src/idl_gen_rust.cpp

/*
 * Copyright 2018 Google Inc. All rights reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

// independent from idl_parser, since this code is not needed for most clients

#include "flatbuffers/code_generators.h"
#include "flatbuffers/flatbuffers.h"
#include "flatbuffers/idl.h"
#include "flatbuffers/util.h"

namespace flatbuffers {

static std::string GeneratedFileName(const std::string &path,
                                     const std::string &file_name) {
  return path + file_name + "_generated.rs";
}

// Convert a camelCaseIdentifier or CamelCaseIdentifier to a
// snake_case_indentifier.
std::string MakeSnakeCase(const std::string &in) {
  std::string s;
  for (size_t i = 0; i < in.length(); i++) {
    if (i == 0) {
      s += static_cast<char>(tolower(in[0]));
    } else if (in[i] == '_') {
      s += '_';
    } else if (!islower(in[i])) {
      // Prevent duplicate underscores for Upper_Snake_Case strings
      // and UPPERCASE strings.
      if (islower(in[i - 1])) {
        s += '_';
      }
      s += static_cast<char>(tolower(in[i]));
    } else {
      s += in[i];
    }
  }
  return s;
}

// Convert a string to all uppercase.
std::string MakeUpper(const std::string &in) {
  std::string s;
  for (size_t i = 0; i < in.length(); i++) {
    s += static_cast<char>(toupper(in[i]));
  }
  return s;
}

// Encapsulate all logical field types in this enum. This allows us to write
// field logic based on type switches, instead of branches on the properties
// set on the Type.
// TODO(rw): for backwards compatibility, we can't use a strict `enum class`
//           declaration here. could we use the `-Wswitch-enum` warning to
//           achieve the same effect?
enum FullType {
  ftInteger = 0,
  ftFloat = 1,
  ftBool = 2,

  ftStruct = 3,
  ftTable = 4,

  ftEnumKey = 5,
  ftUnionKey = 6,

  ftUnionValue = 7,

  // TODO(rw): bytestring?
  ftString = 8,

  ftVectorOfInteger = 9,
  ftVectorOfFloat = 10,
  ftVectorOfBool = 11,
  ftVectorOfEnumKey = 12,
  ftVectorOfStruct = 13,
  ftVectorOfTable = 14,
  ftVectorOfString = 15,
  ftVectorOfUnionValue = 16,
};

// Convert a Type to a FullType (exhaustive).
FullType GetFullType(const Type &type) {
  // N.B. The order of these conditionals matters for some types.

  if (type.base_type == BASE_TYPE_STRING) {
    return ftString;
  } else if (type.base_type == BASE_TYPE_STRUCT) {
    if (type.struct_def->fixed) {
      return ftStruct;
    } else {
      return ftTable;
    }
  } else if (type.base_type == BASE_TYPE_VECTOR) {
    switch (GetFullType(type.VectorType())) {
      case ftInteger: {
        return ftVectorOfInteger;
      }
      case ftFloat: {
        return ftVectorOfFloat;
      }
      case ftBool: {
        return ftVectorOfBool;
      }
      case ftStruct: {
        return ftVectorOfStruct;
      }
      case ftTable: {
        return ftVectorOfTable;
      }
      case ftString: {
        return ftVectorOfString;
      }
      case ftEnumKey: {
        return ftVectorOfEnumKey;
      }
      case ftUnionKey:
      case ftUnionValue: {
        FLATBUFFERS_ASSERT(false && "vectors of unions are unsupported");
        break;
      }
      default: {
        FLATBUFFERS_ASSERT(false && "vector of vectors are unsupported");
      }
    }
  } else if (type.enum_def != nullptr) {
    if (type.enum_def->is_union) {
      if (type.base_type == BASE_TYPE_UNION) {
        return ftUnionValue;
      } else if (IsInteger(type.base_type)) {
        return ftUnionKey;
      } else {
        FLATBUFFERS_ASSERT(false && "unknown union field type");
      }
    } else {
      return ftEnumKey;
    }
  } else if (IsScalar(type.base_type)) {
    if (IsBool(type.base_type)) {
      return ftBool;
    } else if (IsInteger(type.base_type)) {
      return ftInteger;
    } else if (IsFloat(type.base_type)) {
      return ftFloat;
    } else {
      FLATBUFFERS_ASSERT(false && "unknown number type");
    }
  }

  FLATBUFFERS_ASSERT(false && "completely unknown type");

  // this is only to satisfy the compiler's return analysis.
  return ftBool;
}

// If the second parameter is false then wrap the first with Option<...>
std::string WrapInOptionIfNotRequired(std::string s, bool required) {
  if (required) {
    return s;
  } else {
    return "Option<" + s + ">";
  }
}

// If the second parameter is false then add .unwrap()
std::string AddUnwrapIfRequired(std::string s, bool required) {
  if (required) {
    return s + ".unwrap()";
  } else {
    return s;
  }
}

namespace rust {

class RustGenerator : public BaseGenerator {
 public:
  RustGenerator(const Parser &parser, const std::string &path,
                const std::string &file_name)
      : BaseGenerator(parser, path, file_name, "", "::"),
        cur_name_space_(nullptr) {
    const char *keywords[] = {
      // list taken from:
      // https://doc.rust-lang.org/book/second-edition/appendix-01-keywords.html
      //
      // we write keywords one per line so that we can easily compare them with
      // changes to that webpage in the future.

      // currently-used keywords
      "as",
      "break",
      "const",
      "continue",
      "crate",
      "else",
      "enum",
      "extern",
      "false",
      "fn",
      "for",
      "if",
      "impl",
      "in",
      "let",
      "loop",
      "match",
      "mod",
      "move",
      "mut",
      "pub",
      "ref",
      "return",
      "Self",
      "self",
      "static",
      "struct",
      "super",
      "trait",
      "true",
      "type",
      "unsafe",
      "use",
      "where",
      "while",

      // future possible keywords
      "abstract",
      "alignof",
      "become",
      "box",
      "do",
      "final",
      "macro",
      "offsetof",
      "override",
      "priv",
      "proc",
      "pure",
      "sizeof",
      "typeof",
      "unsized",
      "virtual",
      "yield",

      // other rust terms we should not use
      "std",
      "usize",
      "isize",
      "u8",
      "i8",
      "u16",
      "i16",
      "u32",
      "i32",
      "u64",
      "i64",
      "u128",
      "i128",
      "f32",
      "f64",

      // These are terms the code generator can implement on types.
      //
      // In Rust, the trait resolution rules (as described at
      // https://github.com/rust-lang/rust/issues/26007) mean that, as long
      // as we impl table accessors as inherent methods, we'll never create
      // conflicts with these keywords. However, that's a fairly nuanced
      // implementation detail, and how we implement methods could change in
      // the future. as a result, we proactively block these out as reserved
      // words.
      "follow",
      "push",
      "size",
      "alignment",
      "to_little_endian",
      "from_little_endian",
      nullptr };
    for (auto kw = keywords; *kw; kw++) keywords_.insert(*kw);
  }

  // Iterate through all definitions we haven't generated code for (enums,
  // structs, and tables) and output them to a single file.
  bool generate() {
    code_.Clear();
    code_ += "// " + std::string(FlatBuffersGeneratedWarning()) + "\n\n";

    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.
    //
    // TODO(rw): Use a set data structure to reduce namespace evaluations from
    //           O(n**2) to O(n).
    for (auto ns_it = parser_.namespaces_.begin();
         ns_it != parser_.namespaces_.end();
         ++ns_it) {
      const auto &ns = *ns_it;

      // Generate code for all the enum declarations.
      for (auto it = parser_.enums_.vec.begin(); it != parser_.enums_.vec.end();
           ++it) {
        const auto &enum_def = **it;
        if (enum_def.defined_namespace != ns) { continue; }
        if (!enum_def.generated) {
          SetNameSpace(enum_def.defined_namespace);
          GenEnum(enum_def);
        }
      }

      // Generate code for all structs.
      for (auto it = parser_.structs_.vec.begin();
           it != parser_.structs_.vec.end(); ++it) {
        const auto &struct_def = **it;
        if (struct_def.defined_namespace != ns) { continue; }
        if (struct_def.fixed && !struct_def.generated) {
          SetNameSpace(struct_def.defined_namespace);
          GenStruct(struct_def);
        }
      }

      // Generate code for all tables.
      for (auto it = parser_.structs_.vec.begin();
           it != parser_.structs_.vec.end(); ++it) {
        const auto &struct_def = **it;
        if (struct_def.defined_namespace != ns) { continue; }
        if (!struct_def.fixed && !struct_def.generated) {
          SetNameSpace(struct_def.defined_namespace);
          GenTable(struct_def);
        }
      }

