#macros #protobuf #prost


A Macro library for easier mapping between custom models and proto generated code

3 releases

0.1.2 Nov 1, 2023
0.1.1 Oct 31, 2023
0.1.0 Oct 31, 2023

#354 in Rust patterns

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MIT license

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Proto Mapper

github CI/main crates.io

Macro implementation library for mapping between custom models and protobuf generated code


This library is an (almost) complete rewrite of the protobuf-convert library. The purpose of the rewrite is to adapt it to specific needs of our projects. The main concept and the idea remains the same, so the credit goes to the original authors of the protobuf-convert library.

What changed

This library:

  • changes the main name of the macro to ProtoMap
  • changes the main way the macro is used and is interfaced with external traits
  • avoids the use of re implementing ProtoMap trait to client modules
  • is restructured to different crates
  • contains excessive testing for edge cases
  • introduces ProtoScalar types
  • introduces ProtoScalarMap trait for protobuf scalar types
  • handles enumeration protobuf generation code automatically
  • handles option values via scanning the types of the applied struct and chooses different implementation paths
  • supports prost


First, add the dependency in Cargo.toml:

proto-mapper = {version = "0.1.2", features = ["protobuf"] } 


proto-mapper = {version = "0.1.2", features = ["prost"] } 

NOTE: Features prost or protobuf are mutually exclusive and required. Use one of them according to targeted generated code proto framework that you use


A proof of concept that demonstrates the use of this library can be found here. Keep in mind that the PoC is still work in progress.

Mapping scalar values and enumerations

Given the protobuf enumeration and message

syntax = "proto3";

enum EntityStatus {
  STATUS_A = 0;
  STATUS_B = 1;
  STATUS_C = 2;

message ScalarEntity {
  uint32 uint32_f = 1;
  int32 int32_f= 2;
  bool bool_f = 4;
  string string_f = 5;
  int64  int64_f = 6;
  uint64 uint64_f  = 7;
  bytes bytes_f = 8;
  float float_f = 9;
  double double_f = 10;

  EntityStatus status = 11;

After using prost or rust-protobuf library to generate code, you can map your custom model to the generated structs as follows:

#[derive(Debug, Clone, Copy, Default, PartialEq, ProtoMap)]
source = "proto::EntityStatus",
enum EntityStatus {

#[derive(Debug, ProtoMap, Default)]
#[proto_map(source = "proto::ScalarEntity")]
struct ScalarEntity {
    pub uint32_f: u32,
    pub int32_f: i32,
    pub bool_f: bool,
    pub string_f: String,
    pub bytes_f: Vec<u8>,
    pub int64_f: i64,
    pub uint64_f: u64,
    pub float_f: f32,
    pub double_f: f64,
    pub status: EntityStatus,

Then you can convert between your defined struct and the generated code as follows:

let e = ScalarEntity::default();
let p = e.to_proto();

You can also convert a proto instance to your custom struct.

let p = proto::ScalarEntity::default();
let e = ScalarEntity::from_proto(p)?;

Note that the mapping code for the enumeration requires #[proto_map(..., enumeration)] attribute on the rust enumeration and also needs to mark the field inside the ScalarEntity as well.

Mapping optional scalar values and enumerations

Given the same proto file. Out of the box you can map to optional values

That is:

#[derive(Debug, ProtoMap, PartialEq, Default)]
#[proto_map(source = "proto::prost::ScalarEntity")]
struct ScalarEntityOptions {
    pub uint32_f: Option<u32>,
    pub int32_f: Option<i32>,
    pub bool_f: Option<bool>,
    pub string_f: Option<String>,
    pub bytes_f: Option<Vec<u8>>,
    pub int64_f: Option<i64>,
    pub uint64_f: Option<u64>,
    pub float_f: Option<f32>,
    pub double_f: Option<f64>,
    pub status: Option<EntityStatus>,

The macro scans the types of the custom struct that annotates and chooses different implementation paths for the conversion code.

Mapping non scalar values

Given the proto file

syntax = "proto3";

// ... definitions of ScalarEntity

message NestedEntity {
  ScalarEntity first = 1;
  ScalarEntity second = 2;

You can map non scalar values as follows

#[derive(Debug, ProtoMap, PartialEq)]
#[proto_map(source = "proto::NestedEntity")]
struct NestedEntity {
    pub first: ScalarEntity,
    pub second: Option<ScalarEntity>,

Mapping non scalar oneof field to rust enumeration

You can map top level oneof protobuf fields as follows

Given the proto file

syntax = "proto3";

// ... definitions of ScalarEntity

message HierarchyEntity {
  oneof data {
    ScalarEntity first_entity = 1;
    NestedEntity second_entity = 2;

Then one implementation of the custom struct may be

#[derive(Debug, ProtoMap, PartialEq)]
    source = "proto::HierarchyEntity",
    one_of(field = "data"),
    rename_variants = "snake_case"
enum HierarchyEntity {

Note that the rename_variants attribute may take two values snake_case and STREAMING_SNAKE_CASE according to the target generated struct.

Custom mapping scalar values

See examples at tests for prost and for rust-protobuf

Differences between prost and rust-protobuf usage


How it works

Internally the macro discriminates between scalar and not scalar types.

Scalar types are shown below as well as the protobuf types that rust-protobuf and prost autogenerated code maps to.

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>
(table taken from prost project README.md)

All other rust types are considered as non scalar. An exception to that rule is protobuf enum types that need to be marked with meta attribute #[proto_map(enumeration)],

The library implements automatically two traits according to the struct types.

For scalar types:

pub trait ProtoMapScalar<P: ProtoScalar>: Sized {
    /// Converts a reference of [`Self`] to a [`ProtoScalar`]
    fn to_scalar(&self) -> P;

    /// Consumes a [`ProtoScalar`] and returns a [`Self`] or error in the conversion failed
    fn from_scalar(proto: P) -> Result<Self, anyhow::Error>;

For non scalar types:

pub trait ProtoMap
        Self: Sized,
    type ProtoStruct;
    /// Converts a reference of [`Self`] struct to proto [`Self::ProtoStruct`]
    fn to_proto(&self) -> Self::ProtoStruct;

    /// Consumes a proto [`Self::ProtoStruct`] and returns a [`Self`] struct or error in the conversion failed
    fn from_proto(proto: Self::ProtoStruct) -> Result<Self, anyhow::Error>;

Note that protobuf enum types are treated as non scalar with rust-protobuf but as scalar (i32 value) with prost.

Also a third trait named ProtoScalar implementation is provided by the library for all proto scalar types.

To get a rough view of what the macro implement see the prost manual tests or the rust-protobuf manual tests used as guides for creating the implementations.





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