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Rust bindings

Generating FIDL Rust crates

A FIDL Rust crate can be generated from a FIDL library in two ways:

  1. Manually, using the standard FIDL toolchain.
  2. Automatically, using the Fuchsia build system (which under the hood uses the standard FIDL toolchain). This option is only available within the Fuchsia source tree.


A FIDL library maps to a Rust library crate named fidl_, followed by the full library path with underscores instead of dots.

For example, given the library declaration:

library games.tictactoe;

The corresponding FIDL crate is named fidl_games_tictactoe.


Given the constants:

const uint8 BOARD_SIZE = 9;
const string NAME = "Tic-Tac-Toe";

The FIDL toolchain generates the following constants:

  • pub const BOARD_SIZE: u8
  • pub const NAME: &str

The correspondence between FIDL primitive types and Rust types is outlined in built-in types.


This section describes how the FIDL toolchain converts FIDL types to native types in Rust. These types can appear as members in an aggregate type or as parameters to a protocol method.

Built-in types

In following table, when both an "owned" and "borrowed" variant are specified, the "owned" type refers to the type that would appear in an aggregate type (e.g. as the type of a struct field or vector element), and the "borrowed" type refers to the type that would appear if it were used as a protocol method parameter (from the client's perspective) or response tuple value (from the server's perspective). The distinction between owned and borrowed exists in order to take advantage of Rust’s ownership model. When making a request with a parameter of type T, the proxied function call does not need to take ownership of T so the FIDL toolchain needs to generate a borrowed version of T. Borrowed versions often use &mut since the type T may contain handles, in which case the FIDL bindings zero out the handles when encoding which modifies the input. Using &mut instead of taking ownership allows callers to reuse the input value if it does not contain handles.

FIDL Type Rust Type
bool bool
int8 i8
int16 i16
int32 i32
int64 i64
uint8 u8
uint16 u16
uint32 u32
uint64 u64
float32 f32
float64 f64
array:N &mut [T; N] (borrowed)
[T, N] (owned)
vector:N &[T] (borrowed, when T is a numeric primitive)
&mut dyn ExactSizeIterator (borrowed)
Vec (owned)
string &str (borrowed)
String (owned)
request fidl::endpoints::ServerEnd<PMarker>, where PMarker is the marker type for this protocol.
P fidl::endpoints::ClientEnd<PMarker> where PMarker is the marker type for this protocol.
handle fidl::Handle
handle The corresponding handle type is used. For example,fidl::Channel or fidl::Vmo

User defined types

Bits, enums, and tables are always referred to using their generated type T. structs and unions can be either non-nullable or nullable, and used in an owned context or borrowed context, which means that there are four possible equivalent Rust types. For a given struct T or union T, the types are as follows:

owned borrowed
non-nullable T &mut T
nullable Option<T> Option<&mut T>

Request, response, and event parameters

When FIDL needs to generate a single Rust type representing the parameters to a request, response, or event, such as for result types, it uses the following rules:

  • Multiple parameters are represented as a tuple of the parameter types.
  • A single parameter is represented just using the parameter's type.
  • An empty set of parameters is represented using the unit type ().



Given the bits definition:

bits FileMode : uint16 {
    READ = 0b001;
    WRITE = 0b010;
    EXECUTE = 0b100;

The FIDL toolchain generates an equivalent set of bitflags called FileMode with flags FileMode::Read, FileMode::Write, and FileMode::Execute. Bits members using screaming snake case are converted to camel case in the generated Rust code.

The generated bitflags struct always has the complete set of [#derive] rules.


Given the enum definition:

enum Color {
    RED = 1;
    GREEN = 2;
    BLUE = 3;

The FIDL toolchain generates an equivalent Rust enum using the specified underlying type, or u32 if none is specified:

#[derive(Copy, Clone, Debug, Eq, PartialEq, Ord, PartialOrd, Hash)]
pub enum Color {
    Red = 1,
    Green = 2,
    Blue = 3,

With the following methods:

  • from_primitive(prim: u32) -> Option<Color>: Returns Some of the enum variant corresponding to the discriminant value if any, and None otherwise.
  • into_primitive(&self) -> u32: Returns the underlying discriminant value.

The generated enum always has the complete set of [#derive] rules.


Given the struct declaration:

struct Color {
    uint32 id;
    string name = "red";

The FIDL toolchain generates an equivalent Rust struct:

#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
pub struct Color {
    pub id: u32,
    pub name: String,

The generated Color struct follows the [#derive] rules.


Given the union definition:

union JsonValue {
    1: reserved;
    2: int32 int_value;
    3: string string_value;

The FIDL toolchain generates an equivalent Rust enum:

#[derive(Debug, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
pub enum JsonValue {

The generated JsonValue enum follows the [#derive] rules.