      // Generate global helper functions.
      if (parser_.root_struct_def_) {
        auto &struct_def = *parser_.root_struct_def_;
        if (struct_def.defined_namespace != ns) { continue; }
        SetNameSpace(struct_def.defined_namespace);
        GenRootTableFuncs(struct_def);
      }
    }
    if (cur_name_space_) SetNameSpace(nullptr);

    const auto file_path = GeneratedFileName(path_, file_name_);
    const auto final_code = code_.ToString();
    return SaveFile(file_path.c_str(), final_code, false);
  }

 private:
  CodeWriter code_;

  std::set<std::string> keywords_;

  // This tracks the current namespace so we can insert namespace declarations.
  const Namespace *cur_name_space_;

  const Namespace *CurrentNameSpace() const { return cur_name_space_; }

  // Determine if a Type needs a lifetime template parameter when used in the
  // Rust builder args.
  bool TableBuilderTypeNeedsLifetime(const Type &type) const {
    switch (GetFullType(type)) {
      case ftInteger:
      case ftFloat:
      case ftBool:
      case ftEnumKey:
      case ftUnionKey:
      case ftUnionValue: { return false; }
      default: { return true; }
    }
  }

  // Determine if a table args rust type needs a lifetime template parameter.
  bool TableBuilderArgsNeedsLifetime(const StructDef &struct_def) const {
    FLATBUFFERS_ASSERT(!struct_def.fixed);

    for (auto it = struct_def.fields.vec.begin();
         it != struct_def.fields.vec.end(); ++it) {
      const auto &field = **it;
      if (field.deprecated) {
        continue;
      }

      if (TableBuilderTypeNeedsLifetime(field.value.type)) {
        return true;
      }
    }

    return false;
  }

  // Determine if a Type needs to be copied (for endian safety) when used in a
  // Struct.
  bool StructMemberAccessNeedsCopy(const Type &type) const {
    switch (GetFullType(type)) {
      case ftInteger:  // requires endian swap
      case ftFloat: // requires endian swap
      case ftBool: // no endian-swap, but do the copy for UX consistency
      case ftEnumKey: { return true; } // requires endian swap
      case ftStruct: { return false; } // no endian swap
      default: {
        // logic error: no other types can be struct members.
        FLATBUFFERS_ASSERT(false && "invalid struct member type");
        return false; // only to satisfy compiler's return analysis
      }
    }
  }

  std::string EscapeKeyword(const std::string &name) const {
    return keywords_.find(name) == keywords_.end() ? name : name + "_";
  }

  std::string Name(const Definition &def) const {
    return EscapeKeyword(def.name);
  }

  std::string Name(const EnumVal &ev) const { return EscapeKeyword(ev.name); }

  std::string WrapInNameSpace(const Definition &def) const {
    return WrapInNameSpace(def.defined_namespace, Name(def));
  }
  std::string WrapInNameSpace(const Namespace *ns,
                              const std::string &name) const {
    if (CurrentNameSpace() == ns) return name;
    std::string prefix = GetRelativeNamespaceTraversal(CurrentNameSpace(), ns);
    return prefix + name;
  }

  // Determine the namespace traversal needed from the Rust crate root.
  // This may be useful in the future for referring to included files, but is
  // currently unused.
  std::string GetAbsoluteNamespaceTraversal(const Namespace *dst) const {
    std::stringstream stream;

    stream << "::";
    for (auto d = dst->components.begin(); d != dst->components.end(); ++d) {
      stream << MakeSnakeCase(*d) + "::";
    }
    return stream.str();
  }

  // Determine the relative namespace traversal needed to reference one
  // namespace from another namespace. This is useful because it does not force
  // the user to have a particular file layout. (If we output absolute
  // namespace paths, that may require users to organize their Rust crates in a
  // particular way.)
  std::string GetRelativeNamespaceTraversal(const Namespace *src,
                                            const Namespace *dst) const {
    // calculate the path needed to reference dst from src.
    // example: f(A::B::C, A::B::C) -> (none)
    // example: f(A::B::C, A::B)    -> super::
    // example: f(A::B::C, A::B::D) -> super::D
    // example: f(A::B::C, A)       -> super::super::
    // example: f(A::B::C, D)       -> super::super::super::D
    // example: f(A::B::C, D::E)    -> super::super::super::D::E
    // example: f(A, D::E)          -> super::D::E
    // does not include leaf object (typically a struct type).

    size_t i = 0;
    std::stringstream stream;

    auto s = src->components.begin();
    auto d = dst->components.begin();
    for(;;) {
      if (s == src->components.end()) { break; }
      if (d == dst->components.end()) { break; }
      if (*s != *d) { break; }
      ++s;
      ++d;
      ++i;
    }

    for (; s != src->components.end(); ++s) {
      stream << "super::";
    }
    for (; d != dst->components.end(); ++d) {
      stream << MakeSnakeCase(*d) + "::";
    }
    return stream.str();
  }

  // Generate a comment from the schema.
  void GenComment(const std::vector<std::string> &dc, const char *prefix = "") {
    std::string text;
    ::flatbuffers::GenComment(dc, &text, nullptr, prefix);
    code_ += text + "\\";
  }

  // Return a Rust type from the table in idl.h.
  std::string GetTypeBasic(const Type &type) const {
    switch (GetFullType(type)) {
      case ftInteger:
      case ftFloat:
      case ftBool:
      case ftEnumKey:
      case ftUnionKey: { break; }
      default: { FLATBUFFERS_ASSERT(false && "incorrect type given");}
    }

    // clang-format off
    static const char * const ctypename[] = {
    #define FLATBUFFERS_TD(ENUM, IDLTYPE, CTYPE, JTYPE, GTYPE, NTYPE, PTYPE, \
                           RTYPE, KTYPE) \
            #RTYPE,
        FLATBUFFERS_GEN_TYPES(FLATBUFFERS_TD)
    #undef FLATBUFFERS_TD
      // clang-format on
    };

    if (type.enum_def) { return WrapInNameSpace(*type.enum_def); }
    return ctypename[type.base_type];
  }

  // Look up the native type for an enum. This will always be an integer like
  // u8, i32, etc.
  std::string GetEnumTypeForDecl(const Type &type) {
    const auto ft = GetFullType(type);
    if (!(ft == ftEnumKey || ft == ftUnionKey)) {
      FLATBUFFERS_ASSERT(false && "precondition failed in GetEnumTypeForDecl");
    }

    static const char *ctypename[] = {
    // clang-format off
    #define FLATBUFFERS_TD(ENUM, IDLTYPE, CTYPE, JTYPE, GTYPE, NTYPE, PTYPE, \
                           RTYPE, KTYPE) \
            #RTYPE,
        FLATBUFFERS_GEN_TYPES(FLATBUFFERS_TD)
    #undef FLATBUFFERS_TD
      // clang-format on
    };

    // Enums can be bools, but their Rust representation must be a u8, as used
    // in the repr attribute (#[repr(bool)] is an invalid attribute).
    if (type.base_type == BASE_TYPE_BOOL) return "u8";
    return ctypename[type.base_type];
  }

  // Return a Rust type for any type (scalar, table, struct) specifically for
  // using a FlatBuffer.
  std::string GetTypeGet(const Type &type) const {
    switch (GetFullType(type)) {
      case ftInteger:
      case ftFloat:
      case ftBool:
      case ftEnumKey:
      case ftUnionKey: {
        return GetTypeBasic(type); }
      case ftTable: {
        return WrapInNameSpace(type.struct_def->defined_namespace,
                               type.struct_def->name) + "<'a>"; }
      default: {
        return WrapInNameSpace(type.struct_def->defined_namespace,
                               type.struct_def->name); }
    }
  }

  std::string GetEnumValUse(const EnumDef &enum_def,
                            const EnumVal &enum_val) const {
    return Name(enum_def) + "::" + Name(enum_val);
  }

  // Generate an enum declaration,
  // an enum string lookup table,
  // an enum match function,
  // and an enum array of values
  void GenEnum(const EnumDef &enum_def) {
    code_.SetValue("ENUM_NAME", Name(enum_def));
    code_.SetValue("BASE_TYPE", GetEnumTypeForDecl(enum_def.underlying_type));

    GenComment(enum_def.doc_comment);
    code_ += "#[allow(non_camel_case_types)]";
    code_ += "#[repr({{BASE_TYPE}})]";
    code_ += "#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Debug)]";
    code_ += "pub enum " + Name(enum_def) + " {";

    for (auto it = enum_def.Vals().begin(); it != enum_def.Vals().end(); ++it) {
      const auto &ev = **it;

