docs/rust_proto.vm

#[[
## Overview
These build rules are used for building [protobufs][protobuf]/[gRPC][grpc] in [Rust][rust] with Bazel.

There are two rule sets. The first ruleset defines the `rust_proto_library` and `rust_grpc_library`
rules which generate Rust code using the [`rust-protobuf`] dependencies. The second ruleset defines
the `rust_prost_library` which generates Rust code using the [`prost`] and [`tonic`] dependencies.

[rust]: http://www.rust-lang.org/
[protobuf]: https://developers.google.com/protocol-buffers/
[grpc]: https://grpc.io
[`rust-protobuf`]: https://github.com/stepancheg/rust-protobuf/

See the [protobuf example](../examples/proto) for a more complete example of use.

### Setup

To use the Rust proto rules, add the following to your `WORKSPACE` file to add the
external repositories for the Rust proto toolchain (in addition to the [rust rules setup](..)):

```python
load("@rules_rust//proto:repositories.bzl", "rust_proto_repositories")

rust_proto_repositories()

load("@rules_rust//proto:transitive_repositories.bzl", "rust_proto_transitive_repositories")

rust_proto_transitive_repositories()
```

This will load the required dependencies for the Proto, Prost, and Tonic rules. It will also
register a default toolchain for the `rust_proto_library` and `rust_grpc_library` rules. The
`prost` and `tonic` rules do not specify a default toolchain in order to avoid mismatched
dependency issues.

To customize the `rust_proto_library` and `rust_grpc_library` toolchain, please see the section
[Customizing `rust-protobuf` Dependencies](#custom-proto-deps).

To setup the `prost` and `tonic` toolchain, please see the section [Customizing `prost` and `tonic` Dependencies](#custom-prost-deps).

For additional information about Bazel toolchains, see [here](https://docs.bazel.build/versions/master/toolchains.html).

## <a name="custom-proto-deps">Customizing `rust-protobuf` Dependencies

These rules depend on the [`protobuf`](https://crates.io/crates/protobuf) and
the [`grpc`](https://crates.io/crates/grpc) crates in addition to the [protobuf
compiler](https://github.com/google/protobuf). To obtain these crates,
`rust_proto_repositories` imports the given crates using BUILD files generated with
[crate_universe](./crate_universe.md).

If you want to either change the protobuf and gRPC rust compilers, or to
simply use [crate_universe](./crate_universe.md) in a more
complex scenario (with more dependencies), you must redefine those
dependencies.

To do this, once you've imported the needed dependencies (see our
[@rules_rust//proto/3rdparty/BUILD.bazel](https://github.com/bazelbuild/rules_rust/blob/main/proto/3rdparty/BUILD.bazel)
file to see the default dependencies), you need to create your own toolchain. 
To do so you can create a BUILD file with your toolchain definition, for example:

```python
load("@rules_rust//proto:toolchain.bzl", "rust_proto_toolchain")

rust_proto_toolchain(
    name = "proto-toolchain-impl",
    # Path to the protobuf compiler.
    protoc = "@com_google_protobuf//:protoc",
    # Protobuf compiler plugin to generate rust gRPC stubs.
    grpc_plugin = "//3rdparty/crates:cargo_bin_protoc_gen_rust_grpc",
    # Protobuf compiler plugin to generate rust protobuf stubs.
    proto_plugin = "//3rdparty/crates:cargo_bin_protoc_gen_rust",
)

toolchain(
    name = "proto-toolchain",
    toolchain = ":proto-toolchain-impl",
    toolchain_type = "@rules_rust//proto/protobuf:toolchain_type",
)
```

Now that you have your own toolchain, you need to register it by
inserting the following statement in your `WORKSPACE` file:

```python
register_toolchains("//my/toolchains:proto-toolchain")
```

Finally, you might want to set the `rust_deps` attribute in
`rust_proto_library` and `rust_grpc_library` to change the compile-time
dependencies:

```python
rust_proto_library(
    ...
    rust_deps = ["//3rdparty/crates:protobuf"],
    ...
)

rust_grpc_library(
    ...
    rust_deps = [
        "//3rdparty/crates:protobuf",
        "//3rdparty/crates:grpc",
        "//3rdparty/crates:tls_api",
        "//3rdparty/crates:tls_api_stub",
    ],
    ...
)
```

__Note__: Ideally, we would inject those dependencies from the toolchain,
but due to [bazelbuild/bazel#6889](https://github.com/bazelbuild/bazel/issues/6889)
all dependencies added via the toolchain ends-up being in the wrong
configuration.

