Add current_proto_toolchain

After asking in https://bazelbuild.slack.com/archives/C04281DTLH0/p1716279966809639
I gave it a tried, and it seems to be working.

The implementation is based on https://github.com/bazelbuild/rules_python/blob/730a2e39bd2702910f28629d4583b3ec49f4ee5e/python/current_py_toolchain.bzl

proto/defs.bzl

@@ -15,7 +15,7 @@
 """Starlark rules for building protocol buffers."""
 
 load("//proto:proto_lang_toolchain.bzl", _proto_lang_toolchain = "proto_lang_toolchain")
-load("//proto:proto_toolchain.bzl", _proto_toolchain = "proto_toolchain")
+load("//proto:proto_toolchain.bzl", _current_proto_toolchain = "current_proto_toolchain", _proto_toolchain = "proto_toolchain")
 load("//proto/private:native.bzl", "NativeProtoInfo", "native_proto_common")
 load("//proto/private/rules:proto_descriptor_set.bzl", _proto_descriptor_set = "proto_descriptor_set")
 
@@ -45,6 +45,7 @@ proto_descriptor_set = _proto_descriptor_set
 proto_lang_toolchain = _proto_lang_toolchain
 
 proto_toolchain = _proto_toolchain
+current_proto_toolchain = _current_proto_toolchain
 
 # Encapsulates information provided by `proto_library`.
 #

proto/private/rules/proto_toolchain.bzl

@@ -38,3 +38,36 @@ def proto_toolchain(*, name, proto_compiler, exec_compatible_with = []):
         target_compatible_with = [],
         toolchain = name,
     )
+
+def _current_proto_toolchain_impl(ctx):
+    toolchain = ctx.toolchains[ctx.attr._toolchain]
+
+    direct = [toolchain.proto.proto_compiler.executable]
+    transitive = []
+    files = depset(direct, transitive = transitive)
+    return [
+        toolchain,
+        platform_common.TemplateVariableInfo({
+            "PROTOC": str(toolchain.proto.proto_compiler.executable.path),
+        }),
+        DefaultInfo(
+            runfiles = ctx.runfiles(transitive_files = files),
+            files = files,
+        ),
+    ]
+
+current_proto_toolchain = rule(
+    doc = """
+    This rule exists so that the current protoc toolchain can be used in the `toolchains` attribute of
+    other rules, such as genrule. It allows exposing a protoc toolchain after toolchain resolution has
+    happened, to a rule which expects a concrete implementation of a toolchain, rather than a
+    toolchain_type which could be resolved to that toolchain.
+    """,
+    implementation = _current_proto_toolchain_impl,
+    attrs = {
+        "_toolchain": attr.string(default = str(Label("@rules_proto//proto:toolchain_type"))),
+    },
+    toolchains = [
+        str(Label("@rules_proto//proto:toolchain_type")),
+    ],
+)

proto/proto_toolchain.bzl

@@ -14,6 +14,7 @@
 
 """Export for proto_toolchain"""
 
-load("//proto/private/rules:proto_toolchain.bzl", _proto_toolchain_macro = "proto_toolchain")
+load("//proto/private/rules:proto_toolchain.bzl", _current_proto_toolchain = "current_proto_toolchain", _proto_toolchain_macro = "proto_toolchain")
 
 proto_toolchain = _proto_toolchain_macro
+current_proto_toolchain = _current_proto_toolchain