gccrs: const folding port

gccrs: const folding port

This changeset ports cp_global_trees structure which is used throughout the
cp constexpr.cc code. I am not sure what it's purpose is but it seems it is used
to add and manipulate tree information during the c++ compilation process.
It is possible this is not needed in the Rust code and may be taken out later.
Also, the initialization function isn't being called from anywhere yet, so
we will need to find a suitable point for it.

gcc/rust/ChangeLog:
	* backend/rust-tree.cc : Add new includes.
	(scope_chain): New.
	(maybe_add_global): New.
	(init_modules): New (copied from cp)
	(var_in_constexpr_fn): Likewise.
	(member_vec_linear_search): Likewise.
	(member_vec_binary_search): Likewise.
	(is_overloaded_fn): Likewise.
	(ovl_make): Likewise.
	(lookup_add): Likewise.
	(type_memfn_quals): Likewise.
	(struct find_parameter_pack_data): Likewise.
	(struct conv_type_hasher): Likewise.
	(make_conv_op_name): Likewise.
	(builtin_pack_fn_p): Likewise.
	(builtin_pack_call_p): Likewise.
	(has_extra_args_mechanism_p): Likewise.
	(find_parameter_packs_r): Likewise.
	(WALK_SUBTREE): Likewise.
	(type_memfn_rqual): Likewise.
	(maybe_add_lang_type_raw): Likewise.
	(struct c_fileinfo): Likewise.
	(get_fileinfo): Likewise.
	(cxx_make_type): Likewise.
	(build_min_array_type): Likewise.
	(fields_linear_search): Likewise.
	(nothrow_spec_p): Likewise.
	(maybe_get_fns): Likewise.
	(get_fns): Likewise.
	(get_first_fn): Likewise.
	(dependent_name): Likewise.
	(called_fns_equal): Likewise.
	(canonical_eh_spec): Likewise.
	(rs_tree_code_length): Likewise.
	(rs_tree_operand_length): Likewise.
	(rs_tree_equal): Likewise.
	(publicly_uniquely_derived_p): Likewise.
	(comp_except_types): Likewise.
	(comp_except_specs): Likewise.
	(compparms): Likewise.
	(set_array_type_canon): Likewise.
	(struct cplus_array_info): Likewise.
	(struct cplus_array_hasher): Likewise.
	(cplus_array_hasher::hash): Likewise.
	(cplus_array_hasher::equal): Likewise.
	(is_byte_access_type): Likewise.
	(build_cplus_array_type): Likewise.
	(rs_build_qualified_type_real): Likewise.
	(vector_targets_convertible_p): Likewise.
	(comp_array_types): Likewise.
	(same_type_ignoring_top_level_qualifiers_p): Likewise.
	(comp_ptr_ttypes_const): Likewise.
	(similar_type_p): Likewise.
	(structural_comptypes): Likewise.
	(comptypes): Likewise.
	(next_initializable_field): Likewise.
	(sufficient_parms_p): Likewise.
	(default_ctor_p): Likewise.
	(user_provided_p): Likewise.
	(type_has_non_user_provided_default_constructor): Likewise.
	(default_init_uninitialized_part): Likewise.
	(extract_conversion_operator): Likewise.
	(get_class_binding_direct): Likewise.
	(lang_check_failed): Likewise.
	(skip_artificial_parms_for): Likewise.
	(in_class_defaulted_default_constructor): Likewise.
	(is_instantiation_of_constexpr): Likewise.
	(check_for_uninitialized_const_var): Likewise.
	(cv_unqualified): Likewise.
	(make_tree_vector): Likewise.
	(release_tree_vector): Likewise.
	(instantiation_dependent_expression_p): Likewise.
	(cp_get_callee): Likewise.
	(build_nop): Likewise.
	(scalarish_type_p): Likewise.
	(type_has_nontrivial_copy_init): Likewise.
	(build_local_temp): Likewise.
	(is_normal_capture_proxy): Likewise.
	(reject_gcc_builtin): Likewise.
	(is_bitfield_expr_with_lowered_type): Likewise.
	(maybe_undo_parenthesized_ref): Likewise.
