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 ofstd::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.
First, add prost
and its public dependencies to your Cargo.toml
:
[dependencies]
prost = "0.6"
# Only necessary if using Protobuf well-known types:
prost-types = "0.6"
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.
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.
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.
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 in Protobuf messages are translated into Rust as public struct fields of the corresponding type.
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> |
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.
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 are converted to a Rust HashMap
with key and value type converted
from the Protobuf key and value types.
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 convert to a Rust enum. Protobuf oneof
s 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.
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.
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>,
}
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 BTreeMap
s instead of HashMap
s
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.
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.
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,
}
- 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
.
- 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
prost
is distributed under the terms of the Apache License (Version 2.0).
See LICENSE for details.
Copyright 2017 Dan Burkert