Support for adding derive annotations for traits outside prost?

[andrew-otiv]

Prost already can generate types supporting Clone and PartialEq, which is great. We'd like to also be able to derive other traits, like Default, or the various traits used by fuzzers like cargo fuzz. Rust's orphan rules make this hard in the context of code generation.

We currently only have ~50 code generated types, so we could write our own impls manually, but maybe it would possible to add trait derivations to the generated code via configuration? Theres already a mechanism for switching ~off the already supported traits; I am interested in going the other direction.

Related: #493

[romac]

Have you tried using https://docs.rs/prost-build/latest/prost_build/struct.Config.html#method.type_attribute?

[andrew-otiv]

That looks very promising, thanks! We're trying to set this up from bazel rules_rust; hopefully we can drive this through keyword arguments to https://bazelbuild.github.io/rules_rust/rust_proto.html#rust_prost_library.