	(fold_offsetof): Likewise.
	(char_type_p): Likewise.
	(resolve_nondeduced_context): Likewise.
	(instantiate_non_dependent_or_null): Likewise.
	(resolve_nondeduced_context_or_error): Likewise.
	(really_overloaded_fn): Likewise.
	(invalid_nonstatic_memfn_p): Likewise.
	(strip_top_quals): Likewise.
	(cxx_incomplete_type_inform): Likewise.
	(cxx_incomplete_type_diagnostic): Likewise.
	(decl_constant_var_p): Likewise.
	(undeduced_auto_decl): Likewise.
	(require_deduced_type): Likewise.
	* backend/rust-tree.h (enum c_tree_index): Likewise.
	(GTY): Likewise.
	(enum cp_tree_index): Likewise.
	(wchar_decl_node): Likewise.
	(vtable_entry_type): Likewise.
	(delta_type_node): Likewise.
	(vtable_index_type): Likewise.
	(class_type_node): Likewise.
	(unknown_type_node): Likewise.
	(init_list_type_node): Likewise.
	(explicit_void_list_node): Likewise.
	(vtbl_type_node): Likewise.
	(vtbl_ptr_type_node): Likewise.
	(std_node): Likewise.
	(abi_node): Likewise.
	(global_namespace): Likewise.
	(const_type_info_type_node): Likewise.
	(conv_op_marker): Likewise.
	(abort_fndecl): Likewise.
	(current_aggr): Likewise.
	(nullptr_node): Likewise.
	(nullptr_type_node): Likewise.
	(align_type_node): Likewise.
	(char8_type_node): Likewise.
	(char16_type_node): Likewise.
	(char32_type_node): Likewise.
	(wchar_type_node): Likewise.
	(underlying_wchar_type_node): Likewise.
	(wint_type_node): Likewise.
	(signed_size_type_node): Likewise.
	(unsigned_ptrdiff_type_node): Likewise.
	(intmax_type_node): Likewise.
	(uintmax_type_node): Likewise.
	(widest_integer_literal_type_node): Likewise.
	(widest_unsigned_literal_type_node): Likewise.
	(sig_atomic_type_node): Likewise.
	(int8_type_node): Likewise.
	(int16_type_node): Likewise.
	(int32_type_node): Likewise.
	(int64_type_node): Likewise.
	(uint8_type_node): Likewise.
	(c_uint16_type_node): Likewise.
	(c_uint32_type_node): Likewise.
	(c_uint64_type_node): Likewise.
	(int_least8_type_node): Likewise.
	(int_least16_type_node): Likewise.
	(int_least32_type_node): Likewise.
	(int_least64_type_node): Likewise.
	(uint_least8_type_node): Likewise.
	(uint_least16_type_node): Likewise.
	(uint_least32_type_node): Likewise.
	(uint_least64_type_node): Likewise.
	(int_fast8_type_node): Likewise.
	(int_fast16_type_node): Likewise.
	(int_fast32_type_node): Likewise.
	(int_fast64_type_node): Likewise.
	(uint_fast8_type_node): Likewise.
	(uint_fast16_type_node): Likewise.
	(uint_fast32_type_node): Likewise.
	(uint_fast64_type_node): Likewise.
	(intptr_type_node): Likewise.
	(uintptr_type_node): Likewise.
	(truthvalue_type_node): Likewise.
	(truthvalue_true_node): Likewise.
	(truthvalue_false_node): Likewise.
	(char_array_type_node): Likewise.
	(char8_array_type_node): Likewise.
	(char16_array_type_node): Likewise.
	(char32_array_type_node): Likewise.
	(wchar_array_type_node): Likewise.
	(string_type_node): Likewise.
	(const_string_type_node): Likewise.
	(default_function_type): Likewise.
	(function_name_decl_node): Likewise.
	(pretty_function_name_decl_node): Likewise.
	(c99_function_name_decl_node): Likewise.
	(saved_function_name_decls): Likewise.
	(null_node): Likewise.
	(ctor_identifier): Likewise.
	(complete_ctor_identifier): Likewise.
	(base_ctor_identifier): Likewise.
