Support for customizing trait attributes in tonic-build

[ya7on]

Feature Request

Crates

tonic-build

Motivation

Currently, tonic-build generates trait without any way to attach additional attributes or proc-macros.
This makes it difficult to integrate with frameworks or tools that rely on macros, such as automocking struct by trait

Proposal

Add a configuration option in tonic-build to inject custom attributes into generated service traits:

In build options

...
    .trait_atttribute("myservice.MyService", "#[mockall::automock]")
...

In generated code (Before)

/// Generated trait containing gRPC methods that should be implemented for use with ProjectsPrivateServer.
#[async_trait]
pub trait MyServce: std::marker::Send + std::marker::Sync + 'static {
  ...
}

In generated code (After)

/// Generated trait containing gRPC methods that should be implemented for use with ProjectsPrivateServer.
#[async_trait]
#[mockall::automock]
pub trait MyServce: std::marker::Send + std::marker::Sync + 'static {
  ...
}

Alternatives

I ran into this limitation when trying to use #[mockall::automock] for a generated service trait to simplify testing.
In my case:

• I use the trait to describe a client interface for another service.

• Functions in my code accept a generic parameter implementing this trait, so I can pass either:

  • the real gRPC client generated alongside the trait, or
  • a mocked implementation for tests.

Without this feature, mocking is cumbersome because I must manually implement the entire trait for mocks instead of simply deriving it via mockall.

While my primary need is mocking, I believe there are many other potential use cases for injecting proc-macro attributes, such as logging, tracing, or applying custom derives.

---

If this feature aligns with the vision of the project, I am willing to implement the functionality and open a Pull Request