[rust] Add use declarations to Rust-generated bindings for imported FB definitions (#5645)

* Bugfix for Rust generation of union fields named with language keywords

Looking at ParseField, it appears that in the case of unions, an extra field with a `UnionTypeFieldSuffix` is added to the type definition, however, if the name of this field is a keyword in the target language, it isn't escaped.

For example, if generating code for rust for a union field named `type`, flatc will generate a (non-keyword escaped) field named `type_type` for this hidden union field, and one (keyword escaped) called `type_` for the actual union contents.

When the union accessors are generated, they refer to this `type_type` field, but they will escape it mistakenly, generating code like this:

```
  #[inline]
  #[allow(non_snake_case)]
  pub fn type__as_int(&self) -> Option<Int<'a>> {
    if self.type__type() == Type::Int {
      self.type_().map(|u| Int::init_from_table(u))
    } else {
      None
    }
  }
```

Which will fail to build because the field is called `self.type_type()`, not `self.type__type()`.

* [Rust] Add crate-relative use statements for FBS includes.

At present if a flatbuffer description includes a reference to a type in
another file, the generated Rust code needs to be hand-modified to add
the appropriate `use` statements.

This assumes that the dependencies are built into the same crate, which
I think is a reasonable assumption?

* Revert "[Rust] Add crate-relative use statements for FBS includes."

This reverts commit d554d79fec.

* Add updated generated test files.

* Fixing Rust test harness to handle new includes.

Test binaries need to add references to generated code that's
transitively included.

This also has the knock-on in that this code (which is referenced by
include directives directly in the flatbuffer schema files) also needs
to be generated, hence the changes to generate_code.sh.

* Test harnesses expect test data to be checked in.

Put include_test2 files into the same directory as the include_test2
schema definition.

Update all code generation scripts (forgot the batch file from last
time).

Path updates in Rust test.

* Include updated generated code

* Address comments raised in PR

* Fix failing Rust tests.

* Previous merge clobbered this branch change.

* Add updated imports to benchmarks.

* Clarifying comment per PR request

* Update documentation comments per feedback

* Remove non-Rust generated files for include tests, per feedback from @rw/@aardappel

* Broken code generation batch file

* Fix typo

* Add TODO for tidying up use declaration traversal sometime in the future

* Update test files.

src/idl_gen_rust.cpp

@@ -1763,6 +1763,14 @@ class RustGenerator : public BaseGenerator {
   void GenNamespaceImports(const int white_spaces) {
     std::string indent = std::string(white_spaces, ' ');
     code_ += "";
+    for (auto it = parser_.included_files_.begin();
+        it != parser_.included_files_.end(); ++it) {
+      if (it->second.empty()) continue;
+      auto noext = flatbuffers::StripExtension(it->second);
+      auto basename = flatbuffers::StripPath(noext);
+
+      code_ += indent + "use crate::" + basename + "_generated::*;";
+    }
     code_ += indent + "use std::mem;";
     code_ += indent + "use std::cmp::Ordering;";
     code_ += "";
@@ -1847,3 +1855,7 @@ std::string RustMakeRule(const Parser &parser, const std::string &path,
 // TODO(maxburke): There should be test schemas added that use language
 //           keywords as fields of structs, tables, unions, enums, to make sure
 //           that internal code generated references escaped names correctly.
+// TODO(maxburke): We should see if there is a more flexible way of resolving
+//           module paths for use declarations. Right now if schemas refer to
+//           other flatbuffer files, the include paths in emitted Rust bindings
+//           are crate-relative which may undesirable.