	(dtor_identifier): Likewise.
	(complete_dtor_identifier): Likewise.
	(base_dtor_identifier): Likewise.
	(deleting_dtor_identifier): Likewise.
	(conv_op_identifier): Likewise.
	(delta_identifier): Likewise.
	(in_charge_identifier): Likewise.
	(vtt_parm_identifier): Likewise.
	(as_base_identifier): Likewise.
	(this_identifier): Likewise.
	(pfn_identifier): Likewise.
	(vptr_identifier): Likewise.
	(global_identifier): Likewise.
	(anon_identifier): Likewise.
	(auto_identifier): Likewise.
	(decltype_auto_identifier): Likewise.
	(init_list_identifier): Likewise.
	(for_range__identifier): Likewise.
	(for_begin__identifier): Likewise.
	(for_end__identifier): Likewise.
	(for_range_identifier): Likewise.
	(for_begin_identifier): Likewise.
	(for_end_identifier): Likewise.
	(abi_tag_identifier): Likewise.
	(aligned_identifier): Likewise.
	(begin_identifier): Likewise.
	(end_identifier): Likewise.
	(get__identifier): Likewise.
	(gnu_identifier): Likewise.
	(tuple_element_identifier): Likewise.
	(tuple_size_identifier): Likewise.
	(type_identifier): Likewise.
	(value_identifier): Likewise.
	(fun_identifier): Likewise.
	(closure_identifier): Likewise.
	(heap_uninit_identifier): Likewise.
	(heap_identifier): Likewise.
	(heap_deleted_identifier): Likewise.
	(heap_vec_uninit_identifier): Likewise.
	(heap_vec_identifier): Likewise.
	(omp_identifier): Likewise.
	(lang_name_c): Likewise.
	(lang_name_cplusplus): Likewise.
	(empty_except_spec): Likewise.
	(noexcept_true_spec): Likewise.
	(noexcept_false_spec): Likewise.
	(noexcept_deferred_spec): Likewise.
	(terminate_fn): Likewise.
	(call_unexpected_fn): Likewise.
	(get_exception_ptr_fn): Likewise.
	(begin_catch_fn): Likewise.
	(end_catch_fn): Likewise.
	(allocate_exception_fn): Likewise.
	(free_exception_fn): Likewise.
	(throw_fn): Likewise.
	(rethrow_fn): Likewise.
	(atexit_fn_ptr_type_node): Likewise.
	(atexit_node): Likewise.
	(dso_handle_node): Likewise.
	(dynamic_cast_node): Likewise.
	(cleanup_type): Likewise.
	(vtt_parm_type): Likewise.
	(any_targ_node): Likewise.
	(source_location_impl): Likewise.
	(OVL_FUNCTION): Likewise.
	(OVL_CHAIN): Likewise.
	(OVL_DEDUP_P): Likewise.
	(OVL_USING_P): Likewise.
	(OVL_HIDDEN_P): Likewise.
	(OVL_NESTED_P): Likewise.
	(OVL_LOOKUP_P): Likewise.
	(OVL_EXPORT_P): Likewise.
	(OVL_FIRST): Likewise.
	(OVL_NAME): Likewise.
	(OVL_P): Likewise.
	(OVL_SINGLE_P): Likewise.
	(TYPE_HAS_DEFAULT_CONSTRUCTOR): Likewise.
	(CLASSTYPE_LAZY_DEFAULT_CTOR): Likewise.
	(CLASSTYPE_CONSTRUCTORS): Likewise.
	(ATTR_IS_DEPENDENT): Likewise.
	(ABI_TAG_IMPLICIT): Likewise.
	(PARENTHESIZED_LIST_P): Likewise.
	(DECL_DEPENDENT_P): Likewise.
	(USING_DECL_SCOPE): Likewise.
	(USING_DECL_DECLS): Likewise.
	(USING_DECL_TYPENAME_P): Likewise.
	(USING_DECL_UNRELATED_P): Likewise.
	(DECL_DECLARES_FUNCTION_P): Likewise.
	(DECL_DECLARES_TYPE_P): Likewise.
	(IDENTIFIER_KIND_BIT_0): Likewise.