## <a name="custom-prost-deps">Customizing `prost` and `tonic` Dependencies

These rules depend on the [`prost`] and [`tonic`] dependencies. To setup the necessary toolchain
for these rules, you must define a toolchain with the [`prost`], [`prost-types`], [`tonic`],[`protoc-gen-prost`], and [`protoc-gen-tonic`] crates as well as the [`protoc`] binary.

[`prost`]: https://crates.io/crates/prost
[`prost-types`]: https://crates.io/crates/prost-types
[`protoc-gen-prost`]: https://crates.io/crates/protoc-gen-prost
[`protoc-gen-tonic`]: https://crates.io/crates/protoc-gen-tonic
[`tonic`]: https://crates.io/crates/tonic
[`protoc`]: https://github.com/protocolbuffers/protobuf

To get access to these crates, you can use the [crate_universe](./crate_universe.md) repository
rules. For example:

```python
load("//crate_universe:defs.bzl", "crate", "crates_repository")

crates_repository(
    name = "crates_io",
    annotations = {
        "protoc-gen-prost": [crate.annotation(
            gen_binaries = ["protoc-gen-prost"],
            patch_args = [
                "-p1",
            ],
            patches = [
                # Note: You will need to use this patch until a version greater than `0.2.2` of
                # `protoc-gen-prost` is released.
                "@rules_rust//proto/prost/private/3rdparty/patches:protoc-gen-prost.patch",
            ],
        )],
        "protoc-gen-tonic": [crate.annotation(
            gen_binaries = ["protoc-gen-tonic"],
        )],
    },
    cargo_lockfile = "Cargo.Bazel.lock",
    mode = "remote",
    packages = {
        "prost": crate.spec(
            version = "0",
        ),
        "prost-types": crate.spec(
            version = "0",
        ),
        "protoc-gen-prost": crate.spec(
            version = "0",
        ),
        "protoc-gen-tonic": crate.spec(
            version = "0",
        ),
        "tonic": crate.spec(
            version = "0",
        ),
    },
    repository_name = "rules_rust_prost",
    tags = ["manual"],
)
```

You can then define a toolchain with the `rust_prost_toolchain` rule which uses the crates
defined above. For example:

```python
load("@rules_rust//proto/prost:defs.bzl", "rust_prost_toolchain")
load("@rules_rust//rust:defs.bzl", "rust_library_group")

rust_library_group(
    name = "prost_runtime",
    deps = [
        "@crates_io//:prost",
    ],
)

rust_library_group(
    name = "tonic_runtime",
    deps = [
        ":prost_runtime",
        "@crates_io//:tonic",
    ],
)

rust_prost_toolchain(
    name = "prost_toolchain_impl",
    prost_plugin = "@crates_io//:protoc-gen-prost__protoc-gen-prost",
    prost_runtime = ":prost_runtime",
    prost_types = "@crates_io//:prost-types",
    proto_compiler = "@com_google_protobuf//:protoc",
    tonic_plugin = "@crates_io//:protoc-gen-tonic__protoc-gen-tonic",
    tonic_runtime = ":tonic_runtime",
)

toolchain(
    name = "prost_toolchain",
    toolchain = "default_prost_toolchain_impl",
    toolchain_type = "//proto/prost:toolchain_type",
)
```

Lastly, you must register the toolchain in your `WORKSPACE` file. For example:

```python
register_toolchains("//toolchains:prost_toolchain")
```
]]#