Add ability to specify import path instead of hardcode to `tonic-build` codegen

Feature Request

Add ability to specify path to tonic and async_trait imports when referenced in codegen.

Crates

tonic-build

Motivation

I keep tonic and many other dependencies in a separate package. When I import them into a package that uses tonic-build to generate Rust proto, I get an error saying tonic and async_trait cannot be found.

Currently, the only way to use tonic is a direct dependency of the main package not as an import from another package. I haven't come across this issue with other packages. Serde has "crate" container attribute that solves theirs.

Proposal

Add a new method to the tonic_build::configure() -> Config Builder trait.

Alternatives

The alternative way is I clone the repo and hardcode the correct import path.

[davidpdrsn]

I don't follow. Why can't you depend on tonic for the crate that includes the compiled protos?

If I import tonic and prost from another package, the codegen does not find the modules

However, if I include tonic and prost in the same package that compiles protos, it works

I don't know why that is.

[davidpdrsn]

I suppose that is because the client and server codegen nests things in modules. See https://github.com/hyperium/tonic/blob/master/tonic-build/src/server.rs#L67 and https://github.com/hyperium/tonic/blob/master/tonic-build/src/client.rs#L25.

I wonder if adding use super::* to the generated modules would fix it or if a crate config similar to serde's is necessary.

Yes if I refer to tonic as crate::tonic::codegen::*, it works but prost doesn't.
It uses :: (https://github.com/tokio-rs/prost/blob/master/prost-build/src/code_generator.rs#L177) which isn't going to work in this Rust-2018 project according to https://doc.rust-lang.org/reference/visibility-and-privacy.html#pubin-path-pubcrate-pubsuper-and-pubself

[LucioFranco]

Do you have a simple reproduction of your issue? I am not totally following what the issue is.

[JSAbrahams]

Just chiming in, a use case for this I think is also avoiding have to align tonic versions across many crates.
Say I have the following:

• util crate, with tonic version x, and which re-exports tonic, and with some wrapper functions around tonic.
• crate A, which has tonic version x, and which depends on util
• crate B, which has tonic version x, and which depends on util

Now, if I wish to update the tonic version in util, I have to make sure that crate A and crate B's versions are also aligned.
A much more elegant solution would be if I could just depend on the re-export tonic from util as such use util::tonic (in say crate A).
But since tonic_build outputs files where tonic:: is hardcoded, the compiler does not allow this.

In this example, ideally, I would like to be able to tell tonic_build to output files which use util::tonic, for instance.

[lukasweber]

Hi, just created a PR to fix this issue by proposing to use a similar strategy as prost is using with prost_path. I suggest to add a tonic_path method to the code gen builder which allows to configure the path to the tonic crate. Let me know what you think about it.

If you look at the tests/ folder in my changeset, you can see how it would look like.