gccrs: Add HIR to GCC GENERIC lowering entry point

This patch contains the entry point and utilities used for the lowering
of HIR nodes to `tree`s. It also contains a constant evaluator, ported
over from the C++ frontend.

	gcc/rust/
	* backend/rust-compile-context.cc: New.
	* backend/rust-compile-context.h: New.
	* backend/rust-compile.cc: New.
	* backend/rust-compile.h: New.
	* backend/rust-constexpr.cc: New.
	* backend/rust-constexpr.h: New.

Co-authored-by: David Faust <david.faust@oracle.com>
Co-authored-by: Faisal Abbas <90.abbasfaisal@gmail.com>
Signed-off-by: Faisal Abbas <90.abbasfaisal@gmail.com>

6 files changed, 1414 insertions, 0 deletions

diff --git a/gcc/rust/backend/rust-compile-context.cc b/gcc/rust/backend/rust-compile-context.cc
new file mode 100644
index 00000000000..cb2addf6c21
--- /dev/null
+++ b/gcc/rust/backend/rust-compile-context.cc
@@ -0,0 +1,146 @@
+// 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-compile-context.h"
+#include "rust-compile-type.h"
+
+namespace Rust {
+namespace Compile {
+
+Context::Context (::Backend *backend)
+  : backend (backend), resolver (Resolver::Resolver::get ()),
+    tyctx (Resolver::TypeCheckContext::get ()),
+    mappings (Analysis::Mappings::get ()), mangler (Mangler ())
+{
+  setup_builtins ();
+}
+
+void
+Context::setup_builtins ()
+{
+  auto builtins = resolver->get_builtin_types ();
+  for (auto it = builtins.begin (); it != builtins.end (); it++)
+    {
+      HirId ref;
+      bool ok = tyctx->lookup_type_by_node_id ((*it)->get_node_id (), &ref);
+      rust_assert (ok);
+
+      TyTy::BaseType *lookup;
+      ok = tyctx->lookup_type (ref, &lookup);
+      rust_assert (ok);
+
+      TyTyResolveCompile::compile (this, lookup);
+    }
+}
+
+hashval_t
+Context::type_hasher (tree type)
+{
+  inchash::hash hstate;
+
+  hstate.add_int (TREE_CODE (type));
+
+  if (TYPE_NAME (type))
+    {
+      hashval_t record_name_hash
+	= IDENTIFIER_HASH_VALUE (DECL_NAME (TYPE_NAME (type)));
+      hstate.add_object (record_name_hash);
+    }
+
+  for (tree t = TYPE_ATTRIBUTES (type); t; t = TREE_CHAIN (t))
+    /* Just the identifier is adequate to distinguish.  */
+    hstate.add_object (IDENTIFIER_HASH_VALUE (TREE_PURPOSE (t)));
+
+  switch (TREE_CODE (type))
+    {
+    case METHOD_TYPE:
+      hstate.add_object (TYPE_HASH (TYPE_METHOD_BASETYPE (type)));
+      /* FALLTHROUGH. */
+    case FUNCTION_TYPE:
+      for (tree t = TYPE_ARG_TYPES (type); t; t = TREE_CHAIN (t))
+	if (TREE_VALUE (t) != error_mark_node)
+	  hstate.add_object (TYPE_HASH (TREE_VALUE (t)));
+      break;
+
+    case OFFSET_TYPE:
+      hstate.add_object (TYPE_HASH (TYPE_OFFSET_BASETYPE (type)));
+      break;
+
+      case ARRAY_TYPE: {
+	if (TYPE_DOMAIN (type))
+	  hstate.add_object (TYPE_HASH (TYPE_DOMAIN (type)));
+	if (!AGGREGATE_TYPE_P (TREE_TYPE (type)))
+	  {
+	    unsigned typeless = TYPE_TYPELESS_STORAGE (type);
+	    hstate.add_object (typeless);
+	  }
+      }
+      break;
+
+      case INTEGER_TYPE: {
+	tree t = TYPE_MAX_VALUE (type);
+	if (!t)
+	  t = TYPE_MIN_VALUE (type);
+	for (int i = 0; i < TREE_INT_CST_NUNITS (t); i++)
+	  hstate.add_object (TREE_INT_CST_ELT (t, i));
+	break;
+      }
+
+    case REAL_TYPE:
+      case FIXED_POINT_TYPE: {
+	unsigned prec = TYPE_PRECISION (type);
+	hstate.add_object (prec);
+	break;
+      }
+
+    case VECTOR_TYPE:
+      hstate.add_poly_int (TYPE_VECTOR_SUBPARTS (type));
+      break;
+
+    case RECORD_TYPE:
+    case UNION_TYPE:
+      case QUAL_UNION_TYPE: {
+	for (tree t = TYPE_FIELDS (type); t; t = TREE_CHAIN (t))
+	  {
+	    hashval_t name_hash = IDENTIFIER_HASH_VALUE (DECL_NAME (t));
+	    hashval_t type_hash = type_hasher (TREE_TYPE (t));
+	    hstate.add_object (name_hash);
+	    hstate.add_object (type_hash);
+	  }
+      }
+      break;
+
+    case BOOLEAN_TYPE:
+      break;
+
+    case REFERENCE_TYPE:
+      case POINTER_TYPE: {
+	hashval_t type_hash = type_hasher (TREE_TYPE (type));
+	hstate.add_object (type_hash);
+      }
+      break;
+
+    default:
+      break;
+    }
+
+  return hstate.end ();
+}
+
+} // namespace Compile
+} // namespace Rust
diff --git a/gcc/rust/backend/rust-compile-context.h b/gcc/rust/backend/rust-compile-context.h
new file mode 100644
index 00000000000..096b65f8b39
--- /dev/null
+++ b/gcc/rust/backend/rust-compile-context.h
@@ -0,0 +1,343 @@
+// 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_COMPILE_CONTEXT
+#define RUST_COMPILE_CONTEXT
+
+#include "rust-system.h"
+#include "rust-hir-map.h"
+#include "rust-name-resolver.h"
+#include "rust-hir-type-check.h"
+#include "rust-backend.h"