	(IDENTIFIER_KIND_BIT_1): Likewise.
	(IDENTIFIER_KIND_BIT_2): Likewise.
	(IDENTIFIER_MARKED): Likewise.
	(IDENTIFIER_VIRTUAL_P): Likewise.
	(IDENTIFIER_KEYWORD_P): Likewise.
	(IDENTIFIER_CDTOR_P): Likewise.
	(IDENTIFIER_CTOR_P): Likewise.
	(IDENTIFIER_DTOR_P): Likewise.
	(IDENTIFIER_ANY_OP_P): Likewise.
	(IDENTIFIER_OVL_OP_P): Likewise.
	(IDENTIFIER_ASSIGN_OP_P): Likewise.
	(IDENTIFIER_CONV_OP_P): Likewise.
	(IDENTIFIER_NEWDEL_OP_P): Likewise.
	(IDENTIFIER_NEW_OP_P): Likewise.
	(CLASSTYPE_DIAMOND_SHAPED_P): Likewise.
	(CLASSTYPE_REPEATED_BASE_P): Likewise.
	(CLASSTYPE_KEY_METHOD): Likewise.
	(CLASSTYPE_MEMBER_VEC): Likewise.
	(CLASSTYPE_DECL_LIST): Likewise.
	(CLASSTYPE_DESTRUCTOR): Likewise.
	(CLASSTYPE_HAS_PRIMARY_BASE_P): Likewise.
	(CLASSTYPE_PRIMARY_BINFO): Likewise.
	(CLASSTYPE_VBASECLASSES): Likewise.
	(CLASSTYPE_AS_BASE): Likewise.
	(DECL_CONV_FN_P): Likewise.
	(DECL_CONV_FN_TYPE): Likewise.
	(same_type_p): Likewise.
	(WILDCARD_TYPE_P): Likewise.
	(MAYBE_CLASS_TYPE_P): Likewise.
	(FUNCTION_REF_QUALIFIED): Likewise.
	(FUNCTION_RVALUE_QUALIFIED): Likewise.
	(TYPE_PTRMEMFUNC_FN_TYPE): Likewise.
	(TYPE_PTRMEMFUNC_FN_TYPE_RAW): Likewise.
	(TYPE_DEPENDENT_P): Likewise.
	(TYPE_DEPENDENT_P_VALID): Likewise.
	(TYPE_HAS_NONTRIVIAL_DESTRUCTOR): Likewise.
	(TYPE_RAISES_EXCEPTIONS): Likewise.
	(IDENTIFIER_BINDING): Likewise.
	(LANG_IDENTIFIER_CAST): Likewise.
	(IF_COND): Likewise.
	(THEN_CLAUSE): Likewise.
	(ELSE_CLAUSE): Likewise.
	(IF_SCOPE): Likewise.
	(IF_STMT_CONSTEXPR_P): Likewise.
	(IF_STMT_CONSTEVAL_P): Likewise.
	(DECLTYPE_TYPE_EXPR): Likewise.
	(SET_CLASSTYPE_INTERFACE_UNKNOWN_X): Likewise.
	(CLASSTYPE_INTERFACE_ONLY): Likewise.
	(TYPE_NAME_STRING): Likewise.
	(TYPE_NAME_LENGTH): Likewise.
	(CONSTRAINT_VAR_P): Likewise.
	(KOENIG_LOOKUP_P): Likewise.
	(DECL_PARM_INDEX): Likewise.
	(DECL_PARM_LEVEL): Likewise.
	(CONV_IMPLICIT): Likewise.
	(CONV_STATIC): Likewise.
	(CONV_CONST): Likewise.
	(CONV_REINTERPRET): Likewise.
	(CONV_PRIVATE): Likewise.
	(CONV_FORCE_TEMP): Likewise.
	(CONV_FOLD): Likewise.
	(CONV_OLD_CONVERT): Likewise.
	(CONV_C_CAST): Likewise.
	(CONV_BACKEND_CONVERT): Likewise.
	(WANT_INT): Likewise.
	(WANT_FLOAT): Likewise.
	(WANT_ENUM): Likewise.
	(WANT_POINTER): Likewise.
