Add support for rustc flags to rust_proto_library

proto/proto.bzl

@@ -310,6 +310,16 @@ rust_proto_library = rule(
  "rust_deps": attr.label_list(
  doc = "The crates the generated library depends on.",
  ),
+ "rustc_flags": attr.string_list(
+ doc = """\
+ List of compiler flags passed to `rustc`.
+
+ These strings are subject to Make variable expansion for predefined
+ source/output path variables like `$location`, `$execpath`, and 
+ `$rootpath`. This expansion is useful if you wish to pass a generated
+ file of arguments to rustc: `@$(location //package:target)`.
+ """,
+ ),
  "_cc_toolchain": attr.label(
  default = Label("@bazel_tools//tools/cpp:current_cc_toolchain"),
  ),
@@ -388,6 +398,16 @@ rust_grpc_library = rule(
  "rust_deps": attr.label_list(
  doc = "The crates the generated library depends on.",
  ),
+ "rustc_flags": attr.string_list(
+ doc = """\
+ List of compiler flags passed to `rustc`.
+
+ These strings are subject to Make variable expansion for predefined
+ source/output path variables like `$location`, `$execpath`, and 
+ `$rootpath`. This expansion is useful if you wish to pass a generated
+ file of arguments to rustc: `@$(location //package:target)`.
+ """,
+ ),
  "_cc_toolchain": attr.label(
  default = "@bazel_tools//tools/cpp:current_cc_toolchain",
  ),