PROST!

prost is a Protocol Buffers implementation for the Rust Language. prost generates simple, idiomatic Rust code from proto2 and proto3 files.

Compared to other Protocol Buffers implementations, prost

• Generates simple, idiomatic, and readable Rust types by taking advantage of Rust derive attributes.
• Retains comments from .proto files in generated Rust code.
• Allows existing Rust types (not generated from a .proto) to be serialized and deserialized by adding attributes.
• Uses the bytes::{Buf, BufMut} abstractions for serialization instead of std::io::{Read, Write}.
• Respects the Protobuf package specifier when organizing generated code into Rust modules.
• Preserves unknown enum values during deserialization.
• Does not include support for runtime reflection or message descriptors.

Using prost in a Cargo Project

First, add prost and its public dependencies to your Cargo.toml:

[dependencies]
prost = "0.7"
# Only necessary if using Protobuf well-known types:
prost-types = "0.7"

The recommended way to add .proto compilation to a Cargo project is to use the prost-build library. See the prost-build documentation for more details and examples.

Generated Code

prost generates Rust code from source .proto files using the proto2 or proto3 syntax. prost's goal is to make the generated code as simple as possible.

Packages

All .proto files used with prost must contain a package specifier. prost will translate the Protobuf package into a Rust module. For example, given the package specifier:

package foo.bar;

All Rust types generated from the file will be in the foo::bar module.

Messages

Given a simple message declaration:

// Sample message.
message Foo {
}

prost will generate the following Rust struct:

/// Sample message.
#[derive(Clone, Debug, PartialEq, Message)]
pub struct Foo {
}

Fields

Fields in Protobuf messages are translated into Rust as public struct fields of the corresponding type.

Scalar Values

Scalar value types are converted as follows:

Protobuf Type	Rust Type
double	f64
float	f32
int32	i32
int64	i64
uint32	u32
uint64	u64
sint32	i32
sint64	i64
fixed32	u32
fixed64	u64
sfixed32	i32
sfixed64	i64
bool	bool
string	String
bytes	Vec<u8>

Enumerations

All .proto enumeration types convert to the Rust i32 type. Additionally, each enumeration type gets a corresponding Rust enum type, with helper methods to convert i32 values to the enum type. The enum type isn't used directly as a field, because the Protobuf spec mandates that enumerations values are 'open', and decoding unrecognized enumeration values must be possible.

Field Modifiers

Protobuf scalar value and enumeration message fields can have a modifier depending on the Protobuf version. Modifiers change the corresponding type of the Rust field:

.proto Version	Modifier	Rust Type
proto2	optional	Option<T>
proto2	required	T
proto3	default	T
proto2/proto3	repeated	Vec<T>

Map Fields

Map fields are converted to a Rust HashMap with key and value type converted from the Protobuf key and value types.

Message Fields

Message fields are converted to the corresponding struct type. The table of field modifiers above applies to message fields, except that proto3 message fields without a modifier (the default) will be wrapped in an Option. Typically message fields are unboxed. prost will automatically box a message field if the field type and the parent type are recursively nested in order to avoid an infinite sized struct.

Oneof Fields

Oneof fields convert to a Rust enum. Protobuf oneofs types are not named, so prost uses the name of the oneof field for the resulting Rust enum, and defines the enum in a module under the struct. For example, a proto3 message such as:

message Foo {
  oneof widget {
    int32 quux = 1;
    string bar = 2;
  }
}

generates the following Rust[1]:

pub struct Foo {
    pub widget: Option<foo::Widget>,
}
pub mod foo {
    pub enum Widget {
        Quux(i32),
        Bar(String),
    }
}

oneof fields are always wrapped in an Option.

[1] Annotations have been elided for clarity. See below for a full example.

Services

prost-build allows a custom code-generator to be used for processing service definitions. This can be used to output Rust traits according to an application's specific needs.

Generated Code Example

Example .proto file:

syntax = "proto3";
package tutorial;

message Person {
  string name = 1;
  int32 id = 2;  // Unique ID number for this person.
  string email = 3;

  enum PhoneType {
    MOBILE = 0;
    HOME = 1;
    WORK = 2;
  }

  message PhoneNumber {
    string number = 1;
    PhoneType type = 2;
  }

  repeated PhoneNumber phones = 4;
}

// Our address book file is just one of these.
message AddressBook {
  repeated Person people = 1;
}

and the generated Rust code (tutorial.rs):