+#include "rust-hir-full.h"
+#include "rust-mangle.h"
+#include "rust-tree.h"
+
+namespace Rust {
+namespace Compile {
+
+struct fncontext
+{
+  tree fndecl;
+  ::Bvariable *ret_addr;
+};
+
+class Context
+{
+public:
+  Context (::Backend *backend);
+
+  void setup_builtins ();
+
+  bool lookup_compiled_types (tree t, tree *type)
+  {
+    hashval_t h = type_hasher (t);
+    auto it = compiled_type_map.find (h);
+    if (it == compiled_type_map.end ())
+      return false;
+
+    *type = it->second;
+    return true;
+  }
+
+  tree insert_compiled_type (tree type)
+  {
+    hashval_t h = type_hasher (type);
+    auto it = compiled_type_map.find (h);
+    if (it != compiled_type_map.end ())
+      return it->second;
+
+    compiled_type_map.insert ({h, type});
+    push_type (type);
+    return type;
+  }
+
+  tree insert_main_variant (tree type)
+  {
+    hashval_t h = type_hasher (type);
+    auto it = main_variants.find (h);
+    if (it != main_variants.end ())
+      return it->second;
+
+    main_variants.insert ({h, type});
+    return type;
+  }
+
+  ::Backend *get_backend () { return backend; }
+  Resolver::Resolver *get_resolver () { return resolver; }
+  Resolver::TypeCheckContext *get_tyctx () { return tyctx; }
+  Analysis::Mappings *get_mappings () { return mappings; }
+
+  void push_block (tree scope)
+  {
+    scope_stack.push_back (scope);
+    statements.push_back ({});
+  }
+
+  tree pop_block ()
+  {
+    auto block = scope_stack.back ();
+    scope_stack.pop_back ();
+
+    auto stmts = statements.back ();
+    statements.pop_back ();
+
+    backend->block_add_statements (block, stmts);
+
+    return block;
+  }
+
+  tree peek_enclosing_scope ()
+  {
+    if (scope_stack.size () == 0)
+      return nullptr;
+
+    return scope_stack.back ();
+  }
+
+  void add_statement_to_enclosing_scope (tree stmt)
+  {
+    statements.at (statements.size () - 2).push_back (stmt);
+  }
+
+  void add_statement (tree stmt) { statements.back ().push_back (stmt); }
+
+  void insert_var_decl (HirId id, ::Bvariable *decl)
+  {
+    compiled_var_decls[id] = decl;
+  }
+
+  bool lookup_var_decl (HirId id, ::Bvariable **decl)
+  {
+    auto it = compiled_var_decls.find (id);
+    if (it == compiled_var_decls.end ())
+      return false;
+
+    *decl = it->second;
+    return true;
+  }
+
+  void insert_function_decl (const TyTy::FnType *ref, tree fn)
+  {
+    auto id = ref->get_ty_ref ();
+    auto dId = ref->get_id ();
+
+    rust_assert (compiled_fn_map.find (id) == compiled_fn_map.end ());
+    compiled_fn_map[id] = fn;
+
+    auto it = mono_fns.find (dId);
+    if (it == mono_fns.end ())
+      mono_fns[dId] = {};
+
+    mono_fns[dId].push_back ({ref, fn});
+  }
+
+  bool lookup_function_decl (HirId id, tree *fn, DefId dId = UNKNOWN_DEFID,
+			     const TyTy::BaseType *ref = nullptr)
+  {
+    // for for any monomorphized fns
+    if (ref != nullptr)
+      {
+	rust_assert (dId != UNKNOWN_DEFID);
+
+	auto it = mono_fns.find (dId);
+	if (it == mono_fns.end ())
+	  return false;
+
+	for (auto &e : mono_fns[dId])
+	  {
+	    const TyTy::BaseType *r = e.first;
+	    tree f = e.second;
+	    if (ref->is_equal (*r))
+	      {
+		*fn = f;
+		return true;
+	      }
+	  }
+	return false;
+      }
+
+    auto it = compiled_fn_map.find (id);
+    if (it == compiled_fn_map.end ())
+      return false;
+
+    *fn = it->second;
+    return true;
+  }
+
+  void insert_const_decl (HirId id, tree expr) { compiled_consts[id] = expr; }
+
+  bool lookup_const_decl (HirId id, tree *expr)
+  {
+    auto it = compiled_consts.find (id);
+    if (it == compiled_consts.end ())
+      return false;
+
+    *expr = it->second;
+    return true;
+  }
+
+  void insert_label_decl (HirId id, tree label) { compiled_labels[id] = label; }
+
+  bool lookup_label_decl (HirId id, tree *label)
+  {
+    auto it = compiled_labels.find (id);
+    if (it == compiled_labels.end ())
+      return false;
+
+    *label = it->second;
+    return true;
+  }
+
+  void insert_pattern_binding (HirId id, tree binding)
+  {
+    implicit_pattern_bindings[id] = binding;
+  }
+
+  bool lookup_pattern_binding (HirId id, tree *binding)
+  {
+    auto it = implicit_pattern_bindings.find (id);
+    if (it == implicit_pattern_bindings.end ())
+      return false;
+
+    *binding = it->second;
+    return true;
+  }
+
+  void push_fn (tree fn, ::Bvariable *ret_addr)
+  {
+    fn_stack.push_back (fncontext{fn, ret_addr});
+  }
+  void pop_fn () { fn_stack.pop_back (); }
+
+  bool in_fn () { return fn_stack.size () != 0; }
+
+  // Note: it is undefined behavior to call peek_fn () if fn_stack is empty.
