Support for adding attributes to generated *_client/*_server modules

[tarcieri]

Feature Request

Crates

tonic_build

Motivation

We're using tonic_build to compile both proto and service definitions. However, we'd like to gate the service definitions on a cargo feature (e.g. grpc), so it's possible to use the protos independently of Tonic for use in e.g. WASM environments.

Proposal

Add a new method to tonic_builder::Builder similar to the existing field_attribute and type_attribute methods, but instead: module_attribute.

This would make it possible to add an attribute like #[cfg(feature = "grpc")] to the generated modules to gate usages of Tonic.

Alternatives

We managed to "solve" our immediate problem by munging the output of tonic_build, however it's something of a brittle hack. A first-class feature for this would be much more robust.

[davidpdrsn]

I might be wrong but wouldn't this feature require support for module_attribute in prost build?

[davidpdrsn]

Fixed by #684