Bugfix for Rust generation of union fields named with language keywords (#5592)

* 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.

* Address comments raised in PR

* Update documentation comments per feedback

* Fix typo

src/idl_gen_rust.cpp

@@ -1305,6 +1305,7 @@ class RustGenerator : public BaseGenerator {
       auto u = field.value.type.enum_def;
 
       code_.SetValue("FIELD_NAME", Name(field));
+      code_.SetValue("FIELD_TYPE_FIELD_NAME", field.name);
 
       for (auto u_it = u->Vals().begin(); u_it != u->Vals().end(); ++u_it) {
         auto &ev = **u_it;
@@ -1325,8 +1326,19 @@ class RustGenerator : public BaseGenerator {
         code_ +=
             "  pub fn {{FIELD_NAME}}_as_{{U_ELEMENT_NAME}}(&self) -> "
             "Option<{{U_ELEMENT_TABLE_TYPE}}<'a>> {";
+        // If the user defined schemas name a field that clashes with a
+        // language reserved word, flatc will try to escape the field name by
+        // appending an underscore. This works well for most cases, except
+        // one. When generating union accessors (and referring to them 
+        // internally within the code generated here), an extra underscore
+        // will be appended to the name, causing build failures. 
+        //
+        // This only happens when unions have members that overlap with
+        // language reserved words. 
+        // 
+        // To avoid this problem the type field name is used unescaped here:
         code_ +=
-            "    if self.{{FIELD_NAME}}_type() == {{U_ELEMENT_ENUM_TYPE}} {";
+            "    if self.{{FIELD_TYPE_FIELD_NAME}}_type() == {{U_ELEMENT_ENUM_TYPE}} {";
         code_ +=
             "      self.{{FIELD_NAME}}().map(|u| "
             "{{U_ELEMENT_TABLE_TYPE}}::init_from_table(u))";
@@ -1832,3 +1844,6 @@ std::string RustMakeRule(const Parser &parser, const std::string &path,
 // TODO(rw): Generated code should generate endian-safe Debug impls.
 // TODO(rw): Generated code could use a Rust-only enum type to access unions,
 //           instead of making the user use _type() to manually switch.
+// 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.