Flexible unions

Flexible unions (that is, unions that are prefixed with the flexible keyword in their FIDL definition) have an enum additional variant generated to represent the case where the variant is unknown. This variant is considered private - users should instead match against it with a catch all case, which also ensures that adding new variants to a flexible union is source compatible:

// this code will still compile if a new union variant is added
match json_value {
    JsonValue::IntValue(val) => ...,
    JsonValue::StringValue(val) => ...,
    _ => ..., // unknown variant

Encoding a union with an unknown variant writes the unknown data and the original ordinal back onto the wire.


Given the table definition:

table User {
    1: reserved;
    2: uint8 age;
    3: string name;

The FIDL toolchain generates a struct User where each member is optional:

#[derive(Debug, PartialEq)]
pub struct User {
  pub age: Option<u8>,
  pub name: Option<String>,

With the following methods:

  • empty() -> User: Returns a new User, with each member initialized to None.

The generated User struct follows the [#derive] rules.

Table initialization

The recommended way of initializing a table is using the struct update syntax:

let user = User {
    age: Some(20),

This prevents API breakage when new fields are added.


When the FIDL toolchain generates a new struct or enum for a FIDL type, it attempts to derive as many traits from a predefined list of useful traits as it can, including Debug, Copy, Clone, etc. The complete list of default derived traits can be found in the compiler source.

For aggregate types, such as structs, unions, and tables, the set of derives is determined by starting with the list of all possible derives and then removing some based on the fields that are transitively present in the type. For example, aggregate types that transitively contain a vector do not derive Copy, and types that contain a handle do not derive Copy and Clone. Additionally, arrays larger than 32 have no derives. When in doubt, refer to the generated code to check which traits are derived by a specific type. The complete set of rules can be found in fillDerivesForSource.


Given a protocol:

protocol TicTacToe {
    StartGame(bool start_first);
    MakeMove(uint8 row, uint8 col) -> (bool success, GameState? new_state);
    -> OnOpponentMove(GameState new_state);

The main entrypoint for interacting with TicTacToe is the TicTacToeMarker struct, which contains two associated types:

  • Proxy: The associated proxy type for use with async clients. In this example, this is a generated TicTacToeProxy type. Synchronous clients should use TicTacToeSynchronousProxy directly (see Synchronous), which is not stored in an associated type on the TicTacToeMarker.
  • RequestStream: The associated request stream that servers implementing this protocol will need to handle. In this example, this is TicTacToeRequestStream, which is generated by FIDL.

Additionally, TicTacToeMarker has the following associated constants: * DEBUG_NAME: &’static str: The name of the service suitable for debug purposes

Other code may be generated depending on the Protocol and method attributes applied to the protocol or its methods.



For asynchronous clients, the FIDL toolchain generates a TicTacToeProxy struct with the following:

Associated types:

  • TicTacToeProxy::MakeMoveResponseFut: The Future type for the response of a two way method. This type implements std::future::Future<Output = Result<(bool, Option<Box<GameState>>), fidl::Error>> + Send.
  • TicTacToeProxy::OnOpponentMoveResponseFut: The Future type for an incoming event. This type implements std::future::Future<Output = Result<GameState, fidl::Error>> + Send


  • new(channel: fidl::AsyncChannel) -> TicTacToeProxy: Create a new proxy for TicTacToe.
  • into_channel(self) -> Result<fidl::AsyncChannel>: Attempt to convert the proxy back into a channel.
  • take_event_stream(&self) -> TicTacToeEventStream: Get a Stream of events from the server end (see Events).

Methods from implementing TicTacToeProxyInterface:

  • start_game(&self, mut start_first: bool) -> Result<(), fidl::Error>: Proxy method for a fire and forget protocol method. It takes as arguments the request parameters and returns an empty result.
  • make_move(&self, mut row: u8, mut col: u8) -> Self::MakeMoveResponseFut: Proxy method for a two way method. It takes as arguments the request parameters and returns a Future of the response.

An example of setting up an asynchronous proxy is available in the Rust tutorial.

The TicTacToeProxyInterface trait can be useful for testing client code. For example, if you write a function that takes &T as a parameter where T: TicTacToeProxyInterface, you can unit test it with a fake proxy type:

use futures::future::{ready, Ready};

struct FakeTicTacToeProxy {
    move_response: (bool, Option<Box<GameState>>),

impl TicTacToeProxyInterface for FakeTicTacToeProxy {
    fn start_game(&self, mut start_first: bool) -> Result<(), fidl::Error> {}

    type MakeMoveResponseFut = Ready<fidl::Result<(bool, Option<Box<GameState>>)>>;
    fn make_move(&self, mut row: u8, mut col: u8) -> Self::MakeMoveResponseFut {


For synchronous clients of the TicTacToe protocols, the FIDL toolchain generates a TicTacToeSynchronousProxy struct with the following methods:

  • new(channel: fidl::Channel) -> TicTacToeSynchronousProxy: Returns a new synchronous proxy over the client end of a channel. The server end is assumed to implement the TicTacToe protocol.
  • into_channel(self) -> fidl::Channel: Convert the proxy back into a channel.
  • start_game(&mut self, mut a: i64) -> Result<(), fidl::Error>: Proxy method for a fire and forget method: it takes the request parameters as arguments and returns an empty result.
  • make_move(&mut self, mut row: u8, mut col: u8, __deadline: zx::Time) -> Result<(bool, Option<Box<GameState>>), fidl::Error>: Proxy method for a two way method. It takes the request parameters as arguments followed by a deadline parameter which dictates how long the method call will wait for a response (or zx::Time::INFINITE to block indefinitely). It returns a Result of the response parameters.