	(WANT_NULL): Likewise.
	(WANT_VECTOR_OR_COMPLEX): Likewise.
	(WANT_ARITH): Likewise.
	(COMPARE_STRICT): Likewise.
	(COMPARE_BASE): Likewise.
	(COMPARE_DERIVED): Likewise.
	(COMPARE_REDECLARATION): Likewise.
	(COMPARE_STRUCTURAL): Likewise.
	(SF_DEFAULT): Likewise.
	(SF_PRE_PARSED): Likewise.
	(SF_INCLASS_INLINE): Likewise.
	(SD_UNINITIALIZED): Likewise.
	(SD_INITIALIZED): Likewise.
	(SD_DECOMPOSITION): Likewise.
	(SD_DEFAULTED): Likewise.
	(SD_DELETED): Likewise.
	(TYPE_PTRDATAMEM_P): Likewise.
	(RS_TYPE_CONST_P): Likewise.
	(TYPE_MAIN_DECL): Likewise.
	(DECL_NONTRIVIALLY_INITIALIZED_P): Likewise.
	(DECL_DEFAULTED_FN): Likewise.
	(TYPE_HAS_USER_CONSTRUCTOR): Likewise.
	(DECL_INITIALIZED_IN_CLASS_P): Likewise.
	(DECL_DEFAULTED_IN_CLASS_P): Likewise.
	(DECL_NONSTATIC_MEMBER_FUNCTION_P): Likewise.
	(DECL_HAS_IN_CHARGE_PARM_P): Likewise.
	(DECL_HAS_VTT_PARM_P): Likewise.
	(FUNCTION_FIRST_USER_PARMTYPE): Likewise.
	(FUNCTION_FIRST_USER_PARM): Likewise.
	(DECL_CONSTRUCTOR_P): Likewise.
	(DECL_DELETED_FN): Likewise.
	(BRACE_ENCLOSED_INITIALIZER_P): Likewise.
	(DECL_IMMEDIATE_FUNCTION_P): Likewise.
	(SET_DECL_IMMEDIATE_FUNCTION_P): Likewise.
	(CONSTRUCTOR_MUTABLE_POISON): Likewise.
	(PTRMEM_CST_MEMBER): Likewise.
	(REF_PARENTHESIZED_P): Likewise.
	(TYPE_PTRMEM_P): Likewise.
	(TYPE_PTR_OR_PTRMEM_P): Likewise.
	(DECL_DECOMPOSITION_P): Likewise.
	(DECL_DECOMP_BASE): Likewise.
	(DECL_MAYBE_IN_CHARGE_CDTOR_P): Likewise.
	(DECL_MAYBE_IN_CHARGE_DESTRUCTOR_P): Likewise.
	(DECL_CLONED_FUNCTION_P): Likewise.
	(DECL_CLONED_FUNCTION): Likewise.
	(DECL_MAYBE_IN_CHARGE_CONSTRUCTOR_P): Likewise.
	(cp_function_chain): Likewise.
	(cdtor_label): Likewise.
	(current_class_ptr): Likewise.
	(current_class_ref): Likewise.
	(current_eh_spec_block): Likewise.
	(current_in_charge_parm): Likewise.
	(current_vtt_parm): Likewise.
	(current_retval_sentinel): Likewise.
	(current_function_returns_value): Likewise.
	(current_function_returns_null): Likewise.
	(current_function_returns_abnormally): Likewise.
	(current_function_infinite_loop): Likewise.
	(in_base_initializer): Likewise.
	(in_function_try_handler): Likewise.
	(current_function_return_value): Likewise.
	(current_class_type): Likewise.
	(TYPE_BEING_DEFINED): Likewise.
	(DECL_STATIC_FUNCTION_P): Likewise.
	(DECL_FUNCTION_MEMBER_P): Likewise.
	(LANG_DECL_MIN_CHECK): Likewise.
	(LANG_DECL_FN_CHECK): Likewise.
	(LANG_DECL_NS_CHECK): Likewise.
	(LANG_DECL_PARM_CHECK): Likewise.
	(LANG_DECL_DECOMP_CHECK): Likewise.