+  fncontext peek_fn ()
+  {
+    rust_assert (!fn_stack.empty ());
+    return fn_stack.back ();
+  }
+
+  void push_type (tree t) { type_decls.push_back (t); }
+  void push_var (::Bvariable *v) { var_decls.push_back (v); }
+  void push_const (tree c) { const_decls.push_back (c); }
+  void push_function (tree f) { func_decls.push_back (f); }
+
+  void write_to_backend ()
+  {
+    backend->write_global_definitions (type_decls, const_decls, func_decls,
+				       var_decls);
+  }
+
+  bool function_completed (tree fn)
+  {
+    for (auto it = func_decls.begin (); it != func_decls.end (); it++)
+      {
+	tree i = (*it);
+	if (i == fn)
+	  {
+	    return true;
+	  }
+      }
+    return false;
+  }
+
+  void push_loop_context (Bvariable *var) { loop_value_stack.push_back (var); }
+
+  Bvariable *peek_loop_context () { return loop_value_stack.back (); }
+
+  Bvariable *pop_loop_context ()
+  {
+    auto back = loop_value_stack.back ();
+    loop_value_stack.pop_back ();
+    return back;
+  }
+
+  void push_loop_begin_label (tree label)
+  {
+    loop_begin_labels.push_back (label);
+  }
+
+  tree peek_loop_begin_label () { return loop_begin_labels.back (); }
+
+  tree pop_loop_begin_label ()
+  {
+    tree pop = loop_begin_labels.back ();
+    loop_begin_labels.pop_back ();
+    return pop;
+  }
+
+  void push_const_context (void) { const_context++; }
+  void pop_const_context (void)
+  {
+    if (const_context > 0)
+      const_context--;
+  }
+  bool const_context_p (void) { return (const_context > 0); }
+
+  std::string mangle_item (const TyTy::BaseType *ty,
+			   const Resolver::CanonicalPath &path) const
+  {
+    return mangler.mangle_item (ty, path);
+  }
+
+  std::vector<tree> &get_type_decls () { return type_decls; }
+  std::vector<::Bvariable *> &get_var_decls () { return var_decls; }
+  std::vector<tree> &get_const_decls () { return const_decls; }
+  std::vector<tree> &get_func_decls () { return func_decls; }
+
+  static hashval_t type_hasher (tree type);
+
+private:
+  ::Backend *backend;
+  Resolver::Resolver *resolver;
+  Resolver::TypeCheckContext *tyctx;
+  Analysis::Mappings *mappings;
+  Mangler mangler;
+
+  // state
+  std::vector<fncontext> fn_stack;
+  std::map<HirId, ::Bvariable *> compiled_var_decls;
+  std::map<hashval_t, tree> compiled_type_map;
+  std::map<HirId, tree> compiled_fn_map;
+  std::map<HirId, tree> compiled_consts;
+  std::map<HirId, tree> compiled_labels;
+  std::vector<::std::vector<tree>> statements;
+  std::vector<tree> scope_stack;
+  std::vector<::Bvariable *> loop_value_stack;
+  std::vector<tree> loop_begin_labels;
+  std::map<DefId, std::vector<std::pair<const TyTy::BaseType *, tree>>>
+    mono_fns;
+  std::map<HirId, tree> implicit_pattern_bindings;
+  std::map<hashval_t, tree> main_variants;
+
+  // To GCC middle-end
+  std::vector<tree> type_decls;
+  std::vector<::Bvariable *> var_decls;
+  std::vector<tree> const_decls;
+  std::vector<tree> func_decls;
+
+  // Nonzero iff we are currently compiling something inside a constant context.
+  unsigned int const_context = 0;
+};
+
+} // namespace Compile
+} // namespace Rust
+
+#endif // RUST_COMPILE_CONTEXT
diff --git a/gcc/rust/backend/rust-compile.cc b/gcc/rust/backend/rust-compile.cc
new file mode 100644
index 00000000000..0ccb98d9e12
--- /dev/null
+++ b/gcc/rust/backend/rust-compile.cc
@@ -0,0 +1,414 @@
+// 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-compile.h"
+#include "rust-compile-item.h"
+#include "rust-compile-implitem.h"
+#include "rust-compile-expr.h"
+#include "rust-compile-struct-field-expr.h"
+#include "rust-compile-stmt.h"
+#include "rust-hir-trait-resolve.h"
+#include "rust-hir-path-probe.h"
+#include "rust-hir-type-bounds.h"
+#include "rust-hir-dot-operator.h"
+#include "rust-compile-block.h"
+
+namespace Rust {
+namespace Compile {
+
+CompileCrate::CompileCrate (HIR::Crate &crate, Context *ctx)
+  : crate (crate), ctx (ctx)
+{}
+
+CompileCrate::~CompileCrate () {}
+
+void
+CompileCrate::Compile (HIR::Crate &crate, Context *ctx)
+{
+  CompileCrate c (crate, ctx);
+  c.go ();
+}
+
+void
+CompileCrate::go ()
+{
+  for (auto &item : crate.items)
+    CompileItem::compile (item.get (), ctx);
+}
+
+// Shared methods in compilation
+
+tree
+HIRCompileBase::coercion_site (HirId id, tree rvalue,
+			       const TyTy::BaseType *rval,
+			       const TyTy::BaseType *lval,
+			       Location lvalue_locus, Location rvalue_locus)
+{
+  std::vector<Resolver::Adjustment> *adjustments = nullptr;
+  bool ok = ctx->get_tyctx ()->lookup_autoderef_mappings (id, &adjustments);
+  if (ok)
+    {
+      rvalue = resolve_adjustements (*adjustments, rvalue, rvalue_locus);
+    }
+
+  return coercion_site1 (rvalue, rval, lval, lvalue_locus, rvalue_locus);
+}
+
+tree
+HIRCompileBase::coercion_site1 (tree rvalue, const TyTy::BaseType *rval,
+				const TyTy::BaseType *lval,
+				Location lvalue_locus, Location rvalue_locus)
+{
+  if (rvalue == error_mark_node)
+    return error_mark_node;
+
+  const TyTy::BaseType *actual = rval->destructure ();
+  const TyTy::BaseType *expected = lval->destructure ();
+
+  if (expected->get_kind () == TyTy::TypeKind::REF)
+    {
+      // this is a dyn object
+      if (SLICE_TYPE_P (TREE_TYPE (rvalue)))
+	{
+	  return rvalue;
+	}
+
+      // bad coercion... of something to a reference
