[Rust] Added global namespace imports (#5121)

* [Rust] Added global namespace imports

* Documented the need for global imports

* Added white_space params to GenNamespaceImports

* Removed a \n from GenNamespaceImports

src/idl_gen_rust.cpp

@@ -298,6 +298,11 @@ class RustGenerator : public BaseGenerator {
 
     assert(!cur_name_space_);
 
+    // Generate imports for the global scope in case no namespace is used
+    // in the schema file.
+    GenNamespaceImports(0);
+    code_ += "";
+
     // Generate all code in their namespaces, once, because Rust does not
     // permit re-opening modules.
     //
@@ -1740,6 +1745,18 @@ class RustGenerator : public BaseGenerator {
     code_ += "";
   }
 
+  void GenNamespaceImports(const int white_spaces) {
+      std::string indent = std::string(white_spaces, ' ');
+      code_ += indent + "#![allow(dead_code)]";
+      code_ += indent + "#![allow(unused_imports)]";
+      code_ += "";
+      code_ += indent + "use std::mem;";
+      code_ += indent + "use std::cmp::Ordering;";
+      code_ += "";
+      code_ += indent + "extern crate flatbuffers;";
+      code_ += indent + "use self::flatbuffers::EndianScalar;";
+  }
+
   // Set up the correct namespace. This opens a namespace if the current
   // namespace is different from the target namespace. This function
   // closes and opens the namespaces only as necessary.
@@ -1774,14 +1791,8 @@ class RustGenerator : public BaseGenerator {
     // in the previous example, E, then F, then G are opened
     for (auto j = common_prefix_size; j != new_size; ++j) {
       code_ += "pub mod " + MakeSnakeCase(ns->components[j]) + " {";
-      code_ += "  #![allow(dead_code)]";
-      code_ += "  #![allow(unused_imports)]";
-      code_ += "";
-      code_ += "  use std::mem;";
-      code_ += "  use std::cmp::Ordering;";
-      code_ += "";
-      code_ += "  extern crate flatbuffers;";
-      code_ += "  use self::flatbuffers::EndianScalar;";
+      // Generate local namespace imports.
+      GenNamespaceImports(2);
     }
     if (new_size != common_prefix_size) { code_ += ""; }
 

tests/monster_test_generated.rs

 // automatically generated by the FlatBuffers compiler, do not modify
 
 
+#![allow(dead_code)]
+#![allow(unused_imports)]
+
+use std::mem;
+use std::cmp::Ordering;
+
+extern crate flatbuffers;
+use self::flatbuffers::EndianScalar;
+
 pub mod my_game {
   #![allow(dead_code)]
   #![allow(unused_imports)]

tests/namespace_test/namespace_test1_generated.rs

 // automatically generated by the FlatBuffers compiler, do not modify
 
 
+#![allow(dead_code)]
+#![allow(unused_imports)]
+
+use std::mem;
+use std::cmp::Ordering;
+
+extern crate flatbuffers;
+use self::flatbuffers::EndianScalar;
+
 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)]
+
+use std::mem;
+use std::cmp::Ordering;
+
+extern crate flatbuffers;
+use self::flatbuffers::EndianScalar;
+
 pub mod namespace_a {
   #![allow(dead_code)]
   #![allow(unused_imports)]