Please add proto_library attrs to adjust its "virtual path"

When a .proto file contains import "foo/bar/baz.proto", that path doesn’t actually have to exist as-is on the filesystem. It can be mapped to any arbitrary path by protoc by using an equals sign in --proto_path: protoc --proto_path=foo/bar/baz.proto=some/other/path.proto.

Please add attributes to proto_library() to adjust the virtual path used to locate its source proto, similar to how cc_library() allows changing the include path. Good attributes would be:

• import_prefix to add a prefix to the virtual path.
• strip_import_prefix to remove a prefix from the virtual path.

The main use case is cross-repo includes, where we want to use nice short names in the filesystem but URL-prefixed names in the import paths:

# ./external/base/proto/BUILD
proto_library(
  name = "build_info",
  srcs = ["build_info.proto"],
  import_prefix = "github.com/myorg/base",
)

# ./proto/BUILD
proto_library(
  name = "helloworld",
  srcs = ["helloworld.proto"],
  deps = ["@base//proto:build_info],
)

# CURRENT ./proto/helloworld.proto
import "proto/build_info.proto";

# DESIRED ./proto/helloworld.proto
import "github.com/myorg/base/proto/build_info.proto";

[tomwilkie]

Hi there; just to chime in - this issue is preventing us vendoring proto libraries which depend on other proto libraries in go projects - see bazelbuild/rules_go#900. A good example is the go Kubernetes Client library. Is there any chance someone might work on this any time soon, or should I take a stab?

[jayconrod]

Any chance this could be implemented soon?

This blocks Kubernetes and a number of related projects from using proto_library. They are having to pre-generate code with a separate system and check it in.