      GenComment(ev.doc_comment, "  ");
      code_.SetValue("KEY", Name(ev));
      code_.SetValue("VALUE", enum_def.ToString(ev));
      code_ += "  {{KEY}} = {{VALUE}},";
    }
    const EnumVal *minv = enum_def.MinValue();
    const EnumVal *maxv = enum_def.MaxValue();
    FLATBUFFERS_ASSERT(minv && maxv);

    code_ += "";
    code_ += "}";
    code_ += "";

    code_.SetValue("ENUM_NAME", Name(enum_def));
    code_.SetValue("ENUM_NAME_SNAKE", MakeSnakeCase(Name(enum_def)));
    code_.SetValue("ENUM_NAME_CAPS", MakeUpper(MakeSnakeCase(Name(enum_def))));
    code_.SetValue("ENUM_MIN_BASE_VALUE", enum_def.ToString(*minv));
    code_.SetValue("ENUM_MAX_BASE_VALUE", enum_def.ToString(*maxv));

    // Generate enum constants, and impls for Follow, EndianScalar, and Push.
    code_ += "const ENUM_MIN_{{ENUM_NAME_CAPS}}: {{BASE_TYPE}} = \\";
    code_ += "{{ENUM_MIN_BASE_VALUE}};";
    code_ += "const ENUM_MAX_{{ENUM_NAME_CAPS}}: {{BASE_TYPE}} = \\";
    code_ += "{{ENUM_MAX_BASE_VALUE}};";
    code_ += "";
    code_ += "impl<'a> flatbuffers::Follow<'a> for {{ENUM_NAME}} {";
    code_ += "  type Inner = Self;";
    code_ += "  #[inline]";
    code_ += "  fn follow(buf: &'a [u8], loc: usize) -> Self::Inner {";
    code_ += "    flatbuffers::read_scalar_at::<Self>(buf, loc)";
    code_ += "  }";
    code_ += "}";
    code_ += "";
    code_ += "impl flatbuffers::EndianScalar for {{ENUM_NAME}} {";
    code_ += "  #[inline]";
    code_ += "  fn to_little_endian(self) -> Self {";
    code_ += "    let n = {{BASE_TYPE}}::to_le(self as {{BASE_TYPE}});";
    code_ += "    let p = &n as *const {{BASE_TYPE}} as *const {{ENUM_NAME}};";
    code_ += "    unsafe { *p }";
    code_ += "  }";
    code_ += "  #[inline]";
    code_ += "  fn from_little_endian(self) -> Self {";
    code_ += "    let n = {{BASE_TYPE}}::from_le(self as {{BASE_TYPE}});";
    code_ += "    let p = &n as *const {{BASE_TYPE}} as *const {{ENUM_NAME}};";
    code_ += "    unsafe { *p }";
    code_ += "  }";
    code_ += "}";
    code_ += "";
    code_ += "impl flatbuffers::Push for {{ENUM_NAME}} {";
    code_ += "    type Output = {{ENUM_NAME}};";
    code_ += "    #[inline]";
    code_ += "    fn push(&self, dst: &mut [u8], _rest: &[u8]) {";
    code_ += "        flatbuffers::emplace_scalar::<{{ENUM_NAME}}>"
             "(dst, *self);";
    code_ += "    }";
    code_ += "}";
    code_ += "";

    // Generate an array of all enumeration values.
    auto num_fields = NumToString(enum_def.size());
    code_ += "#[allow(non_camel_case_types)]";
    code_ += "const ENUM_VALUES_{{ENUM_NAME_CAPS}}:[{{ENUM_NAME}}; " +
              num_fields + "] = [";
    for (auto it = enum_def.Vals().begin(); it != enum_def.Vals().end(); ++it) {
      const auto &ev = **it;
      auto value = GetEnumValUse(enum_def, ev);
      auto suffix = *it != enum_def.Vals().back() ? "," : "";
      code_ += "  " + value + suffix;
    }
    code_ += "];";
    code_ += "";

    // Generate a string table for enum values.
    // Problem is, if values are very sparse that could generate really big
    // tables. Ideally in that case we generate a map lookup instead, but for
    // the moment we simply don't output a table at all.
    auto range = enum_def.Distance();
    // Average distance between values above which we consider a table
    // "too sparse". Change at will.
    static const uint64_t kMaxSparseness = 5;
    if (range / static_cast<uint64_t>(enum_def.size()) < kMaxSparseness) {
      code_ += "#[allow(non_camel_case_types)]";
      code_ += "const ENUM_NAMES_{{ENUM_NAME_CAPS}}:[&'static str; " +
               NumToString(range + 1) + "] = [";

      auto val = enum_def.Vals().front();
      for (auto it = enum_def.Vals().begin(); it != enum_def.Vals().end();
           ++it) {
        auto ev = *it;
        for (auto k = enum_def.Distance(val, ev); k > 1; --k) {
          code_ += "    \"\",";
        }
        val = ev;
        auto suffix = *it != enum_def.Vals().back() ? "," : "";
        code_ += "    \"" + Name(*ev) + "\"" + suffix;
      }
      code_ += "];";
      code_ += "";

      code_ +=
          "pub fn enum_name_{{ENUM_NAME_SNAKE}}(e: {{ENUM_NAME}}) -> "
          "&'static str {";

      code_ += "  let index = e as {{BASE_TYPE}}\\";
      if (enum_def.MinValue()->IsNonZero()) {
        auto vals = GetEnumValUse(enum_def, *enum_def.MinValue());
        code_ += " - " + vals + " as {{BASE_TYPE}}\\";
      }
      code_ += ";";

      code_ += "  ENUM_NAMES_{{ENUM_NAME_CAPS}}[index as usize]";
      code_ += "}";
      code_ += "";
    }

    if (enum_def.is_union) {
      // Generate tyoesafe offset(s) for unions
      code_.SetValue("NAME", Name(enum_def));
      code_.SetValue("UNION_OFFSET_NAME", Name(enum_def) + "UnionTableOffset");
      code_ += "pub struct {{UNION_OFFSET_NAME}} {}";
    }
  }

  std::string GetFieldOffsetName(const FieldDef &field) {
    return "VT_" + MakeUpper(Name(field));
  }

  std::string GetDefaultConstant(const FieldDef &field) {
    return field.value.type.base_type == BASE_TYPE_FLOAT
               ? field.value.constant + ""
               : field.value.constant;
  }

  std::string GetDefaultScalarValue(const FieldDef &field) {
    switch (GetFullType(field.value.type)) {
      case ftInteger: { return GetDefaultConstant(field); }
      case ftFloat: { return GetDefaultConstant(field); }
      case ftBool: {
        return field.value.constant == "0" ? "false" : "true";
      }
      case ftUnionKey:
      case ftEnumKey: {
        auto ev = field.value.type.enum_def->FindByValue(field.value.constant);
        assert(ev);
        return WrapInNameSpace(field.value.type.enum_def->defined_namespace,
                               GetEnumValUse(*field.value.type.enum_def, *ev));
      }

      // All pointer-ish types have a default value of None, because they are
      // wrapped in Option.
      default: { return "None"; }
    }
  }

  // Create the return type for fields in the *BuilderArgs structs that are
  // used to create Tables.
  //
  // Note: we could make all inputs to the BuilderArgs be an Option, as well
  // as all outputs. But, the UX of Flatbuffers is that the user doesn't get to
  // know if the value is default or not, because there are three ways to
  // return a default value:
  // 1) return a stored value that happens to be the default,
  // 2) return a hardcoded value because the relevant vtable field is not in
  //    the vtable, or
  // 3) return a hardcoded value because the vtable field value is set to zero.
  std::string TableBuilderArgsDefnType(const FieldDef &field,
                                       const std::string &lifetime) {
    const Type& type = field.value.type;

    switch (GetFullType(type)) {
      case ftInteger:
      case ftFloat:
      case ftBool: {
        const auto typname = GetTypeBasic(type);
        return typname;
      }
      case ftStruct: {
        const auto typname = WrapInNameSpace(*type.struct_def);
        return "Option<&" + lifetime + " " + typname + ">";
      }
      case ftTable: {
        const auto typname = WrapInNameSpace(*type.struct_def);
        return "Option<flatbuffers::WIPOffset<" + typname + "<" + lifetime + \
               ">>>";
      }
      case ftString: {
        return "Option<flatbuffers::WIPOffset<&" + lifetime + " str>>";
      }
      case ftEnumKey:
      case ftUnionKey: {
        const auto typname = WrapInNameSpace(*type.enum_def);
        return typname;
      }
      case ftUnionValue: {
        return "Option<flatbuffers::WIPOffset<flatbuffers::UnionWIPOffset>>";
      }