	(RS_INTEGRAL_TYPE_P): Likewise.
	(STAT_HACK_P): Likewise.
	(STAT_TYPE_VISIBLE_P): Likewise.
	(STAT_TYPE): Likewise.
	(STAT_DECL): Likewise.
	(STAT_VISIBLE): Likewise.
	(MAYBE_STAT_DECL): Likewise.
	(MAYBE_STAT_TYPE): Likewise.
	(STAT_TYPE_HIDDEN_P): Likewise.
	(STAT_DECL_HIDDEN_P): Likewise.
	(class warning_sentinel): Likewise.
	(struct uid_sensitive_constexpr_evaluation_checker): Likewise.
	(class iloc_sentinel): Likewise.
	(struct GTY): Likewise.
	(struct named_decl_hash): Likewise.
	(enum lang_decl_selector): Likewise.
	(LANG_DECL_HAS_MIN): Likewise.
	(struct named_label_entry): Likewise.
	(struct named_label_hash): Likewise.
	(enum ref_operator): Likewise.
	(struct c_fileinfo): Likewise.
	(class ovl_iterator): Likewise.
	(class lkp_iterator): Likewise.
	(enum rs_ref_qualifier): Likewise.
	(enum tsubst_flags): Likewise.
	(enum cp_identifier_kind): Likewise.
	(enum tag_types): Likewise.
	(enum compare_bounds_t): Likewise.
	(mark_rvalue_use): Likewise.
	(type_unknown_p): Likewise.
	(init_modules): Likewise.
	(var_in_constexpr_fn): Likewise.
	(ovl_first): Likewise.
	(lookup_add): Likewise.
	(ovl_make): Likewise.
	(is_overloaded_fn): Likewise.
	(maybe_add_lang_type_raw): Likewise.
	(type_memfn_rqual): Likewise.
	(builtin_pack_fn_p): Likewise.
	(make_conv_op_name): Likewise.
	(type_memfn_quals): Likewise.
	(get_fileinfo): Likewise.
	(cxx_make_type): Likewise.
	(build_cplus_array_type): Likewise.
	(is_byte_access_type): Likewise.
	(comptypes): Likewise.
	(canonical_eh_spec): Likewise.
	(cp_tree_operand_length): Likewise.
	(rs_tree_equal): Likewise.
	(compparms): Likewise.
	(rs_build_qualified_type_real): Likewise.
	(rs_build_qualified_type): Likewise.
	(cv_qualified_p): Likewise.
	(similar_type_p): Likewise.
	(vector_targets_convertible_p): Likewise.
	(same_type_ignoring_top_level_qualifiers_p): Likewise.
	(comp_ptr_ttypes_const): Likewise.
	(get_class_binding_direct): Likewise.
	(skip_artificial_parms_for): Likewise.
	(lang_check_failed): Likewise.
	(default_init_uninitialized_part): Likewise.
	(type_has_non_user_provided_default_constructor): Likewise.
	(default_ctor_p): Likewise.
	(user_provided_p): Likewise.
	(sufficient_parms_p): Likewise.
	(next_initializable_field): Likewise.
	(in_class_defaulted_default_constructor): Likewise.
	(is_instantiation_of_constexpr): Likewise.
	(check_for_uninitialized_const_var): Likewise.
	(reduced_constant_expression_p): Likewise.
	(cv_unqualified): Likewise.
	(cp_get_callee): Likewise.
	(cp_get_callee_fndecl_nofold): Likewise.
	(is_nondependent_static_init_expression): Likewise.
	(maybe_constant_init): Likewise.
	(build_nop): Likewise.
	(scalarish_type_p): Likewise.
	(is_bitfield_expr_with_lowered_type): Likewise.
	(convert_bitfield_to_declared_type): Likewise.
	(cp_fold_maybe_rvalue): Likewise.
	(maybe_undo_parenthesized_ref): Likewise.
	(fold_offsetof): Likewise.
	(cp_truthvalue_conversion): Likewise.
	(fold_non_dependent_expr): Likewise.
	(char_type_p): Likewise.
	(instantiation_dependent_expression_p): Likewise.
	(type_has_nontrivial_copy_init): Likewise.