+      if (actual->get_kind () != TyTy::TypeKind::REF)
+	return error_mark_node;
+
+      const TyTy::ReferenceType *exp
+	= static_cast<const TyTy::ReferenceType *> (expected);
+      const TyTy::ReferenceType *act
+	= static_cast<const TyTy::ReferenceType *> (actual);
+
+      tree deref_rvalue = indirect_expression (rvalue, rvalue_locus);
+      tree coerced
+	= coercion_site1 (deref_rvalue, act->get_base (), exp->get_base (),
+			  lvalue_locus, rvalue_locus);
+      if (exp->is_dyn_object () && SLICE_TYPE_P (TREE_TYPE (coerced)))
+	return coerced;
+
+      return address_expression (coerced, rvalue_locus);
+    }
+  else if (expected->get_kind () == TyTy::TypeKind::POINTER)
+    {
+      // this is a dyn object
+      if (SLICE_TYPE_P (TREE_TYPE (rvalue)))
+	{
+	  return rvalue;
+	}
+
+      // bad coercion... of something to a reference
+      bool valid_coercion = actual->get_kind () == TyTy::TypeKind::REF
+			    || actual->get_kind () == TyTy::TypeKind::POINTER;
+      if (!valid_coercion)
+	return error_mark_node;
+
+      const TyTy::ReferenceType *exp
+	= static_cast<const TyTy::ReferenceType *> (expected);
+
+      TyTy::BaseType *actual_base = nullptr;
+      if (actual->get_kind () == TyTy::TypeKind::REF)
+	{
+	  const TyTy::ReferenceType *act
+	    = static_cast<const TyTy::ReferenceType *> (actual);
+
+	  actual_base = act->get_base ();
+	}
+      else if (actual->get_kind () == TyTy::TypeKind::POINTER)
+	{
+	  const TyTy::PointerType *act
+	    = static_cast<const TyTy::PointerType *> (actual);
+
+	  actual_base = act->get_base ();
+	}
+      rust_assert (actual_base != nullptr);
+
+      tree deref_rvalue = indirect_expression (rvalue, rvalue_locus);
+      tree coerced
+	= coercion_site1 (deref_rvalue, actual_base, exp->get_base (),
+			  lvalue_locus, rvalue_locus);
+
+      if (exp->is_dyn_object () && SLICE_TYPE_P (TREE_TYPE (coerced)))
+	return coerced;
+
+      return address_expression (coerced, rvalue_locus);
+    }
+  else if (expected->get_kind () == TyTy::TypeKind::ARRAY)
+    {
+      if (actual->get_kind () != TyTy::TypeKind::ARRAY)
+	return error_mark_node;
+
+      tree tree_rval_type = TyTyResolveCompile::compile (ctx, actual);
+      tree tree_lval_type = TyTyResolveCompile::compile (ctx, expected);
+      if (!verify_array_capacities (tree_lval_type, tree_rval_type,
+				    lvalue_locus, rvalue_locus))
+	return error_mark_node;
+    }
+  else if (expected->get_kind () == TyTy::TypeKind::SLICE)
+    {
+      // bad coercion
+      bool valid_coercion = actual->get_kind () == TyTy::TypeKind::SLICE
+			    || actual->get_kind () == TyTy::TypeKind::ARRAY;
+      if (!valid_coercion)
+	return error_mark_node;
+
+      // nothing to do here
+      if (actual->get_kind () == TyTy::TypeKind::SLICE)
+	return rvalue;
+
+      // return an unsized coercion
+      Resolver::Adjustment unsize_adj (
+	Resolver::Adjustment::AdjustmentType::UNSIZE, actual, expected);
+      return resolve_unsized_adjustment (unsize_adj, rvalue, rvalue_locus);
+    }
+
+  return rvalue;
+}
+
+tree
+HIRCompileBase::coerce_to_dyn_object (tree compiled_ref,
+				      const TyTy::BaseType *actual,
+				      const TyTy::DynamicObjectType *ty,
+				      Location locus)
+{
+  tree dynamic_object = TyTyResolveCompile::compile (ctx, ty);
+  tree dynamic_object_fields = TYPE_FIELDS (dynamic_object);
+  tree vtable_field = DECL_CHAIN (dynamic_object_fields);
+  rust_assert (TREE_CODE (TREE_TYPE (vtable_field)) == ARRAY_TYPE);
+
+  //' this assumes ordering and current the structure is
+  // __trait_object_ptr
+  // [list of function ptrs]
+
+  std::vector<std::pair<Resolver::TraitReference *, HIR::ImplBlock *>>
+    probed_bounds_for_receiver = Resolver::TypeBoundsProbe::Probe (actual);
+
+  tree address_of_compiled_ref = null_pointer_node;
+  if (!actual->is_unit ())
+    address_of_compiled_ref = address_expression (compiled_ref, locus);
+
+  std::vector<tree> vtable_ctor_elems;
+  std::vector<unsigned long> vtable_ctor_idx;
+  unsigned long i = 0;
+  for (auto &bound : ty->get_object_items ())
+    {
+      const Resolver::TraitItemReference *item = bound.first;
+      const TyTy::TypeBoundPredicate *predicate = bound.second;
+
+      auto address = compute_address_for_trait_item (item, predicate,
+						     probed_bounds_for_receiver,
+						     actual, actual, locus);
+      vtable_ctor_elems.push_back (address);
+      vtable_ctor_idx.push_back (i++);
+    }
+
+  tree vtable_ctor = ctx->get_backend ()->array_constructor_expression (
+    TREE_TYPE (vtable_field), vtable_ctor_idx, vtable_ctor_elems, locus);
+
+  std::vector<tree> dyn_ctor = {address_of_compiled_ref, vtable_ctor};
+  return ctx->get_backend ()->constructor_expression (dynamic_object, false,
+						      dyn_ctor, -1, locus);
+}
+
+tree
+HIRCompileBase::compute_address_for_trait_item (
+  const Resolver::TraitItemReference *ref,
+  const TyTy::TypeBoundPredicate *predicate,
+  std::vector<std::pair<Resolver::TraitReference *, HIR::ImplBlock *>>
+    &receiver_bounds,
+  const TyTy::BaseType *receiver, const TyTy::BaseType *root, Location locus)
+{
+  // There are two cases here one where its an item which has an implementation
+  // within a trait-impl-block. Then there is the case where there is a default
+  // implementation for this within the trait.