      case ftVectorOfInteger:
      case ftVectorOfFloat: {
        const auto typname = GetTypeBasic(type.VectorType());
        return "Option<flatbuffers::WIPOffset<flatbuffers::Vector<" + \
               lifetime + ",  " + typname + ">>>";
      }
      case ftVectorOfBool: {
        return "Option<flatbuffers::WIPOffset<flatbuffers::Vector<" + \
               lifetime + ", bool>>>";
      }
      case ftVectorOfEnumKey: {
        const auto typname = WrapInNameSpace(*type.enum_def);
        return "Option<flatbuffers::WIPOffset<flatbuffers::Vector<" + \
               lifetime + ", " + typname + ">>>";
      }
      case ftVectorOfStruct: {
        const auto typname = WrapInNameSpace(*type.struct_def);
        return "Option<flatbuffers::WIPOffset<flatbuffers::Vector<" + \
               lifetime + ", " + typname + ">>>";
      }
      case ftVectorOfTable: {
        const auto typname = WrapInNameSpace(*type.struct_def);
        return "Option<flatbuffers::WIPOffset<flatbuffers::Vector<" + \
               lifetime + ", flatbuffers::ForwardsUOffset<" + typname + \
               "<" + lifetime + ">>>>>";
      }
      case ftVectorOfString: {
        return "Option<flatbuffers::WIPOffset<flatbuffers::Vector<" + \
               lifetime + ", flatbuffers::ForwardsUOffset<&" + lifetime + \
               " str>>>>";
      }
      case ftVectorOfUnionValue: {
        const auto typname = WrapInNameSpace(*type.enum_def) + \
                             "UnionTableOffset";
        return "Option<flatbuffers::WIPOffset<flatbuffers::Vector<" + \
               lifetime + ", flatbuffers::ForwardsUOffset<"
               "flatbuffers::Table<" + lifetime + ">>>>";
      }
    }
    return "INVALID_CODE_GENERATION"; // for return analysis
  }

  std::string TableBuilderArgsDefaultValue(const FieldDef &field) {
    return GetDefaultScalarValue(field);
  }
  std::string TableBuilderAddFuncDefaultValue(const FieldDef &field) {
    // All branches of switch do the same action!
    switch (GetFullType(field.value.type)) {
      case ftUnionKey:
      case ftEnumKey: {
        const std::string basetype = GetTypeBasic(field.value.type); //<- never used
        return GetDefaultScalarValue(field);
      }

      default: { return GetDefaultScalarValue(field); }
    }
  }

  std::string TableBuilderArgsAddFuncType(const FieldDef &field,
                                          const std::string &lifetime) {
    const Type& type = field.value.type;

    switch (GetFullType(field.value.type)) {
      case ftVectorOfStruct: {
        const auto typname = WrapInNameSpace(*type.struct_def);
        return "flatbuffers::WIPOffset<flatbuffers::Vector<" + lifetime + \
               ", " + typname + ">>";
      }
      case ftVectorOfTable: {
        const auto typname = WrapInNameSpace(*type.struct_def);
        return "flatbuffers::WIPOffset<flatbuffers::Vector<" + lifetime + \
               ", flatbuffers::ForwardsUOffset<" + typname + \
               "<" + lifetime + ">>>>";
      }
      case ftVectorOfInteger:
      case ftVectorOfFloat: {
        const auto typname = GetTypeBasic(type.VectorType());
        return "flatbuffers::WIPOffset<flatbuffers::Vector<" + lifetime + \
               ", " + typname + ">>";
      }
      case ftVectorOfBool: {
        return "flatbuffers::WIPOffset<flatbuffers::Vector<" + lifetime + \
               ", bool>>";
      }
      case ftVectorOfString: {
        return "flatbuffers::WIPOffset<flatbuffers::Vector<" + lifetime + \
               ", flatbuffers::ForwardsUOffset<&" + lifetime + " str>>>";
      }
      case ftVectorOfEnumKey: {
        const auto typname = WrapInNameSpace(*type.enum_def);
        return "flatbuffers::WIPOffset<flatbuffers::Vector<" + lifetime + \
               ", " + typname + ">>";
      }
      case ftVectorOfUnionValue: {
        return "flatbuffers::WIPOffset<flatbuffers::Vector<" + lifetime + \
               ", flatbuffers::ForwardsUOffset<flatbuffers::Table<" + \
               lifetime + ">>>";
      }
      case ftEnumKey: {
        const auto typname = WrapInNameSpace(*type.enum_def);
        return typname;
      }
      case ftStruct: {
        const auto typname = WrapInNameSpace(*type.struct_def);
        return "&" + lifetime + " " + typname + "";
      }
      case ftTable: {
        const auto typname = WrapInNameSpace(*type.struct_def);
        return "flatbuffers::WIPOffset<" + typname + "<" + lifetime + ">>";
      }
      case ftInteger:
      case ftFloat: {
        const auto typname = GetTypeBasic(type);
        return typname;
      }
      case ftBool: {
        return "bool";
      }
      case ftString: {
        return "flatbuffers::WIPOffset<&" + lifetime + " str>";
      }
      case ftUnionKey: {
        const auto typname = WrapInNameSpace(*type.enum_def);
        return typname;
      }
      case ftUnionValue: {
        return "flatbuffers::WIPOffset<flatbuffers::UnionWIPOffset>";
      }
    }

    return "INVALID_CODE_GENERATION"; // for return analysis
  }

  std::string TableBuilderArgsAddFuncBody(const FieldDef &field) {
    const Type& type = field.value.type;

    switch (GetFullType(field.value.type)) {
      case ftInteger:
      case ftFloat: {
        const auto typname = GetTypeBasic(field.value.type);
        return "self.fbb_.push_slot::<" + typname + ">";
      }
      case ftBool: {
        return "self.fbb_.push_slot::<bool>";
      }

      case ftEnumKey:
      case ftUnionKey: {
        const auto underlying_typname = GetTypeBasic(type);
        return "self.fbb_.push_slot::<" + underlying_typname + ">";
      }

      case ftStruct: {
        const std::string typname = WrapInNameSpace(*type.struct_def);
        return "self.fbb_.push_slot_always::<&" + typname + ">";
      }
      case ftTable: {
        const auto typname = WrapInNameSpace(*type.struct_def);
        return "self.fbb_.push_slot_always::<flatbuffers::WIPOffset<" + \
               typname +  ">>";
      }

      case ftUnionValue:
      case ftString:
      case ftVectorOfInteger:
      case ftVectorOfFloat:
      case ftVectorOfBool:
      case ftVectorOfEnumKey:
      case ftVectorOfStruct:
      case ftVectorOfTable:
      case ftVectorOfString:
      case ftVectorOfUnionValue: {
        return "self.fbb_.push_slot_always::<flatbuffers::WIPOffset<_>>";
      }
    }
    return "INVALID_CODE_GENERATION"; // for return analysis
  }

  std::string GenTableAccessorFuncReturnType(const FieldDef &field,
                                             const std::string &lifetime) {
    const Type& type = field.value.type;

    switch (GetFullType(field.value.type)) {
      case ftInteger:
      case ftFloat: {
        const auto typname = GetTypeBasic(type);
        return typname;
      }
      case ftBool: {
        return "bool";
      }
      case ftStruct: {
        const auto typname = WrapInNameSpace(*type.struct_def);
        return WrapInOptionIfNotRequired("&" + lifetime + " " + typname, field.required);
      }
      case ftTable: {
        const auto typname = WrapInNameSpace(*type.struct_def);
        return WrapInOptionIfNotRequired(typname + "<" + lifetime + ">", field.required);
      }
      case ftEnumKey:
      case ftUnionKey: {
        const auto typname = WrapInNameSpace(*type.enum_def);
        return typname;
      }

      case ftUnionValue: {
        return WrapInOptionIfNotRequired("flatbuffers::Table<" + lifetime + ">", field.required);
      }
      case ftString: {
         return WrapInOptionIfNotRequired("&" + lifetime + " str", field.required);
      }
      case ftVectorOfInteger:
      case ftVectorOfFloat: {
        const auto typname = GetTypeBasic(type.VectorType());
        if (IsOneByte(type.VectorType().base_type)) {
          return WrapInOptionIfNotRequired("&" + lifetime + " [" + typname + "]", field.required);
        }
        return WrapInOptionIfNotRequired("flatbuffers::Vector<" + lifetime + ", " + typname + ">", field.required);
      }
      case ftVectorOfBool: {
        return WrapInOptionIfNotRequired("&" + lifetime + " [bool]", field.required);
      }
      case ftVectorOfEnumKey: {
        const auto typname = WrapInNameSpace(*type.enum_def);
        return WrapInOptionIfNotRequired("flatbuffers::Vector<" + lifetime + ", " + typname + ">", field.required);
      }
      case ftVectorOfStruct: {
        const auto typname = WrapInNameSpace(*type.struct_def);
        return WrapInOptionIfNotRequired("&" + lifetime + " [" + typname + "]", field.required);
      }
      case ftVectorOfTable: {
        const auto typname = WrapInNameSpace(*type.struct_def);
        return WrapInOptionIfNotRequired("flatbuffers::Vector<" + lifetime + ", flatbuffers::ForwardsUOffset<" + \
               typname + "<" + lifetime + ">>>", field.required);
      }
      case ftVectorOfString: {
        return WrapInOptionIfNotRequired("flatbuffers::Vector<" + lifetime + ", flatbuffers::ForwardsUOffset<&" + \
               lifetime + " str>>", field.required);
      }
      case ftVectorOfUnionValue: {
        FLATBUFFERS_ASSERT(false && "vectors of unions are not yet supported");
        // TODO(rw): when we do support these, we should consider using the
        //           Into trait to convert tables to typesafe union values.
        return "INVALID_CODE_GENERATION"; // for return analysis
      }
    }
    return "INVALID_CODE_GENERATION"; // for return analysis
  }