	(build_local_temp): Likewise.
	(is_normal_capture_proxy): Likewise.
	(reject_gcc_builtin): Likewise.
	(resolve_nondeduced_context): Likewise.
	(cxx_incomplete_type_diagnostic): Likewise.
	(cxx_incomplete_type_error): Likewise.
	(invalid_nonstatic_memfn_p): Likewise.
	(really_overloaded_fn): Likewise.
	(resolve_nondeduced_context_or_error): Likewise.
	(instantiate_non_dependent_or_null): Likewise.
	(cxx_incomplete_type_inform): Likewise.
	(strip_top_quals): Likewise.
	(undeduced_auto_decl): Likewise.
	(require_deduced_type): Likewise.
	(decl_constant_var_p): Likewise.
	(type_of_this_parm): Likewise.
	(class_of_this_parm): Likewise.
	(identifier_p): Likewise.
	(gnu_vector_type_p): Likewise.
	(make_tree_vector): Likewise.
	(release_tree_vector): Likewise.
	(class releasing_vec): Likewise.
	(vec_safe_push): Likewise.
	(null_node_p): Likewise.
	* backend/rust-compile-base.cc (HIRCompileBase::setup_fndecl): Correctly
	set fndecl as constexpr from qualifiers.
	(HIRCompileBase::compile_function): Test if fndecl is constexpr.
	(HIRCompileBase::compile_constant_item): Set fndecl as constexpr.
	* backend/rust-compile-intrinsic.cc : Update include.
	(finalize_intrinsic_block): Set fndecl as constexpr.
	* backend/rust-constexpr.cc : Update include.
	(VERIFY_CONSTANT): New.
	(is_instantiation_of_constexpr): New.
	(literal_type_p): New.
	(verify_constant): New.
	(find_array_ctor_elt): New.
	(array_index_cmp): New.
	(potential_constant_expression_1): New.
	(get_nth_callarg): New.
	(unshare_constructor): New.
	(maybe_save_constexpr_fundef): New.
	(returns): New.
	(breaks): New.
	(continues): New.
	(switches): New.
	(struct constexpr_global_ctx): Add new fields.
	(constexpr_fundef): New.
	(struct constexpr_call): New.
	(constexpr_call_hasher : ggc_ptr_hash<constexpr_call>): New.
	(enum constexpr_switch_state): New.
	(struct constexpr_ctx): Add new fields.
	(struct constexpr_fundef_hasher): New.
	(constexpr_fundef_table): New.
	(constexpr_fundef_hasher::equal): New.
	(constexpr_fundef_hasher::hash): New.
	(retrieve_constexpr_fundef): New.
	(uid_sensitive_constexpr_evaluation_value): New.
	(uid_sensitive_constexpr_evaluation_true_counter): New.
	(uid_sensitive_constexpr_evaluation_p): New.
	(class temp_override): New.
	(struct uid_sensitive_constexpr_evaluation_sentinel): New.
	(struct uid_sensitive_constexpr_evaluation_checker): New.
	(::uid_sensitive_constexpr_evaluation_sentinel): New.
	(::uid_sensitive_constexpr_evaluation_checker): New.
	(uid_sensitive_constexpr_evaluation_checker::evaluation_restricted_p): New.
	(constexpr_call_table): New.
	(constexpr_call_hasher::hash): New.
	(constexpr_call_hasher::equal): New.
	(maybe_initialize_constexpr_call_table): New.
	(fundef_copies_table): New.
	(get_fundef_copy): New.
	(save_fundef_copy): New.
	(constexpr_expression): Refactor in ...
	(eval_constant_expression): ...this.
	(eval_store_expression): Add 3 bool params.
	(eval_call_expression): Likewise.
	(eval_binary_expression): Likewise.
	(eval_statement_list): New.
	(extract_string_elt): New.
	(eval_conditional_expression): New.
	(eval_bit_field_ref): New.
	(eval_loop_expr): New.
	(eval_switch_expr): New.
	(eval_unary_expression): New.
	(call_stack): New.
	(call_stack_tick): New.
	(last_cx_error_tick): New.
	(push_cx_call_context): New.