+  //
+  // The awkward part here is that this might be a generic trait and we need to
+  // figure out the correct monomorphized type for this so we can resolve the
+  // address of the function , this is stored as part of the
+  // type-bound-predicate
+  //
+  // Algo:
+  // check if there is an impl-item for this trait-item-ref first
+  // else assert that the trait-item-ref has an implementation
+
+  TyTy::TypeBoundPredicateItem predicate_item
+    = predicate->lookup_associated_item (ref->get_identifier ());
+  rust_assert (!predicate_item.is_error ());
+
+  // this is the expected end type
+  TyTy::BaseType *trait_item_type = predicate_item.get_tyty_for_receiver (root);
+  rust_assert (trait_item_type->get_kind () == TyTy::TypeKind::FNDEF);
+  TyTy::FnType *trait_item_fntype
+    = static_cast<TyTy::FnType *> (trait_item_type);
+
+  // find impl-block for this trait-item-ref
+  HIR::ImplBlock *associated_impl_block = nullptr;
+  const Resolver::TraitReference *predicate_trait_ref = predicate->get ();
+  for (auto &item : receiver_bounds)
+    {
+      Resolver::TraitReference *trait_ref = item.first;
+      HIR::ImplBlock *impl_block = item.second;
+      if (predicate_trait_ref->is_equal (*trait_ref))
+	{
+	  associated_impl_block = impl_block;
+	  break;
+	}
+    }
+
+  // FIXME this probably should just return error_mark_node but this helps
+  // debug for now since we are wrongly returning early on type-resolution
+  // failures, until we take advantage of more error types and error_mark_node
+  rust_assert (associated_impl_block != nullptr);
+
+  // lookup self for the associated impl
+  std::unique_ptr<HIR::Type> &self_type_path
+    = associated_impl_block->get_type ();
+  TyTy::BaseType *self = nullptr;
+  bool ok = ctx->get_tyctx ()->lookup_type (
+    self_type_path->get_mappings ().get_hirid (), &self);
+  rust_assert (ok);
+
+  // lookup the predicate item from the self
+  TyTy::TypeBoundPredicate *self_bound = nullptr;
+  for (auto &bound : self->get_specified_bounds ())
+    {
+      const Resolver::TraitReference *bound_ref = bound.get ();
+      const Resolver::TraitReference *specified_ref = predicate->get ();
+      if (bound_ref->is_equal (*specified_ref))
+	{
+	  self_bound = &bound;
+	  break;
+	}
+    }
+  rust_assert (self_bound != nullptr);
+
+  // lookup the associated item from the associated impl block
+  TyTy::TypeBoundPredicateItem associated_self_item
+    = self_bound->lookup_associated_item (ref->get_identifier ());
+  rust_assert (!associated_self_item.is_error ());
+
+  TyTy::BaseType *mono1 = associated_self_item.get_tyty_for_receiver (self);
+  rust_assert (mono1 != nullptr);
+  rust_assert (mono1->get_kind () == TyTy::TypeKind::FNDEF);
+  TyTy::FnType *assocated_item_ty1 = static_cast<TyTy::FnType *> (mono1);
+
+  // Lookup the impl-block for the associated impl_item if it exists
+  HIR::Function *associated_function = nullptr;
+  for (auto &impl_item : associated_impl_block->get_impl_items ())
+    {
+      bool is_function = impl_item->get_impl_item_type ()
+			 == HIR::ImplItem::ImplItemType::FUNCTION;
+      if (!is_function)
+	continue;
+
+      HIR::Function *fn = static_cast<HIR::Function *> (impl_item.get ());
+      bool found_associated_item
+	= fn->get_function_name ().compare (ref->get_identifier ()) == 0;
+      if (found_associated_item)
+	associated_function = fn;
+    }
+
+  // we found an impl_item for this
+  if (associated_function != nullptr)
+    {
+      // lookup the associated type for this item
+      TyTy::BaseType *lookup = nullptr;
+      bool ok = ctx->get_tyctx ()->lookup_type (
+	associated_function->get_mappings ().get_hirid (), &lookup);
+      rust_assert (ok);
+      rust_assert (lookup->get_kind () == TyTy::TypeKind::FNDEF);
+      TyTy::FnType *lookup_fntype = static_cast<TyTy::FnType *> (lookup);
+
+      if (lookup_fntype->needs_substitution ())
+	{
+	  TyTy::SubstitutionArgumentMappings mappings
+	    = assocated_item_ty1->solve_missing_mappings_from_this (
+	      *trait_item_fntype, *lookup_fntype);
+	  lookup_fntype = lookup_fntype->handle_substitions (mappings);
+	}
+
+      return CompileInherentImplItem::Compile (associated_function, ctx,
+					       lookup_fntype, true, locus);
+    }
+
+  // we can only compile trait-items with a body
+  bool trait_item_has_definition = ref->is_optional ();