  std::string GenTableAccessorFuncBody(const FieldDef &field,
                                       const std::string &lifetime,
                                       const std::string &offset_prefix) {
    const std::string offset_name = offset_prefix + "::" + \
                                    GetFieldOffsetName(field);
    const Type& type = field.value.type;

    switch (GetFullType(field.value.type)) {
      case ftInteger:
      case ftFloat:
      case ftBool: {
        const auto typname = GetTypeBasic(type);
        const auto default_value = GetDefaultScalarValue(field);
        return "self._tab.get::<" + typname + ">(" + offset_name + ", Some(" + \
               default_value + ")).unwrap()";
      }
      case ftStruct: {
        const auto typname = WrapInNameSpace(*type.struct_def);
        return AddUnwrapIfRequired("self._tab.get::<" + typname + ">(" + offset_name + ", None)", field.required);
      }
      case ftTable: {
        const auto typname = WrapInNameSpace(*type.struct_def);
        return AddUnwrapIfRequired("self._tab.get::<flatbuffers::ForwardsUOffset<" + \
               typname + "<" + lifetime + ">>>(" + offset_name + ", None)", field.required);
      }
      case ftUnionValue: {
        return AddUnwrapIfRequired("self._tab.get::<flatbuffers::ForwardsUOffset<"
               "flatbuffers::Table<" + lifetime + ">>>(" + offset_name + \
               ", None)", field.required);
      }
      case ftUnionKey:
      case ftEnumKey: {
        const auto underlying_typname = GetTypeBasic(type); //<- never used
        const auto typname = WrapInNameSpace(*type.enum_def);
        const auto default_value = GetDefaultScalarValue(field);
        return "self._tab.get::<" + typname + ">(" + offset_name + \
               ", Some(" + default_value + ")).unwrap()";
      }
      case ftString: {
        return AddUnwrapIfRequired("self._tab.get::<flatbuffers::ForwardsUOffset<&str>>(" + \
               offset_name + ", None)", field.required);
      }

      case ftVectorOfInteger:
      case ftVectorOfFloat: {
        const auto typname = GetTypeBasic(type.VectorType());
        std::string s = "self._tab.get::<flatbuffers::ForwardsUOffset<"
                        "flatbuffers::Vector<" + lifetime + ", " + typname + \
                        ">>>(" + offset_name + ", None)";
        // single-byte values are safe to slice
        if (IsOneByte(type.VectorType().base_type)) {
          s += ".map(|v| v.safe_slice())";
        }
        return AddUnwrapIfRequired(s, field.required);
      }
      case ftVectorOfBool: {
        return AddUnwrapIfRequired("self._tab.get::<flatbuffers::ForwardsUOffset<"
               "flatbuffers::Vector<" + lifetime + ", bool>>>(" + \
               offset_name + ", None).map(|v| v.safe_slice())", field.required);
      }
      case ftVectorOfEnumKey: {
        const auto typname = WrapInNameSpace(*type.enum_def);
        return AddUnwrapIfRequired("self._tab.get::<flatbuffers::ForwardsUOffset<"
               "flatbuffers::Vector<" + lifetime + ", " + typname + ">>>(" + \
               offset_name + ", None)", field.required);
      }
      case ftVectorOfStruct: {
        const auto typname = WrapInNameSpace(*type.struct_def);
        return AddUnwrapIfRequired("self._tab.get::<flatbuffers::ForwardsUOffset<"
               "flatbuffers::Vector<" + typname + ">>>(" + \
               offset_name + ", None).map(|v| v.safe_slice() )", field.required);
      }
      case ftVectorOfTable: {
        const auto typname = WrapInNameSpace(*type.struct_def);
        return AddUnwrapIfRequired("self._tab.get::<flatbuffers::ForwardsUOffset<"
               "flatbuffers::Vector<flatbuffers::ForwardsUOffset<" + typname + \
               "<" + lifetime + ">>>>>(" + offset_name + ", None)", field.required);
      }
      case ftVectorOfString: {
        return AddUnwrapIfRequired("self._tab.get::<flatbuffers::ForwardsUOffset<"
               "flatbuffers::Vector<flatbuffers::ForwardsUOffset<&" + \
               lifetime + " str>>>>(" + offset_name + ", None)", field.required);
      }
      case ftVectorOfUnionValue: {
        FLATBUFFERS_ASSERT(false && "vectors of unions are not yet supported");
        return "INVALID_CODE_GENERATION"; // for return analysis
      }
    }
    return "INVALID_CODE_GENERATION"; // for return analysis
  }

  bool TableFieldReturnsOption(const Type& type) {
    switch (GetFullType(type)) {
      case ftInteger:
      case ftFloat:
      case ftBool:
      case ftEnumKey:
      case ftUnionKey:
        return false;
      default: return true;
    }
  }

  // Generate an accessor struct, builder struct, and create function for a
  // table.
  void GenTable(const StructDef &struct_def) {
    code_.SetValue("STRUCT_NAME", Name(struct_def));
    code_.SetValue("OFFSET_TYPELABEL", Name(struct_def) + "Offset");
    code_.SetValue("STRUCT_NAME_SNAKECASE", MakeSnakeCase(Name(struct_def)));

    // Generate an offset type, the base type, the Follow impl, and the
    // init_from_table impl.
    code_ += "pub enum {{OFFSET_TYPELABEL}} {}";
    code_ += "#[derive(Copy, Clone, Debug, PartialEq)]";
    code_ += "";

    GenComment(struct_def.doc_comment);

    code_ += "pub struct {{STRUCT_NAME}}<'a> {";
    code_ += "  pub _tab: flatbuffers::Table<'a>,";
    code_ += "}";
    code_ += "";
    code_ += "impl<'a> flatbuffers::Follow<'a> for {{STRUCT_NAME}}<'a> {";
    code_ += "    type Inner = {{STRUCT_NAME}}<'a>;";
    code_ += "    #[inline]";
    code_ += "    fn follow(buf: &'a [u8], loc: usize) -> Self::Inner {";
    code_ += "        Self {";
    code_ += "            _tab: flatbuffers::Table { buf: buf, loc: loc },";
    code_ += "        }";
    code_ += "    }";
    code_ += "}";
    code_ += "";
    code_ += "impl<'a> {{STRUCT_NAME}}<'a> {";
    code_ += "    #[inline]";
    code_ += "    pub fn init_from_table(table: flatbuffers::Table<'a>) -> "
             "Self {";
    code_ += "        {{STRUCT_NAME}} {";
    code_ += "            _tab: table,";
    code_ += "        }";
    code_ += "    }";

    // Generate a convenient create* function that uses the above builder
    // to create a table in one function call.
    code_.SetValue("MAYBE_US",
        struct_def.fields.vec.size() == 0 ? "_" : "");
    code_.SetValue("MAYBE_LT",
        TableBuilderArgsNeedsLifetime(struct_def) ? "<'args>" : "");
    code_ += "    #[allow(unused_mut)]";
    code_ += "    pub fn create<'bldr: 'args, 'args: 'mut_bldr, 'mut_bldr>(";
    code_ += "        _fbb: "
             "&'mut_bldr mut flatbuffers::FlatBufferBuilder<'bldr>,";
    code_ += "        {{MAYBE_US}}args: &'args {{STRUCT_NAME}}Args{{MAYBE_LT}})"
             " -> flatbuffers::WIPOffset<{{STRUCT_NAME}}<'bldr>> {";

    code_ += "      let mut builder = {{STRUCT_NAME}}Builder::new(_fbb);";
    for (size_t size = struct_def.sortbysize ? sizeof(largest_scalar_t) : 1;
         size; size /= 2) {
      for (auto it = struct_def.fields.vec.rbegin();
           it != struct_def.fields.vec.rend(); ++it) {
        const auto &field = **it;
        // TODO(rw): fully understand this sortbysize usage
        if (!field.deprecated && (!struct_def.sortbysize ||
                                  size == SizeOf(field.value.type.base_type))) {
          code_.SetValue("FIELD_NAME", Name(field));
          if (TableFieldReturnsOption(field.value.type)) {
            code_ += "      if let Some(x) = args.{{FIELD_NAME}} "
                     "{ builder.add_{{FIELD_NAME}}(x); }";
          } else {
            code_ += "      builder.add_{{FIELD_NAME}}(args.{{FIELD_NAME}});";
          }
        }
      }
    }
    code_ += "      builder.finish()";
    code_ += "    }";
    code_ += "";

