message: Use keep case for Rust union discriminant type.
  Fixes
  I realize that Rust doesn't really follow the Namer convention, since
  it uses Field case for methods... that's a future problem.
diff: @@ -1605,7 +1605,7 @@ class RustGenerator : public BaseGenerator {
  code_.SetValue("OFFSET_VALUE", NumToString(field.value.offset));
  code_.SetValue("FIELD", namer_.Field(field));
  code_.SetValue("BLDR_DEF_VAL", GetDefaultValue(field, kBuilder));
  - code_.SetValue("DISCRIMINANT", namer_.Method(field) + "_type");
  + code_.SetValue("DISCRIMINANT", namer_.Field(field) + "_type");
  code_.IncrementIdentLevel();
  cb(field);
  code_.DecrementIdentLevel();
  