#[derive(Clone, Debug, PartialEq, Message)]
pub struct Person {
    #[prost(string, tag="1")]
    pub name: String,
    /// Unique ID number for this person.
    #[prost(int32, tag="2")]
    pub id: i32,
    #[prost(string, tag="3")]
    pub email: String,
    #[prost(message, repeated, tag="4")]
    pub phones: Vec<person::PhoneNumber>,
}
pub mod person {
    #[derive(Clone, Debug, PartialEq, Message)]
    pub struct PhoneNumber {
        #[prost(string, tag="1")]
        pub number: String,
        #[prost(enumeration="PhoneType", tag="2")]
        pub type_: i32,
    }
    #[derive(Clone, Copy, Debug, PartialEq, Eq, Enumeration)]
    pub enum PhoneType {
        Mobile = 0,
        Home = 1,
        Work = 2,
    }
}
/// Our address book file is just one of these.
#[derive(Clone, Debug, PartialEq, Message)]
pub struct AddressBook {
    #[prost(message, repeated, tag="1")]
    pub people: Vec<Person>,
}

Accessing the protoc FileDescriptorSet

The prost_build::Config::file_descriptor_set_path option can be used to emit a file descriptor set during the build & code generation step. When used in conjunction with the std::include_bytes macro and the prost_types::FileDescriptorSet type, applications and libraries using Prost can implement introspection capabilities requiring details from the original .proto files.

Using prost in a no_std Crate

prost is compatible with no_std crates. To enable no_std support, disable the std features in prost and prost-types:

[dependencies]
prost = { version = "0.6", default-features = false, features = ["prost-derive"] }
# Only necessary if using Protobuf well-known types:
prost-types = { version = "0.6", default-features = false }

Additionally, configure prost-buid to output BTreeMaps instead of HashMaps for all Protobuf map fields in your build.rs:

let mut config = prost_build::Config::new();
config.btree_map(&["."]);

When using edition 2015, it may be necessary to add an extern crate core; directive to the crate which includes prost-generated code.

Serializing Existing Types

prost uses a custom derive macro to handle encoding and decoding types, which means that if your existing Rust type is compatible with Protobuf types, you can serialize and deserialize it by adding the appropriate derive and field annotations.

Currently the best documentation on adding annotations is to look at the generated code examples above.

Tag Inference for Existing Types

Prost automatically infers tags for the struct.

Fields are tagged sequentially in the order they are specified, starting with 1.

You may skip tags which have been reserved, or where there are gaps between sequentially occurring tag values by specifying the tag number to skip to with the tag attribute on the first field after the gap. The following fields will be tagged sequentially starting from the next number.

use prost;
use prost::{Enumeration, Message};

#[derive(Clone, PartialEq, Message)]
struct Person {
    #[prost(string, tag = "1")]
    pub id: String, // tag=1
    // NOTE: Old "name" field has been removed
    // pub name: String, // tag=2 (Removed)
    #[prost(string, tag = "6")]
    pub given_name: String, // tag=6
    #[prost(string)]
    pub family_name: String, // tag=7
    #[prost(string)]
    pub formatted_name: String, // tag=8
    #[prost(uint32, tag = "3")]
    pub age: u32, // tag=3
    #[prost(uint32)]
    pub height: u32, // tag=4
    #[prost(enumeration = "Gender")]
    pub gender: i32, // tag=5
    // NOTE: Skip to less commonly occurring fields
    #[prost(string, tag = "16")]
    pub name_prefix: String, // tag=16  (eg. mr/mrs/ms)
    #[prost(string)]
    pub name_suffix: String, // tag=17  (eg. jr/esq)
    #[prost(string)]
    pub maiden_name: String, // tag=18
}

#[derive(Clone, Copy, Debug, PartialEq, Eq, Enumeration)]
pub enum Gender {
    Unknown = 0,
    Female = 1,
    Male = 2,
}

FAQ

1. Could prost be implemented as a serializer for Serde?

Probably not, however I would like to hear from a Serde expert on the matter. There are two complications with trying to serialize Protobuf messages with Serde:

• Protobuf fields require a numbered tag, and curently there appears to be no mechanism suitable for this in serde.
• The mapping of Protobuf type to Rust type is not 1-to-1. As a result, trait-based approaches to dispatching don't work very well. Example: six different Protobuf field types correspond to a Rust Vec<i32>: repeated int32, repeated sint32, repeated sfixed32, and their packed counterparts.

But it is possible to place serde derive tags onto the generated types, so the same structure can support both prost and Serde.

2. I get errors when trying to run cargo test on MacOS

If the errors are about missing autoreconf or similar, you can probably fix them by running

brew install automake
brew install libtool

License

prost is distributed under the terms of the Apache License (Version 2.0).

See LICENSE for details.

Copyright 2017 Dan Burkert