    // Generate field id constants.
    if (struct_def.fields.vec.size() > 0) {
      for (auto it = struct_def.fields.vec.begin();
           it != struct_def.fields.vec.end(); ++it) {
        const auto &field = **it;
        if (field.deprecated) {
          // Deprecated fields won't be accessible.
          continue;
        }

        code_.SetValue("OFFSET_NAME", GetFieldOffsetName(field));
        code_.SetValue("OFFSET_VALUE", NumToString(field.value.offset));
        code_ += "    pub const {{OFFSET_NAME}}: flatbuffers::VOffsetT = "
                 "{{OFFSET_VALUE}};";
      }
      code_ += "";
    }

    // Generate the accessors. Each has one of two forms:
    //
    // If a value can be None:
    //   pub fn name(&'a self) -> Option<user_facing_type> {
    //     self._tab.get::<internal_type>(offset, defaultval)
    //   }
    //
    // If a value is always Some:
    //   pub fn name(&'a self) -> user_facing_type {
    //     self._tab.get::<internal_type>(offset, defaultval).unwrap()
    //   }
    const auto offset_prefix = Name(struct_def);
    for (auto it = struct_def.fields.vec.begin();
         it != struct_def.fields.vec.end(); ++it) {
      const auto &field = **it;
      if (field.deprecated) {
        // Deprecated fields won't be accessible.
        continue;
      }

      code_.SetValue("FIELD_NAME", Name(field));
      code_.SetValue("RETURN_TYPE",
                     GenTableAccessorFuncReturnType(field, "'a"));
      code_.SetValue("FUNC_BODY",
                     GenTableAccessorFuncBody(field, "'a", offset_prefix));

      GenComment(field.doc_comment, "  ");
      code_ += "  #[inline]";
      code_ += "  pub fn {{FIELD_NAME}}(&self) -> {{RETURN_TYPE}} {";
      code_ += "    {{FUNC_BODY}}";
      code_ += "  }";

      // Generate a comparison function for this field if it is a key.
      if (field.key) {
        GenKeyFieldMethods(field);
      }

      // Generate a nested flatbuffer field, if applicable.
      auto nested = field.attributes.Lookup("nested_flatbuffer");
      if (nested) {
        std::string qualified_name = nested->constant;
        auto nested_root = parser_.LookupStruct(nested->constant);
        if (nested_root == nullptr) {
          qualified_name = parser_.current_namespace_->GetFullyQualifiedName(
              nested->constant);
          nested_root = parser_.LookupStruct(qualified_name);
        }
        FLATBUFFERS_ASSERT(nested_root);  // Guaranteed to exist by parser.
        (void)nested_root;

        code_.SetValue("OFFSET_NAME",
                       offset_prefix + "::" + GetFieldOffsetName(field));
        code_ += "  pub fn {{FIELD_NAME}}_nested_flatbuffer(&'a self) -> "
                 " Option<{{STRUCT_NAME}}<'a>> {";
        code_ += "     match self.{{FIELD_NAME}}() {";
        code_ += "         None => { None }";
        code_ += "         Some(data) => {";
        code_ += "             use self::flatbuffers::Follow;";
        code_ += "             Some(<flatbuffers::ForwardsUOffset"
                 "<{{STRUCT_NAME}}<'a>>>::follow(data, 0))";
        code_ += "         },";
        code_ += "     }";
        code_ += "  }";
      }
    }

    // Explicit specializations for union accessors
    for (auto it = struct_def.fields.vec.begin();
         it != struct_def.fields.vec.end(); ++it) {
      const auto &field = **it;
      if (field.deprecated || field.value.type.base_type != BASE_TYPE_UNION) {
        continue;
      }

      auto u = field.value.type.enum_def;

      code_.SetValue("FIELD_NAME", Name(field));

      for (auto u_it = u->Vals().begin(); u_it != u->Vals().end(); ++u_it) {
        auto &ev = **u_it;
        if (ev.union_type.base_type == BASE_TYPE_NONE) { continue; }

        auto table_init_type = WrapInNameSpace(
          ev.union_type.struct_def->defined_namespace,
          ev.union_type.struct_def->name);

          code_.SetValue("U_ELEMENT_ENUM_TYPE",
              WrapInNameSpace(u->defined_namespace, GetEnumValUse(*u, ev)));
        code_.SetValue("U_ELEMENT_TABLE_TYPE", table_init_type);
        code_.SetValue("U_ELEMENT_NAME", MakeSnakeCase(Name(ev)));

        code_ += "  #[inline]";
        code_ += "  #[allow(non_snake_case)]";
        code_ += "  pub fn {{FIELD_NAME}}_as_{{U_ELEMENT_NAME}}(&self) -> "
                 "Option<{{U_ELEMENT_TABLE_TYPE}}<'a>> {";
        code_ += "    if self.{{FIELD_NAME}}_type() == {{U_ELEMENT_ENUM_TYPE}} {";
        code_ += "      self.{{FIELD_NAME}}().map(|u| "
                 "{{U_ELEMENT_TABLE_TYPE}}::init_from_table(u))";
        code_ += "    } else {";
        code_ += "      None";
        code_ += "    }";
        code_ += "  }";
        code_ += "";
      }
    }

    code_ += "}";  // End of table impl.
    code_ += "";

    // Generate an args struct:
    code_.SetValue("MAYBE_LT",
        TableBuilderArgsNeedsLifetime(struct_def) ? "<'a>" : "");
    code_ += "pub struct {{STRUCT_NAME}}Args{{MAYBE_LT}} {";
    for (auto it = struct_def.fields.vec.begin();
         it != struct_def.fields.vec.end(); ++it) {
      const auto &field = **it;
      if (!field.deprecated) {
        code_.SetValue("PARAM_NAME", Name(field));
        code_.SetValue("PARAM_TYPE", TableBuilderArgsDefnType(field, "'a "));
        code_ += "    pub {{PARAM_NAME}}: {{PARAM_TYPE}},";
      }
    }
    code_ += "}";

    // Generate an impl of Default for the *Args type:
    code_ += "impl<'a> Default for {{STRUCT_NAME}}Args{{MAYBE_LT}} {";
    code_ += "    #[inline]";
    code_ += "    fn default() -> Self {";
    code_ += "        {{STRUCT_NAME}}Args {";
    for (auto it = struct_def.fields.vec.begin();
        it != struct_def.fields.vec.end(); ++it) {
      const auto &field = **it;
      if (!field.deprecated) {
        code_.SetValue("PARAM_VALUE", TableBuilderArgsDefaultValue(field));
        code_.SetValue("REQ", field.required ? " // required field" : "");
        code_.SetValue("PARAM_NAME", Name(field));
        code_ += "            {{PARAM_NAME}}: {{PARAM_VALUE}},{{REQ}}";
      }
    }
    code_ += "        }";
    code_ += "    }";
    code_ += "}";

    // Generate a builder struct:
    code_ += "pub struct {{STRUCT_NAME}}Builder<'a: 'b, 'b> {";
    code_ += "  fbb_: &'b mut flatbuffers::FlatBufferBuilder<'a>,";
    code_ += "  start_: flatbuffers::WIPOffset<"
             "flatbuffers::TableUnfinishedWIPOffset>,";
    code_ += "}";

    // Generate builder functions:
    code_ += "impl<'a: 'b, 'b> {{STRUCT_NAME}}Builder<'a, 'b> {";
    for (auto it = struct_def.fields.vec.begin();
         it != struct_def.fields.vec.end(); ++it) {
      const auto &field = **it;
      if (!field.deprecated) {
        const bool is_scalar = IsScalar(field.value.type.base_type);

        std::string offset = GetFieldOffsetName(field);

        // Generate functions to add data, which take one of two forms.
        //
        // If a value has a default:
        //   fn add_x(x_: type) {
        //     fbb_.push_slot::<type>(offset, x_, Some(default));
        //   }
        //
        // If a value does not have a default:
        //   fn add_x(x_: type) {
        //     fbb_.push_slot_always::<type>(offset, x_);
        //   }
        code_.SetValue("FIELD_NAME", Name(field));
        code_.SetValue("FIELD_OFFSET", Name(struct_def) + "::" + offset);
        code_.SetValue("FIELD_TYPE", TableBuilderArgsAddFuncType(field, "'b "));
        code_.SetValue("FUNC_BODY", TableBuilderArgsAddFuncBody(field));
        code_ += "  #[inline]";
        code_ += "  pub fn add_{{FIELD_NAME}}(&mut self, {{FIELD_NAME}}: "
                 "{{FIELD_TYPE}}) {";
        if (is_scalar) {
          code_.SetValue("FIELD_DEFAULT_VALUE",
                         TableBuilderAddFuncDefaultValue(field));
          code_ += "    {{FUNC_BODY}}({{FIELD_OFFSET}}, {{FIELD_NAME}}, "
                   "{{FIELD_DEFAULT_VALUE}});";
        } else {
          code_ += "    {{FUNC_BODY}}({{FIELD_OFFSET}}, {{FIELD_NAME}});";
        }
        code_ += "  }";
      }
    }