An example of setting up a synchronous proxy is available in the Rust tutorial.


Protocol request stream

To represent the stream of incoming requests to a server, the FIDL toolchain generates a TicTacToeRequestStream type that implements futures::Stream<Item = Result<TicTacToeRequest, fidl::Error>> as well as fidl::endpoints::RequestStream. Each protocol has a corresponding request stream type.

Request enum

TicTacToeRequest is an enum representing the possible requests of the TicTacToe protocol. It has the following variants:

  • StartGame { start_first: bool, control_handle: TicTacToeControlHandle }: A fire and forget request, which contains the request parameters and a control handle.
  • MakeMove { row: u8, col: u8, responder: TicTacToeMakeMoveResponder }: A two way method request, which contains the request parameters and a responder.

One such enum is generated for each protocol.

Request responder

Each two way method has a corresponding generated responder type which the server uses to respond to a request. In this example, which only has one two way method, the FIDL toolchain generates TicTacToeMakeMoveResponder, which provides the following methods:

  • send(self, mut success: bool, mut new_state: Option<&mut GameState>) -> Result<(), fidl::Error>: Sends a response.
  • send_no_shutdown_on_err(self, mut success: bool, mut new_state: Option<&mut GameState>) -> Result<(), fidl::Error>: Similar to send but does not shut down the channel if an error occurs.
  • control_handle(&self) -> &TicTacToeControlHandle: Get the underlying control handle.
  • drop_without_shutdown(mut self): Drop the Responder without shutting down the channel.

Protocol control handle

The FIDL toolchain generates TicTacToeControlHandle to encapsulate the client endpoint of the TicTacToe protocol on the server side. It contains the following methods:

  • shutdown(&self): Shut down the channel.
  • shutdown_with_epitaph(&self, status: zx_status::Status): Send an epitaph and then shut down the channel.
  • send_on_opponent_move(&self, mut new_state: &mut GameState) -> Result<(), fidl::Error>: Proxy method for an event, which takes as arguments the event’s parameters and returns an empty result (see Events).



For receiving events on the client, the FIDL toolchain generates a TicTacToeEventStream, which can be obtained using the take_event_stream() method on the TicTacToeProxy. TicTacToeEventStream implements futures::Stream<Item = Result<TicTacToeEvent, fidl::Error>>.

TicTacToeEvent is an enum representing the possible events. It has the following variants:

  • OnOpponentMove { new_state: GameState }: Discriminant for the TicTacToeEvent event.

And provides the following methods:

  • into_on_opponent_move(self) -> Option<GameState>: Return Some of the parameters of the event, or None if the variant does not match the method call.


Servers can send events by using the control handle corresponding to the protocol. The control handle can be obtained through a TicTacToeRequest received from the client. For fire and forget methods, the control handle is available through the control_handle field, and for two way methods, it is available through the control_handle() method on the responder. A control handle for a protocol can also be obtained through the corresponding request stream (in this example, TicTacToeRequestStream), since it implements fidl::endpoints::RequestStream.


For a method with an error type:

protocol TicTacToe {
    MakeMove(uint8 row, uint8 col) -> (GameState new_state) error MoveError;

The FIDL toolchain generates a public TicTacToeMakeMoveResult type alias for std::result::Result<GameState, MoveError>. The rest of the bindings code for this method is generated as if it has a single response parameter result of type TicTacToeMakeMoveResult. The type used for a successful result follows the parameter type conversion rules.

Protocol composition

FIDL does not have a concept of inheritance, and generates full code as described above for all composed protocols. In other words, the code generated for the following:

protocol A {

protocol B {
    compose A;

Is the same as the following code:

protocol A {

protocol B {

Protocol and method attributes


The "Transitional" attribute only affects the ProxyInterface trait, which is sometimes used in test code. For non-test code, protocols can be transitioned on the server side by having request handlers temporarily use a catch-all match arm in the Request handler. Client code does not need to be soft transitioned since the generated proxy will always implement all methods.

For methods annotated with the "Transitional" attribute, the ProxyInterface trait for asynchronous clients provides default implementations that call unimplemented!(). As noted earlier, this has no effect on the Proxy type, which always implements all the trait's methods. However, it can help for soft transitions when the ProxyInterface trait is used for fake proxies in client-side unit tests.


For protocols annotated with the "Discoverable" attribute, the Marker type additionally implements the fidl::endpoints::DiscoverableService trait.