Fix extern crate in root namespace (#4905)

Imports the Rust FlatBuffers runtime crate even when not using a namespace in a schema.

src/idl_gen_rust.cpp

@@ -271,6 +271,9 @@ class RustGenerator : public BaseGenerator {
   bool generate() {
     code_.Clear();
     code_ += "// " + std::string(FlatBuffersGeneratedWarning()) + "\n\n";
+    code_ += "#![allow(dead_code)]";
+    code_ += "#![allow(unused_imports)]";
+    code_ += "extern crate flatbuffers;\n";
 
     assert(!cur_name_space_);
 

tests/monster_test_generated.rs

 // automatically generated by the FlatBuffers compiler, do not modify
 
 
+#![allow(dead_code)]
+#![allow(unused_imports)]
+extern crate flatbuffers;
+
 pub mod my_game {
   #![allow(dead_code)]
   #![allow(unused_imports)]

tests/monster_test_my_game.example2_generated.dart

@@ -6,8 +6,8 @@ library my_game.example2;
 import 'dart:typed_data' show Uint8List;
 import 'package:flat_buffers/flat_buffers.dart' as fb;
 
-import './monster_test_my_game.example_generated.dart' as my_game_example;
 import './monster_test_my_game_generated.dart' as my_game;
+import './monster_test_my_game.example_generated.dart' as my_game_example;
 
 class Monster {
   Monster._(this._bc, this._bcOffset);

tests/monster_test_my_game_generated.dart

@@ -6,8 +6,8 @@ library my_game;
 import 'dart:typed_data' show Uint8List;
 import 'package:flat_buffers/flat_buffers.dart' as fb;
 
-import './monster_test_my_game.example_generated.dart' as my_game_example;
 import './monster_test_my_game.example2_generated.dart' as my_game_example2;
+import './monster_test_my_game.example_generated.dart' as my_game_example;
 
 class InParentNamespace {
   InParentNamespace._(this._bc, this._bcOffset);

tests/namespace_test/namespace_test1_generated.rs

 // automatically generated by the FlatBuffers compiler, do not modify
 
 
+#![allow(dead_code)]
+#![allow(unused_imports)]
+extern crate flatbuffers;
+
 pub mod namespace_a {
   #![allow(dead_code)]
   #![allow(unused_imports)]

tests/namespace_test/namespace_test2_generated.rs

 // automatically generated by the FlatBuffers compiler, do not modify
 
 
+#![allow(dead_code)]
+#![allow(unused_imports)]
+extern crate flatbuffers;
+
 pub mod namespace_a {
   #![allow(dead_code)]
   #![allow(unused_imports)]