+  rust_assert (trait_item_has_definition);
+
+  HIR::TraitItem *trait_item = ref->get_hir_trait_item ();
+  return CompileTraitItem::Compile (trait_item, ctx, trait_item_fntype, true,
+				    locus);
+}
+
+bool
+HIRCompileBase::verify_array_capacities (tree ltype, tree rtype,
+					 Location lvalue_locus,
+					 Location rvalue_locus)
+{
+  rust_assert (ltype != NULL_TREE);
+  rust_assert (rtype != NULL_TREE);
+
+  // lets just return ok as other errors have already occurred
+  if (ltype == error_mark_node || rtype == error_mark_node)
+    return true;
+
+  tree ltype_domain = TYPE_DOMAIN (ltype);
+  if (!ltype_domain)
+    return false;
+
+  if (!TREE_CONSTANT (TYPE_MAX_VALUE (ltype_domain)))
+    return false;
+
+  unsigned HOST_WIDE_INT ltype_length
+    = wi::ext (wi::to_offset (TYPE_MAX_VALUE (ltype_domain))
+		 - wi::to_offset (TYPE_MIN_VALUE (ltype_domain)) + 1,
+	       TYPE_PRECISION (TREE_TYPE (ltype_domain)),
+	       TYPE_SIGN (TREE_TYPE (ltype_domain)))
+	.to_uhwi ();
+
+  tree rtype_domain = TYPE_DOMAIN (rtype);
+  if (!rtype_domain)
+    return false;
+
+  if (!TREE_CONSTANT (TYPE_MAX_VALUE (rtype_domain)))
+    return false;
+
+  unsigned HOST_WIDE_INT rtype_length
+    = wi::ext (wi::to_offset (TYPE_MAX_VALUE (rtype_domain))
+		 - wi::to_offset (TYPE_MIN_VALUE (rtype_domain)) + 1,
+	       TYPE_PRECISION (TREE_TYPE (rtype_domain)),
+	       TYPE_SIGN (TREE_TYPE (rtype_domain)))
+	.to_uhwi ();
+
+  if (ltype_length != rtype_length)
+    {
+      rust_error_at (
+	rvalue_locus,
+	"expected an array with a fixed size of " HOST_WIDE_INT_PRINT_UNSIGNED
+	" elements, found one with " HOST_WIDE_INT_PRINT_UNSIGNED " elements",
+	ltype_length, rtype_length);
+      return false;
+    }
+
+  return true;
+}
+
+} // namespace Compile
+} // namespace Rust
diff --git a/gcc/rust/backend/rust-compile.h b/gcc/rust/backend/rust-compile.h
new file mode 100644
index 00000000000..62ebac69cc1
--- /dev/null
+++ b/gcc/rust/backend/rust-compile.h
@@ -0,0 +1,47 @@
+// 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_COMPILE_H
+#define RUST_COMPILE_H
+
+#include "rust-system.h"
+#include "rust-hir-full.h"
+#include "rust-compile-context.h"
+
+namespace Rust {
+namespace Compile {
+
+class CompileCrate
+{
+public:
+  static void Compile (HIR::Crate &crate, Context *ctx);
+
+  ~CompileCrate ();
+
+private:
+  CompileCrate (HIR::Crate &crate, Context *ctx);
+  void go ();
+
+  HIR::Crate &crate;
+  Context *ctx;
+};
+
+} // namespace Compile
+} // namespace Rust
+
+#endif // RUST_COMPILE_H
diff --git a/gcc/rust/backend/rust-constexpr.cc b/gcc/rust/backend/rust-constexpr.cc
new file mode 100644
index 00000000000..5aa10d92e15
--- /dev/null
+++ b/gcc/rust/backend/rust-constexpr.cc
@@ -0,0 +1,433 @@
+// 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-constexpr.h"
+#include "rust-location.h"
+#include "rust-diagnostics.h"
+#include "rust-tree.h"
+
+#include "fold-const.h"
+#include "realmpfr.h"
+#include "convert.h"
+#include "print-tree.h"
+#include "gimplify.h"
+#include "tree-iterator.h"
+
+namespace Rust {
+namespace Compile {
+
+struct constexpr_global_ctx
+{
+  HOST_WIDE_INT constexpr_ops_count;
+
+  constexpr_global_ctx () : constexpr_ops_count (0) {}
+};
+
+struct constexpr_ctx
+{
+  constexpr_global_ctx *global;
+};
+
+static tree
+constant_value_1 (tree decl, bool strict_p, bool return_aggregate_cst_ok_p,
+		  bool unshare_p);
+tree
+decl_constant_value (tree decl, bool unshare_p);
+
+static void
+non_const_var_error (location_t loc, tree r);
+
+static tree
+constexpr_expression (const constexpr_ctx *ctx, tree);
+
+static tree
+constexpr_fn_retval (const constexpr_ctx *ctx, tree r);
+
+static tree
+eval_store_expression (const constexpr_ctx *ctx, tree r);
+
+static tree
+eval_call_expression (const constexpr_ctx *ctx, tree r);
+
+static tree
+eval_binary_expression (const constexpr_ctx *ctx, tree r);
+
+static tree
+get_function_named_in_call (tree t);
+
+tree
+fold_expr (tree expr)
+{
+  constexpr_global_ctx global_ctx;
+  constexpr_ctx ctx = {&global_ctx};
+
+  tree folded = constexpr_expression (&ctx, expr);
+  rust_assert (folded != NULL_TREE);
+  return folded;
+}
+
+static tree
+constexpr_expression (const constexpr_ctx *ctx, tree t)
+{
+  location_t loc = EXPR_LOCATION (t);
+
+  if (CONSTANT_CLASS_P (t))
+    {
+      if (TREE_OVERFLOW (t))
