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 = "0.14"
# Only necessary if using Protobuf well-known types:
prost-types = "0.14"

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.

See the snazzy repository for a simple start-to-finish example.

MSRV

prost follows the tokio-rs project’s MSRV model and supports 1.82. For more information on the tokio msrv policy you can check it out here

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.

protoc

With prost-build v0.11 release, protoc will be required to invoke compile_protos (unless skip_protoc is enabled). Prost will no longer provide bundled protoc or attempt to compile protoc for users. For install instructions for protoc, please check out the protobuf install instructions.

Packages

Prost can now generate code for .proto files that don’t have a package spec. 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. For example, this proto enum:

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

gets this corresponding Rust enum 1:

pub enum PhoneType {
Mobile = 0,
Home = 1,
Work = 2,
}

You can convert a PhoneType value to an i32 by doing:

PhoneType::Mobile as i32

The #[derive(::prost::Enumeration)] annotation added to the generated PhoneType adds these associated functions to the type:

impl PhoneType {
pub fn is_valid(value: i32) -> bool { ... }
#[deprecated]
pub fn from_i32(value: i32) -> Option<PhoneType> { ... }
}

It also adds an impl TryFrom<i32> for PhoneType, so you can convert an i32 to its corresponding PhoneType value by doing, for example:

let phone_type = 2i32;

match PhoneType::try_from(phone_type) {
Ok(PhoneType::Mobile) => ...,
Ok(PhoneType::Home) => ...,
Ok(PhoneType::Work) => ...,
Err(_) => ...,
}

Additionally, wherever a proto enum is used as a field in a Message, the message will have ‘accessor’ methods to get/set the value of the field as the Rust enum type. For instance, this proto PhoneNumber message that has a field named type of type PhoneType:

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

will become the following Rust type 2 with methods type and set_type:

pub struct PhoneNumber {
pub number: String,
pub r#type: i32, // the `r#` is needed because `type` is a Rust keyword
}

impl PhoneNumber {
pub fn r#type(&self) -> PhoneType { ... }
pub fn set_type(&mut self, value: PhoneType) { ... }
}

Note that the getter methods will return the Rust enum’s default value if the field has an invalid i32 value.

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 for scalar types, Option<T> otherwise
proto3	optional	Option<T>
proto2/proto3	repeated	Vec<T>

Note that in proto3 the default representation for all user-defined message types is Option<T>, and for scalar types just T (during decoding, a missing value is populated by T::default()). If you need a witness of the presence of a scalar type T, use the optional modifier to enforce an Option<T> representation in the generated Rust struct.

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 Rust3:

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.

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, PartialEq, ::prost::Message)]
pub struct Person {
#[prost(string, tag="1")]
pub name: ::prost::alloc::string::String,
/// Unique ID number for this person.
#[prost(int32, tag="2")]
pub id: i32,
#[prost(string, tag="3")]
pub email: ::prost::alloc::string::String,
#[prost(message, repeated, tag="4")]
pub phones: ::prost::alloc::vec::Vec<person::PhoneNumber>,
}
/// Nested message and enum types in `Person`.
pub mod person {
#[derive(Clone, PartialEq, ::prost::Message)]
pub struct PhoneNumber {
#[prost(string, tag="1")]
pub number: ::prost::alloc::string::String,
#[prost(enumeration="PhoneType", tag="2")]
pub r#type: i32,
}
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, PartialOrd, Ord, ::prost::Enumeration)]
#[repr(i32)]
pub enum PhoneType {
Mobile = 0,
Home = 1,
Work = 2,
}
}
/// Our address book file is just one of these.
#[derive(Clone, PartialEq, ::prost::Message)]
pub struct AddressBook {
#[prost(message, repeated, tag="1")]
pub people: ::prost::alloc::vec::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 = "0.14.3", default-features = false, features = ["derive"] }
# Only necessary if using Protobuf well-known types:
prost-types = { version = "0.14.3", default-features = false }

Additionally, configure prost-build 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,
}

Nix

The prost project maintains flakes support for local development. Once you have nix and nix flakes setup you can just run nix develop to get a shell configured with the required dependencies to compile the whole project.

Feature Flags

• std: Enable integration with standard library. Disable this feature for no_std support. This feature is enabled by default.
• derive: Enable integration with prost-derive. Disable this feature to reduce compile times. This feature is enabled by default.
• prost-derive: Deprecated. Alias for derive feature.
• no-recursion-limit: Disable the recursion limit. The recursion limit is 100 and cannot be customized.

Contributing

The current maintainer is not contributing new features and doesn’t have the time to review new features. Bug fixes and small improvements are welcome. Feel free to contribute small and easily reviewable PRs.

Bug fixes are still important, and security fixes will be released as soon as possible. Contact the #prost channel in Tokio discord if you feel a bug or security fix is not getting enough attention.

The maintainer expects the official protobuf project to release their rust library soon and expects it to be as fully featured as the C++ library. See their source code and crate for more information.

FAQ

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 currently 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. 1.

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

Why are most fields are wrapped in an Option?

In the protobuf wire protocol all fields are optional. The design of prost choose to expose this to the user of the types. prost leans toward correctness and safety, aligned with Rust’s philosophy — even at the cost of verbosity.

License

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

See LICENSE for details.

Copyright 2022 Dan Burkert & Tokio Contributors

Footnotes

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

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

Annotations have been elided for clarity. See below for a full example. ↩