override;
+  virtual void visit (EnumItemStruct &item) override;
+  virtual void visit (EnumItemDiscriminant &item) override;
+  virtual void visit (Enum &enum_item) override;
+  virtual void visit (Union &union_item) override;
+  virtual void visit (ConstantItem &const_item) override;
+  virtual void visit (StaticItem &static_item) override;
+  virtual void visit (TraitItemFunc &item) override;
+  virtual void visit (TraitItemConst &item) override;
+  virtual void visit (TraitItemType &item) override;
+  virtual void visit (Trait &trait) override;
+  virtual void visit (ImplBlock &impl) override;
+  virtual void visit (ExternalStaticItem &item) override;
+  virtual void visit (ExternalFunctionItem &item) override;
+  virtual void visit (ExternBlock &block) override;
+  virtual void visit (LiteralPattern &pattern) override;
+  virtual void visit (IdentifierPattern &pattern) override;
+  virtual void visit (WildcardPattern &pattern) override;
+  virtual void visit (RangePatternBoundLiteral &bound) override;
+  virtual void visit (RangePatternBoundPath &bound) override;
+  virtual void visit (RangePatternBoundQualPath &bound) override;
+  virtual void visit (RangePattern &pattern) override;
+  virtual void visit (ReferencePattern &pattern) override;
+  virtual void visit (StructPatternFieldTuplePat &field) override;
+  virtual void visit (StructPatternFieldIdentPat &field) override;
+  virtual void visit (StructPatternFieldIdent &field) override;
+  virtual void visit (StructPattern &pattern) override;
+  virtual void visit (TupleStructItemsNoRange &tuple_items) override;
+  virtual void visit (TupleStructItemsRange &tuple_items) override;
+  virtual void visit (TupleStructPattern &pattern) override;
+  virtual void visit (TuplePatternItemsMultiple &tuple_items) override;
+  virtual void visit (TuplePatternItemsRanged &tuple_items) override;
+  virtual void visit (TuplePattern &pattern) override;
+  virtual void visit (GroupedPattern &pattern) override;
+  virtual void visit (SlicePattern &pattern) override;
+  virtual void visit (EmptyStmt &stmt) override;
+  virtual void visit (LetStmt &stmt) override;
+  virtual void visit (ExprStmtWithoutBlock &stmt) override;
+  virtual void visit (ExprStmtWithBlock &stmt) override;
+  virtual void visit (TraitBound &bound) override;
+  virtual void visit (ImplTraitType &type) override;
+  virtual void visit (TraitObjectType &type) override;
+  virtual void visit (ParenthesisedType &type) override;
+  virtual void visit (ImplTraitTypeOneBound &type) override;
+  virtual void visit (TupleType &type) override;
+  virtual void visit (NeverType &type) override;
+  virtual void visit (RawPointerType &type) override;
+  virtual void visit (ReferenceType &type) override;
+  virtual void visit (ArrayType &type) override;
+  virtual void visit (SliceType &type) override;
+  virtual void visit (InferredType &type) override;
+  virtual void visit (BareFunctionType &type) override;
+};
+
+} // namespace HIR
+} // namespace Rust
+
+#endif /* !RUST_CONST_CHECKER_H */
diff --git a/gcc/rust/checks/errors/rust-unsafe-checker.h b/gcc/rust/checks/errors/rust-unsafe-checker.h
index 38b9019..21ec0f4 100644
--- a/gcc/rust/checks/errors/rust-unsafe-checker.h
+++ b/gcc/rust/checks/errors/rust-unsafe-checker.h
@@ -59,8 +59,8 @@ private:
   void check_use_of_static (HirId node_id, Location locus);
 
   Resolver::TypeCheckContext &context;
-  Resolver::Resolver resolver;
-  Analysis::Mappings mappings;
+  Resolver::Resolver &resolver;
+  Analysis::Mappings &mappings;
 
   virtual void visit (IdentifierExpr &ident_expr) override;
   virtual void visit (Lifetime &lifetime) override;
diff --git a/gcc/rust/hir/rust-ast-lower-expr.h b/gcc/rust/hir/rust-ast-lower-expr.h
index 166a44b..b420a4d 100644
--- a/gcc/rust/hir/rust-ast-lower-expr.h
+++ b/gcc/rust/hir/rust-ast-lower-expr.h
@@ -307,7 +307,14 @@ public:
 	elements.push_back (std::unique_ptr<HIR::Expr> (translated_elem));
       }
 