    // Struct initializer (all fields required);
    code_ += "  #[inline]";
    code_ +=
        "  pub fn new(_fbb: &'b mut flatbuffers::FlatBufferBuilder<'a>) -> "
        "{{STRUCT_NAME}}Builder<'a, 'b> {";
    code_.SetValue("NUM_FIELDS", NumToString(struct_def.fields.vec.size()));
    code_ += "    let start = _fbb.start_table();";
    code_ += "    {{STRUCT_NAME}}Builder {";
    code_ += "      fbb_: _fbb,";
    code_ += "      start_: start,";
    code_ += "    }";
    code_ += "  }";

    // finish() function.
    code_ += "  #[inline]";
    code_ += "  pub fn finish(self) -> "
             "flatbuffers::WIPOffset<{{STRUCT_NAME}}<'a>> {";
    code_ += "    let o = self.fbb_.end_table(self.start_);";

    for (auto it = struct_def.fields.vec.begin();
         it != struct_def.fields.vec.end(); ++it) {
      const auto &field = **it;
      if (!field.deprecated && field.required) {
        code_.SetValue("FIELD_NAME", MakeSnakeCase(Name(field)));
        code_.SetValue("OFFSET_NAME", GetFieldOffsetName(field));
        code_ += "    self.fbb_.required(o, {{STRUCT_NAME}}::{{OFFSET_NAME}},"
                 "\"{{FIELD_NAME}}\");";
      }
    }
    code_ += "    flatbuffers::WIPOffset::new(o.value())";
    code_ += "  }";
    code_ += "}";
    code_ += "";
  }

  // Generate functions to compare tables and structs by key. This function
  // must only be called if the field key is defined.
  void GenKeyFieldMethods(const FieldDef &field) {
    FLATBUFFERS_ASSERT(field.key);

    code_.SetValue("KEY_TYPE", GenTableAccessorFuncReturnType(field, ""));

    code_ += "  #[inline]";
    code_ += "  pub fn key_compare_less_than(&self, o: &{{STRUCT_NAME}}) -> "
             " bool {";
    code_ += "    self.{{FIELD_NAME}}() < o.{{FIELD_NAME}}()";
    code_ += "  }";
    code_ += "";
    code_ += "  #[inline]";
    code_ += "  pub fn key_compare_with_value(&self, val: {{KEY_TYPE}}) -> "
             " ::std::cmp::Ordering {";
    code_ += "    let key = self.{{FIELD_NAME}}();";
    code_ += "    key.cmp(&val)";
    code_ += "  }";
  }

  // Generate functions for accessing the root table object. This function
  // must only be called if the root table is defined.
  void GenRootTableFuncs(const StructDef &struct_def) {
    FLATBUFFERS_ASSERT(parser_.root_struct_def_ && "root table not defined");
    auto name = Name(struct_def);

    code_.SetValue("STRUCT_NAME", name);
    code_.SetValue("STRUCT_NAME_SNAKECASE", MakeSnakeCase(name));
    code_.SetValue("STRUCT_NAME_CAPS", MakeUpper(MakeSnakeCase(name)));

    // The root datatype accessors:
    code_ += "#[inline]";
    code_ +=
        "pub fn get_root_as_{{STRUCT_NAME_SNAKECASE}}<'a>(buf: &'a [u8])"
        " -> {{STRUCT_NAME}}<'a> {";
    code_ += "  flatbuffers::get_root::<{{STRUCT_NAME}}<'a>>(buf)";
    code_ += "}";
    code_ += "";

    code_ += "#[inline]";
    code_ += "pub fn get_size_prefixed_root_as_{{STRUCT_NAME_SNAKECASE}}"
             "<'a>(buf: &'a [u8]) -> {{STRUCT_NAME}}<'a> {";
    code_ += "  flatbuffers::get_size_prefixed_root::<{{STRUCT_NAME}}<'a>>"
             "(buf)";
    code_ += "}";
    code_ += "";

    if (parser_.file_identifier_.length()) {
      // Declare the identifier
      code_ += "pub const {{STRUCT_NAME_CAPS}}_IDENTIFIER: &'static str\\";
      code_ += " = \"" + parser_.file_identifier_ + "\";";
      code_ += "";

      // Check if a buffer has the identifier.
      code_ += "#[inline]";
      code_ += "pub fn {{STRUCT_NAME_SNAKECASE}}_buffer_has_identifier\\";
      code_ += "(buf: &[u8]) -> bool {";
      code_ += "  return flatbuffers::buffer_has_identifier(buf, \\";
      code_ += "{{STRUCT_NAME_CAPS}}_IDENTIFIER, false);";
      code_ += "}";
      code_ += "";
      code_ += "#[inline]";
      code_ += "pub fn {{STRUCT_NAME_SNAKECASE}}_size_prefixed\\";
      code_ += "_buffer_has_identifier(buf: &[u8]) -> bool {";
      code_ += "  return flatbuffers::buffer_has_identifier(buf, \\";
      code_ += "{{STRUCT_NAME_CAPS}}_IDENTIFIER, true);";
      code_ += "}";
      code_ += "";
    }

    if (parser_.file_extension_.length()) {
      // Return the extension
      code_ += "pub const {{STRUCT_NAME_CAPS}}_EXTENSION: &'static str = \\";
      code_ += "\"" + parser_.file_extension_ + "\";";
      code_ += "";
    }

    // Finish a buffer with a given root object:
    code_.SetValue("OFFSET_TYPELABEL", Name(struct_def) + "Offset");
    code_ += "#[inline]";
    code_ += "pub fn finish_{{STRUCT_NAME_SNAKECASE}}_buffer<'a, 'b>(";
    code_ += "    fbb: &'b mut flatbuffers::FlatBufferBuilder<'a>,";
    code_ += "    root: flatbuffers::WIPOffset<{{STRUCT_NAME}}<'a>>) {";
    if (parser_.file_identifier_.length()) {
      code_ += "  fbb.finish(root, Some({{STRUCT_NAME_CAPS}}_IDENTIFIER));";
    } else {
      code_ += "  fbb.finish(root, None);";
    }
    code_ += "}";
    code_ += "";
    code_ += "#[inline]";
    code_ += "pub fn finish_size_prefixed_{{STRUCT_NAME_SNAKECASE}}_buffer"
             "<'a, 'b>("
             "fbb: &'b mut flatbuffers::FlatBufferBuilder<'a>, "
             "root: flatbuffers::WIPOffset<{{STRUCT_NAME}}<'a>>) {";
    if (parser_.file_identifier_.length()) {
      code_ += "  fbb.finish_size_prefixed(root, "
               "Some({{STRUCT_NAME_CAPS}}_IDENTIFIER));";
    } else {
      code_ += "  fbb.finish_size_prefixed(root, None);";
    }
    code_ += "}";
  }

  static void GenPadding(
      const FieldDef &field, std::string *code_ptr, int *id,
      const std::function<void(int bits, std::string *code_ptr, int *id)> &f) {
    if (field.padding) {
      for (int i = 0; i < 4; i++) {
        if (static_cast<int>(field.padding) & (1 << i)) {
          f((1 << i) * 8, code_ptr, id);
        }
      }
      assert(!(field.padding & ~0xF));
    }
  }

  static void PaddingDefinition(int bits, std::string *code_ptr, int *id) {
    *code_ptr += "  padding" + NumToString((*id)++) + "__: u" + \
                 NumToString(bits) + ",";
  }

  static void PaddingInitializer(int bits, std::string *code_ptr, int *id) {
    (void)bits;
    *code_ptr += "padding" + NumToString((*id)++) + "__: 0,";
  }

  // Generate an accessor struct with constructor for a flatbuffers struct.
  void GenStruct(const StructDef &struct_def) {
    // Generates manual padding and alignment.
    // Variables are private because they contain little endian data on all
    // platforms.
    GenComment(struct_def.doc_comment);
    code_.SetValue("ALIGN", NumToString(struct_def.minalign));
    code_.SetValue("STRUCT_NAME", Name(struct_def));

    code_ += "// struct {{STRUCT_NAME}}, aligned to {{ALIGN}}";
    code_ += "#[repr(C, align({{ALIGN}}))]";

