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,
}

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

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 const 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.

[^2]: Annotations have been elided for clarity. See below for a full example.

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 Rust[^3]:

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.

[^3]: 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, 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 {