-    translated_array_elems = new HIR::ArrayElemsValues (std::move (elements));
+    auto crate_num = mappings->get_current_crate ();
+    Analysis::NodeMapping mapping (mappings->get_current_crate (),
+				   elems.get_node_id (),
+				   mappings->get_next_hir_id (crate_num),
+				   UNKNOWN_LOCAL_DEFID);
+
+    translated_array_elems
+      = new HIR::ArrayElemsValues (mapping, std::move (elements));
   }
 
   void visit (AST::ArrayElemsCopied &elems) override
@@ -317,8 +324,15 @@ public:
     HIR::Expr *num_copies
       = ASTLoweringExpr::translate (elems.get_num_copies ().get ());
 
+    auto crate_num = mappings->get_current_crate ();
+    Analysis::NodeMapping mapping (mappings->get_current_crate (),
+				   elems.get_node_id (),
+				   mappings->get_next_hir_id (crate_num),
+				   UNKNOWN_LOCAL_DEFID);
+
     translated_array_elems
-      = new HIR::ArrayElemsCopied (std::unique_ptr<HIR::Expr> (element),
+      = new HIR::ArrayElemsCopied (mapping,
+				   std::unique_ptr<HIR::Expr> (element),
 				   std::unique_ptr<HIR::Expr> (num_copies));
   }
 
diff --git a/gcc/rust/hir/tree/rust-hir-expr.h b/gcc/rust/hir/tree/rust-hir-expr.h
index 327a9ac..d16ac92 100644
--- a/gcc/rust/hir/tree/rust-hir-expr.h
+++ b/gcc/rust/hir/tree/rust-hir-expr.h
@@ -849,6 +849,8 @@ public:
     COPIED,
   };
 
+  ArrayElems (Analysis::NodeMapping mappings) : mappings (mappings){};
+
   virtual ~ArrayElems () {}
 
   // Unique pointer custom clone ArrayElems function
@@ -863,9 +865,13 @@ public:
 
   virtual ArrayExprType get_array_expr_type () const = 0;
 
+  Analysis::NodeMapping &get_mappings () { return mappings; }
+
 protected:
   // pure virtual clone implementation
   virtual ArrayElems *clone_array_elems_impl () const = 0;
+
+  Analysis::NodeMapping mappings;
 };
 
 // Value array elements
@@ -876,12 +882,13 @@ class ArrayElemsValues : public ArrayElems
   // TODO: should this store location data?
 
 public:
-  ArrayElemsValues (std::vector<std::unique_ptr<Expr> > elems)
-    : values (std::move (elems))
+  ArrayElemsValues (Analysis::NodeMapping mappings,
+		    std::vector<std::unique_ptr<Expr> > elems)
+    : ArrayElems (mappings), values (std::move (elems))
   {}
 
   // copy constructor with vector clone
-  ArrayElemsValues (ArrayElemsValues const &other)
+  ArrayElemsValues (ArrayElemsValues const &other) : ArrayElems (other)
   {
     values.reserve (other.values.size ());
     for (const auto &e : other.values)
@@ -930,15 +937,16 @@ class ArrayElemsCopied : public ArrayElems
 
 public:
   // Constructor requires pointers for polymorphism
-  ArrayElemsCopied (std::unique_ptr<Expr> copied_elem,
+  ArrayElemsCopied (Analysis::NodeMapping mappings,
+		    std::unique_ptr<Expr> copied_elem,
 		    std::unique_ptr<Expr> copy_amount)
-    : elem_to_copy (std::move (copied_elem)),
+    : ArrayElems (mappings), elem_to_copy (std::move (copied_elem)),
       num_copies (std::move (copy_amount))
   {}
 
   // Copy constructor required due to unique_ptr - uses custom clone
   ArrayElemsCopied (ArrayElemsCopied const &other)
-    : elem_to_copy (other.elem_to_copy->clone_expr ()),
+    : ArrayElems (other), elem_to_copy (other.elem_to_copy->clone_expr ()),
       num_copies (other.num_copies->clone_expr ())
   {}
 
diff --git a/gcc/rust/rust-session-manager.cc b/gcc/rust/rust-session-manager.cc
index 6a2c1b6..e3c211e 100644
--- a/gcc/rust/rust-session-manager.cc
+++ b/gcc/rust/rust-session-manager.cc
@@ -17,8 +17,8 @@
 // <http://www.gnu.org/licenses/>.
 