    // PartialEq is useful to derive because we can correctly compare structs
    // for equality by just comparing their underlying byte data. This doesn't
    // hold for PartialOrd/Ord.
    code_ += "#[derive(Clone, Copy, Debug, PartialEq)]";
    code_ += "pub struct {{STRUCT_NAME}} {";

    int padding_id = 0;
    for (auto it = struct_def.fields.vec.begin();
         it != struct_def.fields.vec.end(); ++it) {
      const auto &field = **it;
      code_.SetValue("FIELD_TYPE", GetTypeGet(field.value.type));
      code_.SetValue("FIELD_NAME", Name(field));
      code_ += "  {{FIELD_NAME}}_: {{FIELD_TYPE}},";

      if (field.padding) {
        std::string padding;
        GenPadding(field, &padding, &padding_id, PaddingDefinition);
        code_ += padding;
      }
    }

    code_ += "} // pub struct {{STRUCT_NAME}}";

    // Generate impls for SafeSliceAccess (because all structs are endian-safe),
    // Follow for the value type, Follow for the reference type, Push for the
    // value type, and Push for the reference type.
    code_ += "impl flatbuffers::SafeSliceAccess for {{STRUCT_NAME}} {}";
    code_ += "impl<'a> flatbuffers::Follow<'a> for {{STRUCT_NAME}} {";
    code_ += "  type Inner = &'a {{STRUCT_NAME}};";
    code_ += "  #[inline]";
    code_ += "  fn follow(buf: &'a [u8], loc: usize) -> Self::Inner {";
    code_ += "    <&'a {{STRUCT_NAME}}>::follow(buf, loc)";
    code_ += "  }";
    code_ += "}";
    code_ += "impl<'a> flatbuffers::Follow<'a> for &'a {{STRUCT_NAME}} {";
    code_ += "  type Inner = &'a {{STRUCT_NAME}};";
    code_ += "  #[inline]";
    code_ += "  fn follow(buf: &'a [u8], loc: usize) -> Self::Inner {";
    code_ += "    flatbuffers::follow_cast_ref::<{{STRUCT_NAME}}>(buf, loc)";
    code_ += "  }";
    code_ += "}";
    code_ += "impl<'b> flatbuffers::Push for {{STRUCT_NAME}} {";
    code_ += "    type Output = {{STRUCT_NAME}};";
    code_ += "    #[inline]";
    code_ += "    fn push(&self, dst: &mut [u8], _rest: &[u8]) {";
    code_ += "        let src = unsafe {";
    code_ += "            ::std::slice::from_raw_parts("
             "self as *const {{STRUCT_NAME}} as *const u8, Self::size())";
    code_ += "        };";
    code_ += "        dst.copy_from_slice(src);";
    code_ += "    }";
    code_ += "}";
    code_ += "impl<'b> flatbuffers::Push for &'b {{STRUCT_NAME}} {";
    code_ += "    type Output = {{STRUCT_NAME}};";
    code_ += "";
    code_ += "    #[inline]";
    code_ += "    fn push(&self, dst: &mut [u8], _rest: &[u8]) {";
    code_ += "        let src = unsafe {";
    code_ += "            ::std::slice::from_raw_parts("
             "*self as *const {{STRUCT_NAME}} as *const u8, Self::size())";
    code_ += "        };";
    code_ += "        dst.copy_from_slice(src);";
    code_ += "    }";
    code_ += "}";
    code_ += "";
    code_ += "";

    // Generate a constructor that takes all fields as arguments.
    code_ += "impl {{STRUCT_NAME}} {";
    std::string arg_list;
    std::string init_list;
    padding_id = 0;
    for (auto it = struct_def.fields.vec.begin();
         it != struct_def.fields.vec.end(); ++it) {
      const auto &field = **it;
      const auto member_name = Name(field) + "_";
      const auto reference = StructMemberAccessNeedsCopy(field.value.type)
                             ? "" : "&'a ";
      const auto arg_name = "_" + Name(field);
      const auto arg_type = reference + GetTypeGet(field.value.type);

      if (it != struct_def.fields.vec.begin()) {
        arg_list += ", ";
      }
      arg_list += arg_name + ": ";
      arg_list += arg_type;
      init_list += "      " + member_name;
      if (StructMemberAccessNeedsCopy(field.value.type)) {
        init_list += ": " + arg_name + ".to_little_endian(),\n";
      } else {
        init_list += ": *" + arg_name + ",\n";
      }
    }

    code_.SetValue("ARG_LIST", arg_list);
    code_.SetValue("INIT_LIST", init_list);
    code_ += "  pub fn new<'a>({{ARG_LIST}}) -> Self {";
    code_ += "    {{STRUCT_NAME}} {";
    code_ += "{{INIT_LIST}}";
    padding_id = 0;
    for (auto it = struct_def.fields.vec.begin();
         it != struct_def.fields.vec.end(); ++it) {
      const auto &field = **it;
      if (field.padding) {
        std::string padding;
        GenPadding(field, &padding, &padding_id, PaddingInitializer);
        code_ += "      " + padding;
      }
    }
    code_ += "    }";
    code_ += "  }";

    // Generate accessor methods for the struct.
    for (auto it = struct_def.fields.vec.begin();
         it != struct_def.fields.vec.end(); ++it) {
      const auto &field = **it;

      auto field_type = TableBuilderArgsAddFuncType(field, "'a");
      auto member = "self." + Name(field) + "_";
      auto value = StructMemberAccessNeedsCopy(field.value.type) ?
        member + ".from_little_endian()" : member;

      code_.SetValue("FIELD_NAME", Name(field));
      code_.SetValue("FIELD_TYPE", field_type);
      code_.SetValue("FIELD_VALUE", value);
      code_.SetValue("REF", IsStruct(field.value.type) ? "&" : "");

      GenComment(field.doc_comment, "  ");
      code_ += "  pub fn {{FIELD_NAME}}<'a>(&'a self) -> {{FIELD_TYPE}} {";
      code_ += "    {{REF}}{{FIELD_VALUE}}";
      code_ += "  }";

      // Generate a comparison function for this field if it is a key.
      if (field.key) {
        GenKeyFieldMethods(field);
      }
    }
    code_ += "}";
    code_ += "";
  }

  void GenNamespaceImports(const int white_spaces) {
      std::string indent = std::string(white_spaces, ' ');
      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.
  //
  // The file must start and end with an empty (or null) namespace so that
  // namespaces are properly opened and closed.
  void SetNameSpace(const Namespace *ns) {
    if (cur_name_space_ == ns) { return; }

    // Compute the size of the longest common namespace prefix.
    // If cur_name_space is A::B::C::D and ns is A::B::E::F::G,
    // the common prefix is A::B:: and we have old_size = 4, new_size = 5
    // and common_prefix_size = 2
    size_t old_size = cur_name_space_ ? cur_name_space_->components.size() : 0;
    size_t new_size = ns ? ns->components.size() : 0;

    size_t common_prefix_size = 0;
    while (common_prefix_size < old_size && common_prefix_size < new_size &&
           ns->components[common_prefix_size] ==
               cur_name_space_->components[common_prefix_size]) {
      common_prefix_size++;
    }

    // Close cur_name_space in reverse order to reach the common prefix.
    // In the previous example, D then C are closed.
    for (size_t j = old_size; j > common_prefix_size; --j) {
      code_ += "}  // pub mod " + cur_name_space_->components[j - 1];
    }
    if (old_size != common_prefix_size) { code_ += ""; }

    // open namespace parts to reach the ns namespace
    // in the previous example, E, then F, then G are opened
    for (auto j = common_prefix_size; j != new_size; ++j) {
      code_ += "#[allow(unused_imports, dead_code)]";
      code_ += "pub mod " + MakeSnakeCase(ns->components[j]) + " {";
      // Generate local namespace imports.
      GenNamespaceImports(2);
    }
    if (new_size != common_prefix_size) { code_ += ""; }

    cur_name_space_ = ns;
  }
};

}  // namespace rust

bool GenerateRust(const Parser &parser, const std::string &path,
                  const std::string &file_name) {
  rust::RustGenerator generator(parser, path, file_name);
  return generator.generate();
}

std::string RustMakeRule(const Parser &parser, const std::string &path,
                         const std::string &file_name) {
  std::string filebase =
      flatbuffers::StripPath(flatbuffers::StripExtension(file_name));
  std::string make_rule = GeneratedFileName(path, filebase) + ": ";

  auto included_files = parser.GetIncludedFilesRecursive(file_name);
  for (auto it = included_files.begin(); it != included_files.end(); ++it) {
    make_rule += " " + *it;
  }
  return make_rule;
}

}  // namespace flatbuffers

// TODO(rw): Generated code should import other generated files.
// TODO(rw): Generated code should refer to namespaces in included files in a
//           way that makes them referrable.
// TODO(rw): Generated code should indent according to nesting level.
// 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.