+	{
+	  error_at (loc, "overflow in constant expression");
+	  return t;
+	}
+
+      return t;
+    }
+
+  // Avoid excessively long constexpr evaluations
+  if (++ctx->global->constexpr_ops_count >= constexpr_ops_limit)
+    {
+      rust_error_at (
+	Location (loc),
+	"%<constexpr%> evaluation operation count exceeds limit of "
+	"%wd (use %<-fconstexpr-ops-limit=%> to increase the limit)",
+	constexpr_ops_limit);
+
+      return t;
+    }
+
+  tree r = t;
+  tree_code tcode = TREE_CODE (t);
+  switch (tcode)
+    {
+      case CONST_DECL: {
+	r = decl_constant_value (t, /*unshare_p=*/false);
+	if (TREE_CODE (r) == TARGET_EXPR
+	    && TREE_CODE (TARGET_EXPR_INITIAL (r)) == CONSTRUCTOR)
+	  r = TARGET_EXPR_INITIAL (r);
+	if (DECL_P (r))
+	  {
+	    non_const_var_error (loc, r);
+	    return r;
+	  }
+      }
+      break;
+
+    case POINTER_PLUS_EXPR:
+    case POINTER_DIFF_EXPR:
+    case PLUS_EXPR:
+    case MINUS_EXPR:
+    case MULT_EXPR:
+    case TRUNC_DIV_EXPR:
+    case CEIL_DIV_EXPR:
+    case FLOOR_DIV_EXPR:
+    case ROUND_DIV_EXPR:
+    case TRUNC_MOD_EXPR:
+    case CEIL_MOD_EXPR:
+    case ROUND_MOD_EXPR:
+    case RDIV_EXPR:
+    case EXACT_DIV_EXPR:
+    case MIN_EXPR:
+    case MAX_EXPR:
+    case LSHIFT_EXPR:
+    case RSHIFT_EXPR:
+    case LROTATE_EXPR:
+    case RROTATE_EXPR:
+    case BIT_IOR_EXPR:
+    case BIT_XOR_EXPR:
+    case BIT_AND_EXPR:
+    case TRUTH_XOR_EXPR:
+    case LT_EXPR:
+    case LE_EXPR:
+    case GT_EXPR:
+    case GE_EXPR:
+    case EQ_EXPR:
+    case NE_EXPR:
+    case SPACESHIP_EXPR:
+    case UNORDERED_EXPR:
+    case ORDERED_EXPR:
+    case UNLT_EXPR:
+    case UNLE_EXPR:
+    case UNGT_EXPR:
+    case UNGE_EXPR:
+    case UNEQ_EXPR:
+    case LTGT_EXPR:
+    case RANGE_EXPR:
+    case COMPLEX_EXPR:
+      r = eval_binary_expression (ctx, t);
+      break;
+
+    case CALL_EXPR:
+      r = eval_call_expression (ctx, t);
+      break;
+
+    case RETURN_EXPR:
+      rust_assert (TREE_OPERAND (t, 0) != NULL_TREE);
+      r = constexpr_expression (ctx, TREE_OPERAND (t, 0));
+      break;
+
+    case MODIFY_EXPR:
+      r = eval_store_expression (ctx, t);
+      break;
+
+    default:
+      break;
+    }
+
+  return r;
+}
+
+static tree
+eval_store_expression (const constexpr_ctx *ctx, tree t)
+{
+  tree init = TREE_OPERAND (t, 1);
+  if (TREE_CLOBBER_P (init))
+    /* Just ignore clobbers.  */
+    return void_node;
+
+  /* First we figure out where we're storing to.  */
+  tree target = TREE_OPERAND (t, 0);
+
+  tree type = TREE_TYPE (target);
+  bool preeval = SCALAR_TYPE_P (type) || TREE_CODE (t) == MODIFY_EXPR;
+  if (preeval)
+    {
+      /* Evaluate the value to be stored without knowing what object it will be
+	 stored in, so that any side-effects happen first.  */
+      init = fold_expr (init);
+    }
+
+  bool evaluated = false;
+  tree object = NULL_TREE;
+  for (tree probe = target; object == NULL_TREE;)
+    {
+      switch (TREE_CODE (probe))
+	{
+	default:
+	  if (evaluated)
+	    object = probe;
+	  else
+	    {
+	      probe = constexpr_expression (ctx, probe);
+	      evaluated = true;
+	    }
+	  break;
+	}
+    }
+
+  return init;
+}
+
+/* Subroutine of cxx_eval_constant_expression.
+ Like cxx_eval_unary_expression, except for binary expressions.  */
+static tree
+eval_binary_expression (const constexpr_ctx *ctx, tree t)
+{
+  tree orig_lhs = TREE_OPERAND (t, 0);
+  tree orig_rhs = TREE_OPERAND (t, 1);
+  tree lhs, rhs;
+
+  lhs = constexpr_expression (ctx, orig_lhs);
+  rhs = constexpr_expression (ctx, orig_rhs);
+
+  location_t loc = EXPR_LOCATION (t);
+  enum tree_code code = TREE_CODE (t);
+  tree type = TREE_TYPE (t);
+
+  return fold_binary_loc (loc, code, type, lhs, rhs);
+}
+
+// Subroutine of cxx_eval_constant_expression.
+// Evaluate the call expression tree T in the context of OLD_CALL expression
+// evaluation.
+static tree
+eval_call_expression (const constexpr_ctx *ctx, tree t)
+{
+  tree fun = get_function_named_in_call (t);
+  return constexpr_fn_retval (ctx, DECL_SAVED_TREE (fun));
+}
+
+// Subroutine of check_constexpr_fundef.  BODY is the body of a function
+// declared to be constexpr, or a sub-statement thereof.  Returns the
+// return value if suitable, error_mark_node for a statement not allowed in
+// a constexpr function, or NULL_TREE if no return value was found.