 #include "rust-session-manager.h"
-#include "rust-unsafe-checker.h"
 #include "rust-diagnostics.h"
+#include "rust-unsafe-checker.h"
 #include "rust-lex.h"
 #include "rust-parse.h"
 #include "rust-macro-expand.h"
@@ -26,6 +26,7 @@
 #include "rust-ast-lower.h"
 #include "rust-hir-type-check.h"
 #include "rust-privacy-check.h"
+#include "rust-const-checker.h"
 #include "rust-tycheck-dump.h"
 #include "rust-compile.h"
 #include "rust-cfg-parser.h"
@@ -796,6 +797,8 @@ Session::parse_file (const char *filename)
     return;
 
   HIR::UnsafeChecker ().go (hir);
+  HIR::ConstChecker ().go (hir);
+
   if (saw_errors ())
     return;
 
diff --git a/gcc/rust/util/rust-stacked-contexts.h b/gcc/rust/util/rust-stacked-contexts.h
new file mode 100644
index 0000000..c34eb90
--- /dev/null
+++ b/gcc/rust/util/rust-stacked-contexts.h
@@ -0,0 +1,86 @@
+// Copyright (C) 2020-2022 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_CONTEXT_STACK_H
+#define RUST_CONTEXT_STACK_H
+
+#include "rust-system.h"
+
+namespace Rust {
+
+/**
+ * Context stack util class. This class is useful for situations where you can
+ * enter the same kind of context multiple times. For example, when dealing with
+ * unsafe contexts, you might be tempted to simply keep a boolean value.
+ *
+ * ```rust
+ * let a = 15;
+ * unsafe { // we set the boolean to true
+ *     // Now unsafe operations are allowed!
+ *     let b = *(&a as *const i32);
+ *     let c = std::mem::transmute<i32, f32>(b); // Urgh!
+ * } // we set it to false
+ * ```
+ *
+ * However, since the language allows nested unsafe blocks, you may run into
+ * this situation:
+ *
+ * ```rust
+ * unsafe { // we set the boolean to true
+ *     unsafe { // we set the boolean to true
+ *     } // we set it to false
+ *
+ *     // Now unsafe operations are forbidden again, the boolean is false
+ *     let f = std::mem::transmute<i32, f32>(15); // Error!
+ * } // we set it to false
+ * ```
+ */
+template <typename T> class StackedContexts
+{
+public:
+  /**
+   * Enter a special context
+   */
+  void enter (T value) { stack.emplace_back (value); }
+
+  /**
+   * Exit a special context
+   */
+  T exit ()
+  {
+    rust_assert (!stack.empty ());
+
+    auto last = stack.back ();
+    stack.pop_back ();
+
+    return last;
+  }
+
+  /**
+   * Are we currently inside of a special context?
+   */
+  bool is_in_context () const { return !stack.empty (); }
+
+private:
+  /* Actual data */
+  std::vector<T> stack;
+};
+
+} // namespace Rust
+
+#endif /* !RUST_CONTEXT_STACK_H */
diff --git a/gcc/testsuite/rust/compile/const1.rs b/gcc/testsuite/rust/compile/const1.rs
new file mode 100644
index 0000000..5f19c67
--- /dev/null
+++ b/gcc/testsuite/rust/compile/const1.rs
@@ -0,0 +1,6 @@
+fn bar() {}
+
+const fn foo() {
+    bar(); // { dg-error "only functions marked as .const. are allowed to be called from constant contexts" }
+}
+
diff --git a/gcc/testsuite/rust/compile/const2.rs b/gcc/testsuite/rust/compile/const2.rs
new file mode 100644
index 0000000..17b6de5
--- /dev/null
+++ b/gcc/testsuite/rust/compile/const2.rs
@@ -0,0 +1,7 @@
+// { dg-additional-options "-w" }
+
+const fn foo() {
+    const fn bar() {}
+
+    bar();
+}
diff --git a/gcc/testsuite/rust/compile/const3.rs b/gcc/testsuite/rust/compile/const3.rs
new file mode 100644
index 0000000..22dc3d3
--- /dev/null
+++ b/gcc/testsuite/rust/compile/const3.rs
@@ -0,0 +1,7 @@
+fn size() -> usize {
+    15
+}
+
+fn main() {
+    let a = [15; size()]; // { dg-error "only functions marked as .const. are allowed to be called from constant contexts" }
+}