Apply protobuf#19679, register toolchain early

This is an experiment to see if applying
protocolbuffers/protobuf#19679 via a patch and registering
`@rules_scala_toolchains//protoc/...:all` will fix Windows MSVC builds.

Local experiments suggest that this does indeed enable the toolchainized
`protoc` to take precedence and avoid compiling protobuf.

Author: mbland

WORKSPACE

@@ -5,6 +5,8 @@ load("//scala:deps.bzl", "rules_scala_dependencies")
 
 rules_scala_dependencies()
 
+register_toolchains("@rules_scala_toolchains//protoc/...:all")
+
 # The next two calls instantiate the `@host_platform` repo to work around:
 # - https://github.com/bazelbuild/bazel/issues/22558
 # Only required if using `--incompatible_enable_proto_toolchain_resolution`.

patches/0001-protobuf-rm-protoc-dep.patch

@@ -0,0 +1,91 @@
+diff --git a/protobuf.bzl b/protobuf.bzl
+index 283c85850..ad91faba6 100644
+--- a/protobuf.bzl
++++ b/protobuf.bzl
+@@ -1,7 +1,9 @@
+ load("@bazel_skylib//lib:versions.bzl", "versions")
+ load("@rules_cc//cc:defs.bzl", "objc_library")
+ load("@rules_python//python:defs.bzl", "py_library")
++load("//bazel/common:proto_common.bzl", "proto_common")
+ load("//bazel/common:proto_info.bzl", "ProtoInfo")
++load("//bazel/private:toolchain_helpers.bzl", "toolchains")
+ 
+ def _GetPath(ctx, path):
+     if ctx.label.workspace_root:
+@@ -71,6 +73,26 @@ def _CsharpOuts(srcs):
+         for src in srcs
+     ]
+ 
++_PROTOC_ATTRS = toolchains.if_legacy_toolchain({
++    "_proto_compiler": attr.label(
++        cfg = "exec",
++        executable = True,
++        allow_files = True,
++        default = configuration_field("proto", "proto_compiler"),
++    ),
++})
++_PROTOC_FRAGMENTS = ["proto"]
++_PROTOC_TOOLCHAINS = toolchains.use_toolchain(toolchains.PROTO_TOOLCHAIN)
++
++def _protoc_files_to_run(ctx):
++    if proto_common.INCOMPATIBLE_ENABLE_PROTO_TOOLCHAIN_RESOLUTION:
++        toolchain = ctx.toolchains[toolchains.PROTO_TOOLCHAIN]
++        if not toolchain:
++            fail("Protocol compiler toolchain could not be resolved.")
++        return toolchain.proto.proto_compiler
++    else:
++        return ctx.attr._proto_compiler[DefaultInfo].files_to_run
++
+ ProtoGenInfo = provider(
+     fields = ["srcs", "import_flags", "deps"],
+ )
+@@ -310,7 +332,7 @@ def _internal_gen_well_known_protos_java_impl(ctx):
+             args.add_all([src.path[offset:] for src in dep.direct_sources])
+ 
+     ctx.actions.run(
+-        executable = ctx.executable._protoc,
++        executable = _protoc_files_to_run(ctx),
+         inputs = descriptors,
+         outputs = [srcjar],
+         arguments = [args],
+@@ -334,12 +356,9 @@ internal_gen_well_known_protos_java = rule(
+         "javalite": attr.bool(
+             default = False,
+         ),
+-        "_protoc": attr.label(
+-            executable = True,
+-            cfg = "exec",
+-            default = "//:protoc",
+-        ),
+-    },
++    } | _PROTOC_ATTRS,
++    fragments = _PROTOC_FRAGMENTS,
++    toolchains = _PROTOC_TOOLCHAINS,
+ )
+ 
+ def _internal_gen_kt_protos(ctx):
+@@ -373,7 +392,7 @@ def _internal_gen_kt_protos(ctx):
+             args.add_all([src.path[offset:] for src in dep.direct_sources])
+ 
+     ctx.actions.run(
+-        executable = ctx.executable._protoc,
++        executable = _protoc_files_to_run(ctx),
+         inputs = descriptors,
+         outputs = [srcjar],
+         arguments = [args],
+@@ -397,12 +416,9 @@ internal_gen_kt_protos = rule(
+         "lite": attr.bool(
+             default = False,
+         ),
+-        "_protoc": attr.label(
+-            executable = True,
+-            cfg = "exec",
+-            default = "//:protoc",
+-        ),
+-    },
++    } | _PROTOC_ATTRS,
++    fragments = _PROTOC_FRAGMENTS,
++    toolchains = _PROTOC_TOOLCHAINS,
+ )
+ 
+ def internal_objc_proto_library(

patches/BUILD

@@ -57,6 +57,8 @@ def rules_scala_dependencies():
         strip_prefix = "protobuf-29.3",
         url = "https://github.com/protocolbuffers/protobuf/archive/refs/tags/v29.3.tar.gz",
         repo_mapping = {"@com_google_absl": "@abseil-cpp"},
+        patches = ["//patches:0001-protobuf-rm-protoc-dep.patch"],
+        patch_args = ["-p1"],
     )
 
     maybe(