Dependencies for host are used for target when set in grpc_deps

[ignasl]

I have defined and registered my own scala_proto_toolchain:

scala_proto_toolchain(
    name = "my_shiny_toolchain",
    grpc_deps = [
        "//important/dependency"
    ],
    code_generator = "//important/generator:my_generator"
)

Naturally, I am declaring the rule for proto and another scala library that depends on it:

scalapb_proto_library(
   name = "my_proto",
   deps = [...]
)

scala_library(
   name = "my_library",
   deps = [
      ":my_proto",
      "//important/dependency"
   ]
)

So dependencies are eventually set-up like this:

1. my_proto -> //important/dependency
2. my_library -> //important/dependency
3. my_library -> my_proto

Now, the problem arises that when "my_library" gets built, the classpath contains two copies of the //important/dependency jars:

1. /bazel-out/host/bin/important/dependency/dependency.jar
2. /bazel-out/darwin-fastbuild/bin/important/dependency/dependency.jar

This is a problem, because jars meant for the host find their way into dependency list meant for the target. I cannot remove the //important/dependency' from the grpc_deps` list, because generated code requires it and this causes compile failures.

I believe (correct me if I am wrong) that bazel or rules_scala incorrectly resolves libraries' paths to host. With JVM languages compilers can happily accept them because they are essentially different. However, if this was, for example, C++ cross-platform build (host is x86 and target ARM) compiler would not allow it. Since bazel supports cross-compiling, the issue must be with scala_rules. Otherwise, please, let me know what am doing wrong.

[ignasl]

@ianoc-stripe, @ianoc could you provide some insights?

[ianoc-stripe]

Hi @ignasl it would be great if you could add a test in a PR for this? it should be possible to write one that fails i believe? it would make it much easier to debug/fix.

[ianoc-stripe]

@ignasl merged a changed yesterday i think should act as a workaround for you, unfortunately the api's changed slightly due to limitations in bazel in making this nicer at the moment. But it should stop the host deps coming for target

[ignasl]

Thanks a lot for solving this one. API change is not a problem at all we are still in POC mode. I have already started experimenting.