Missing rust code when compiling a rust_grpc_library target

[satreix]

Hi team 👋

Thanks for the awesome work invested in the rust ruleset.

I am attempting to migrate from rules_rust 0.47.1 to 0.49.3 (this is a PR with a repro).
I get the following error:

$ bazel test //...
ERROR: /home/runner/work/everest/everest/src/proto/helloworld/BUILD.bazel:37:18: Generating Rust protobuf stubs failed: (Exit 255): optional_output_wrapper failed: error executing RustProtocGen command (from target //src/proto/helloworld:helloworld_rust_grpc) 
  (cd /home/runner/.cache/bazel/_bazel_runner/b10ccf95f70cafd45527ff3170f0ab76/sandbox/linux-sandbox/611/execroot/_main && \
  exec env - \
  bazel-out/k8-opt-exec-ST-d57f47055a04/bin/external/rules_rust~/proto/protobuf/optional_output_wrapper bazel-out/k8-fastbuild/bin/src/proto/helloworld/helloworld_rust_grpc.grpc.rust/helloworld_grpc.rs -- 'com.google.devtools.build.lib.analysis.FilesToRunProvider$FullFilesToRunProvider@4f19aaaf' '--plugin=protoc-gen-grpc-rules_rust=bazel-out/k8-opt-exec-ST-d57f47055a04/bin/external/rules_rust~~i~rules_rust_proto__grpc-compiler-0.6.2/protoc-gen-rust-grpc__bin' '--grpc-rules_rust_out=bazel-out/k8-fastbuild/bin/src/proto/helloworld/helloworld_rust_grpc.grpc.rust' '--plugin=protoc-gen-rules_rust=bazel-out/k8-opt-exec-ST-d57f47055a04/bin/external/rules_rust~~i~rules_rust_proto__protobuf-codegen-2.8.2/protoc-gen-rust__bin' '--rules_rust_out=bazel-out/k8-fastbuild/bin/src/proto/helloworld/helloworld_rust_grpc.grpc.rust' '--descriptor_set_in=bazel-out/k8-fastbuild/bin/src/proto/helloworld/helloworld_proto-descriptor-set.proto.bin' helloworld/helloworld.proto)
# Configuration: 5c8ee76a4e8e7a83bc2d45bc81bdf4944e47f188efe88858782477e5980db997
# Execution platform: @@platforms//host:host

Use --sandbox_debug to see verbose messages from the sandbox and retain the sandbox build root for debugging
Usage: [optional_output1...optional_outputN] -- program [arg1...argn]
Args { inner: ["bazel-out/k8-opt-exec-ST-d57f47055a04/bin/external/rules_rust~/proto/protobuf/optional_output_wrapper", "bazel-out/k8-fastbuild/bin/src/proto/helloworld/helloworld_rust_grpc.grpc.rust/helloworld_grpc.rs", "--", "com.google.devtools.build.lib.analysis.FilesToRunProvider$FullFilesToRunProvider@4f19aaaf", "--plugin=protoc-gen-grpc-rules_rust=bazel-out/k8-opt-exec-ST-d57f47055a04/bin/external/rules_rust~~i~rules_rust_proto__grpc-compiler-0.6.2/protoc-gen-rust-grpc__bin", "--grpc-rules_rust_out=bazel-out/k8-fastbuild/bin/src/proto/helloworld/helloworld_rust_grpc.grpc.rust", "--plugin=protoc-gen-rules_rust=bazel-out/k8-opt-exec-ST-d57f47055a04/bin/external/rules_rust~~i~rules_rust_proto__protobuf-codegen-2.8.2/protoc-gen-rust__bin", "--rules_rust_out=bazel-out/k8-fastbuild/bin/src/proto/helloworld/helloworld_rust_grpc.grpc.rust", "--descriptor_set_in=bazel-out/k8-fastbuild/bin/src/proto/helloworld/helloworld_proto-descriptor-set.proto.bin", "helloworld/helloworld.proto"] }
Os { code: 2, kind: NotFound, message: "No such file or directory" }
Analyzing: 166 targets (1108 packages loaded, 45770 targets configured)
[1,026 / 1,167] 11 / 43 tests; checking cached actions
ERROR: /home/runner/work/everest/everest/src/proto/helloworld/BUILD.bazel:37:18 Compiling Rust rlib helloworld_rust_grpc (3 files) failed: (Exit 255): optional_output_wrapper failed: error executing RustProtocGen command (from target //src/proto/helloworld:helloworld_rust_grpc) 
  (cd /home/runner/.cache/bazel/_bazel_runner/b10ccf95f70cafd45527ff3170f0ab76/sandbox/linux-sandbox/611/execroot/_main && \
  exec env - \
  bazel-out/k8-opt-exec-ST-d57f47055a04/bin/external/rules_rust~/proto/protobuf/optional_output_wrapper bazel-out/k8-fastbuild/bin/src/proto/helloworld/helloworld_rust_grpc.grpc.rust/helloworld_grpc.rs -- 'com.google.devtools.build.lib.analysis.FilesToRunProvider$FullFilesToRunProvider@4f19aaaf' '--plugin=protoc-gen-grpc-rules_rust=bazel-out/k8-opt-exec-ST-d57f47055a04/bin/external/rules_rust~~i~rules_rust_proto__grpc-compiler-0.6.2/protoc-gen-rust-grpc__bin' '--grpc-rules_rust_out=bazel-out/k8-fastbuild/bin/src/proto/helloworld/helloworld_rust_grpc.grpc.rust' '--plugin=protoc-gen-rules_rust=bazel-out/k8-opt-exec-ST-d57f47055a04/bin/external/rules_rust~~i~rules_rust_proto__protobuf-codegen-2.8.2/protoc-gen-rust__bin' '--rules_rust_out=bazel-out/k8-fastbuild/bin/src/proto/helloworld/helloworld_rust_grpc.grpc.rust' '--descriptor_set_in=bazel-out/k8-fastbuild/bin/src/proto/helloworld/helloworld_proto-descriptor-set.proto.bin' helloworld/helloworld.proto)
# Configuration: 5c8ee76a4e8e7a83bc2d45bc81bdf4944e47f188efe88858782477e5980db997
# Execution platform: @@platforms//host:host

Inspecting the sandbox it looks like the .rs file is indeed missing:

$ tree $sandbox/darwin-sandbox/1907/execroot/_main
$sandbox/darwin-sandbox/1907/execroot/_main
└── bazel-out
    ├── darwin_x86_64-fastbuild
    │   └── bin
    │       └── src
    │           └── proto
    │               └── helloworld
    │                   ├── _virtual_imports
    │                   │   └── helloworld_proto
    │                   │       └── helloworld
    │                   │           └── helloworld.proto -> $execroot/_main/bazel-out/darwin_x86_64-fastbuild/bin/src/proto/helloworld/_virtual_imports/helloworld_proto/helloworld/helloworld.proto
    │                   ├── helloworld_proto-descriptor-set.proto.bin -> $execroot/_main/bazel-out/darwin_x86_64-fastbuild/bin/src/proto/helloworld/helloworld_proto-descriptor-set.proto.bin
    │                   └── helloworld_rust_grpc.grpc.rust
    └── darwin_x86_64-opt-exec-ST-d57f47055a04
        └── bin
            └── external
... more stuff

Going from there:

1. could we get the error to tell the user what file is missing?
2. am I using the ruleset wrong or is this a regression?