	(pop_cx_call_context): New.
	(cx_error_context): New.
	(fold_expr): Adjust call to eval_constant_expression.
	(same_type_ignoring_tlq_and_bounds_p): New.
	(union_active_member): Port more for cxx constexpr.
	(fold_indirect_ref_1): New.
	(rs_fold_indirect_ref): Likewise.
	(rs_eval_indirect_ref): New (from corresponding cxx FE file).
	(eval_logical_expression): Adjust parameter list.
	(lookup_placeholder): New (from corresponding cxx FE file).
	(constant_value_1): Remove.
	(inline_asm_in_constexpr_error): New (from corresponding cxx FE file).
	(verify_ctor_sanity): New.
	(get_callee): Remove.
	(initialized_type): New.
	(maybe_constexpr_fn): Remove.
	(init_subob_ctx): New.
	(base_field_constructor_elt): New.
	(get_or_insert_ctor_field): New.
	(eval_vector_conditional_expression): New (from correponding cxx FE file).
	(eval_bare_aggregate): New.
	(cxx_eval_trinary_expression): New.
	(reduced_constant_expression_p): New.
	(adjust_temp_type): New.
	(free_constructor): New.
	(eval_and_check_array_index): New.
	(eval_array_reference): New.
	(eval_component_reference): New.
	(label_matches): New.
	(eval_constant_expression): New.
	(modifying_const_object_error): New.
	(is_empty_field): New.
	(eval_store_expression): New.
	(eval_binary_expression): New.
	(addr_of_non_const_var): New.
	(rs_bind_parameters_in_call): New.
	(eval_builtin_function_call): New.
	(eval_call_expression): New.
	(build_anon_member_initialization): New.
	(build_data_member_initialization): New.
	(constexpr_fn_retval): New.
	(constant_value_1): New.
	(decl_constant_value): New.
	(non_const_var_error): New.
	(get_callee): New.
	(get_function_named_in_call): New.
	(maybe_constexpr_fn): New.
	(get_nth_callarg): New.
	(var_in_maybe_constexpr_fn): New.
	(instantiate_cx_fn_r): New.
	(instantiate_constexpr_fns): New.
	(array_index_cmp): New.
	(unshare_constructor): New.
	(find_array_ctor_elt): New.
	(verify_constant): New.
	(find_heap_var_refs): New.
	(find_immediate_fndecl): New.
	(diag_array_subscript): New.
	(get_array_or_vector_nelts): New.
	(eval_and_check_array_index): New.
	(extract_string_elt): New.
	(is_valid_constexpr_fn): New.
	(explain_invalid_constexpr_fn): New.
	(register_constexpr_fundef): New.
	(maybe_save_constexpr_fundef): New.
	(eval_statement_list): New.
	(eval_conditional_expression): New.
	(eval_bit_field_ref): New.
	(returns): New.
	(breaks): New.
	(continues): New.
	(switches): New.
	(eval_loop_expr): New.
	(eval_switch_expr): New.
	(eval_unary_expression): New.
	(cxx_eval_outermost_constant_expr): New.
	(is_static_init_expression): New.
	(is_constant_expression): New.
	(is_nondependent_static_init_expression): New.
	(maybe_constant_init_1): New.
	(maybe_constant_init): New.
	(is_nondependent_constant_expression): New.
	(cv_cache): New.
	(maybe_constant_value): New.
	(potential_constant_expression): New.
	(struct check_for_return_continue_data): New.
	(check_for_return_continue): New.
	(decl_namespace_context): New.
	(decl_in_std_namespace_p): New.
	(is_std_construct_at): New.
	(cxx_dynamic_cast_fn_p): New.
	(is_std_allocator_allocate): New.
	(is_std_allocator_allocate): New.
	(potential_constant_expression_1): New.
	(potential_constant_expression_1): New.
	(fold_non_dependent_init): New.
	* backend/rust-constexpr.h (maybe_save_constexpr_fundef): New.

gcc/testsuite:
	* rust/compile/const4.rs: New.
	* rust/compile/const5.rs: New.

Signed-off-by: Faisal Abbas <90.abbasfaisal@gmail.com>