+static tree
+constexpr_fn_retval (const constexpr_ctx *ctx, tree body)
+{
+  switch (TREE_CODE (body))
+    {
+      case STATEMENT_LIST: {
+	tree expr = NULL_TREE;
+	for (tree stmt : tsi_range (body))
+	  {
+	    tree s = constexpr_fn_retval (ctx, stmt);
+	    if (s == error_mark_node)
+	      return error_mark_node;
+	    else if (s == NULL_TREE)
+	      /* Keep iterating.  */;
+	    else if (expr)
+	      /* Multiple return statements.  */
+	      return error_mark_node;
+	    else
+	      expr = s;
+	  }
+	return expr;
+      }
+
+    case RETURN_EXPR:
+      return constexpr_expression (ctx, body);
+
+      case DECL_EXPR: {
+	tree decl = DECL_EXPR_DECL (body);
+	if (TREE_CODE (decl) == USING_DECL
+	    /* Accept __func__, __FUNCTION__, and __PRETTY_FUNCTION__.  */
+	    || DECL_ARTIFICIAL (decl))
+	  return NULL_TREE;
+	return error_mark_node;
+      }
+
+    case CLEANUP_POINT_EXPR:
+      return constexpr_fn_retval (ctx, TREE_OPERAND (body, 0));
+
+      case BIND_EXPR: {
+	tree b = BIND_EXPR_BODY (body);
+	return constexpr_fn_retval (ctx, b);
+      }
+      break;
+
+    default:
+      return error_mark_node;
+    }
+  return error_mark_node;
+}
+
+// Taken from cp/constexpr.cc
+//
+// If DECL is a scalar enumeration constant or variable with a
+// constant initializer, return the initializer (or, its initializers,
+// recursively); otherwise, return DECL.  If STRICT_P, the
+// initializer is only returned if DECL is a
+// constant-expression.  If RETURN_AGGREGATE_CST_OK_P, it is ok to
+// return an aggregate constant.  If UNSHARE_P, return an unshared
+// copy of the initializer.
+static tree
+constant_value_1 (tree decl, bool strict_p, bool return_aggregate_cst_ok_p,
+		  bool unshare_p)
+{
+  while (TREE_CODE (decl) == CONST_DECL)
+    {
+      tree init;
+      /* If DECL is a static data member in a template
+	 specialization, we must instantiate it here.  The
+	 initializer for the static data member is not processed
+	 until needed; we need it now.  */
+
+      init = DECL_INITIAL (decl);
+      if (init == error_mark_node)
+	{
+	  if (TREE_CODE (decl) == CONST_DECL)
+	    /* Treat the error as a constant to avoid cascading errors on
+	       excessively recursive template instantiation (c++/9335).  */
+	    return init;
+	  else
+	    return decl;
+	}
+
+      decl = init;
+    }
+  return unshare_p ? unshare_expr (decl) : decl;
+}
+
+// A more relaxed version of decl_really_constant_value, used by the
+// common C/C++ code.
+tree
+decl_constant_value (tree decl, bool unshare_p)
+{
+  return constant_value_1 (decl, /*strict_p=*/false,
+			   /*return_aggregate_cst_ok_p=*/true,
+			   /*unshare_p=*/unshare_p);
+}
+
+static void
+non_const_var_error (location_t loc, tree r)
+{
+  error_at (loc,
+	    "the value of %qD is not usable in a constant "
+	    "expression",
+	    r);
+  /* Avoid error cascade.  */
+  if (DECL_INITIAL (r) == error_mark_node)
+    return;
+
+  // more in cp/constexpr.cc
+}
+
+static tree
+get_callee (tree call)
+{
+  if (call == NULL_TREE)
+    return call;
+  else if (TREE_CODE (call) == CALL_EXPR)
+    return CALL_EXPR_FN (call);
+
+  return NULL_TREE;
+}
+
+// We have an expression tree T that represents a call, either CALL_EXPR
+// or AGGR_INIT_EXPR. If the call is lexically to a named function,
+// return the _DECL for that function.
+static tree
+get_function_named_in_call (tree t)
+{
+  tree fun = get_callee (t);
+  if (fun && TREE_CODE (fun) == ADDR_EXPR
+      && TREE_CODE (TREE_OPERAND (fun, 0)) == FUNCTION_DECL)
+    fun = TREE_OPERAND (fun, 0);
+  return fun;
+}
+
+// forked from gcc/cp/constexpr.cc maybe_constexpr_fn
+
+/* True if a function might be declared constexpr  */
+
+bool
+maybe_constexpr_fn (tree t)
+{
+  return (DECL_DECLARED_CONSTEXPR_P (t));
+}
+
+// forked from gcc/cp/constexpr.cc get_nth_callarg
+
+/* We have an expression tree T that represents a call, either CALL_EXPR.
+  Return the Nth argument.  */
+
+inline tree
+get_nth_callarg (tree t, int n)
+{
+  return CALL_EXPR_ARG (t, n);
+}
+
+// forked from gcc/cp/constexpr.cc var_in_maybe_constexpr_fn
+
+/* True if T was declared in a function that might be constexpr: either a
+   function that was declared constexpr.  */
+
+bool
+var_in_maybe_constexpr_fn (tree t)
+{
+  return (DECL_FUNCTION_SCOPE_P (t) && maybe_constexpr_fn (DECL_CONTEXT (t)));
+}
+
+} // namespace Compile
+} // namespace Rust
diff --git a/gcc/rust/backend/rust-constexpr.h b/gcc/rust/backend/rust-constexpr.h
new file mode 100644
index 00000000000..3cfcec817a9
--- /dev/null
+++ b/gcc/rust/backend/rust-constexpr.h
@@ -0,0 +1,31 @@
+// 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_CONSTEXPR
+#define RUST_CONSTEXPR
+
+#include "rust-system.h"
+#include "tree.h"
+
+namespace Rust {
+namespace Compile {
+
+extern tree fold_expr (tree);
+
+} // namespace Compile
+} // namespace Rust
+
+#endif // RUST_CONSTEXPR