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Feature Request: Generate domain-specific Rust types for scalar fields

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
35/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Active
Tech stack
rust
Domain
devtools

Research direction

Start by reviewing buffa-codegen and buffa_build::Config to understand how scalar fields and configuration are currently generated. Evaluate the proposed scalar_option, scalar_adapter, and fail_missing_adapter APIs, including ScalarAdapter conversions for binary and ProtoJSON paths. Done means domain-specific Rust scalar types are exposed while preserving the original wire and ProtoJSON representations.

Written by the indexing model from the issue text.

Description

Is there currently a practical way to generate domain-specific Rust types for protobuf scalar fields while preserving their original wire and ProtoJSON representations?

For example, given this protobuf definition:

message Dummy {
  int32 id = 1;
  double mass = 2;
}

I would like buffa-codegen to generate:

pub struct Dummy {
    pub id: ::my_types::DummyId,
    pub mass: ::uom::si::f64::Mass, // https://crates.io/crates/uom
}

As of Buffa 0.9.2, my understanding is that this is not supported.
extern_path can replace protobuf message or enum types, but it cannot replace an individual scalar field's Rust representation.
Implementing the entire containing message manually appears possible, but it is error-prone.
Replacing the scalar with a wrapper message would also change its wire and ProtoJSON representations.

Proposal:

One way this feature could work is through a custom protobuf option.
A custom option could be defined once and reused to attach semantic information to fields:

edition = "2024";

import "google/protobuf/descriptor.proto";

extend google.protobuf.FieldOptions {
  string semantic_type = 51000 [retention = RETENTION_SOURCE];
}

message Dummy {
  int32 id = 1 [(semantic_type) = "dummy.id"];
  double mass = 2 [(semantic_type) = "mass.kilogram"];
}

Buffa could then provide an API mapping option values to Rust types and adapters:

buffa_build::Config::new()
    .scalar_option(".semantic_type")
    .scalar_adapter(
        "dummy.id",
        "::my_types::DummyId",
        "::my_types::DummyIdAdapter",
    )
    .scalar_adapter(
        "mass.kilogram",
        "::uom::si::f64::Mass",
        "::my_types::KilogramAdapter",
    )
    .fail_missing_adapter(); // fails if a field has a 'semantic_type' that is not mapped via 'scalar_adapter'

The user would select or define the domain types and implement the adapters:

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct DummyId(pub i32);

pub struct DummyIdAdapter;

impl buffa::ScalarAdapter<i32> for DummyIdAdapter {
    type Value = DummyId;

    fn from_wire(value: i32) -> DummyId {
        DummyId(value)
    }

    fn to_wire(value: &DummyId) -> i32 {
        value.0
    }
}

pub struct KilogramAdapter;

impl buffa::ScalarAdapter<f64> for KilogramAdapter {
    type Value = uom::si::f64::Mass;

    fn from_wire(value: f64) -> Self::Value {
        uom::si::f64::Mass::new::<uom::si::mass::kilogram>(value)
    }

    fn to_wire(value: &Self::Value) -> f64 {
        value.get::<uom::si::mass::kilogram>()
    }
}

Generated binary and ProtoJSON implementations would convert through the adapters, while owned and view APIs would expose the domain types.

Would this use case fit Buffa's direction?

Dominant language
Rust
Stars
900
Forks
95
Avg merge
5d 10h
Merged PRs (30d)
57

Getting set up

First steps

  1. Read the whole issue, then the project's contributing guide.
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  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

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