Add precompiled protoc toolchain

Registers a precompiled protocol compiler toolchain when
`--incompatible_enable_proto_toolchain_resolution` is `True`.
Part of bazel-contrib#1482 and bazel-contrib#1652.

Stops `protoc` recompilation, and fixes the build breakage in bazel-contrib#1710 due
to `protobuf` include paths exceeding the Visual Studio path length
limit.

The updates to `scala_proto/scala_proto_toolchain.bzl` were inspired by:

- protocolbuffers/protobuf: bazel: Remove hardcoded dependency on
  //:protoc from language runtimes #19679
  protocolbuffers/protobuf#19679

The `proto_lang_toolchain` call was inspired by the `README` from:

- https://github.com/aspect-build/toolchains_protoc/

Adds `scripts/update_protoc_integrity.py` to automatically update
`scala/private/protoc/protoc_integrity.bzl`.

This should make builds of `rules_scala` much faster all around. Given
the fact that this feature depends on recent `protobuf` versions, and
the Windows `protobuf` build breaks without it, we have a catch-22. It
likely can't be separated from the rest of bazel-contrib#1710, though I would prefer
that.

It also seems likely that we'd eventually need to do this to continue
supporting Windows, per:

- protocolbuffers/protobuf#12947
- https://protobuf.dev/news/v30/#poison-msvc--bazel
- protocolbuffers/protobuf#20085

More background on proto toolchainization:

- Proto Toolchainisation Design Doc
  https://docs.google.com/document/d/1CE6wJHNfKbUPBr7-mmk_0Yo3a4TaqcTPE0OWNuQkhPs/edit

- bazelbuild/bazel: Protobuf repo recompilation sensitivity
  bazelbuild/bazel#7095

- bazelbuild/rules_proto: Implement proto toolchainisation
  bazelbuild/rules_proto#179

- rules_proto 6.0.0 release notes mentioning Protobuf Toolchainisation
  https://github.com/bazelbuild/rules_proto/releases/tag/6.0.0

Author: mbland

.bazelrc

@@ -2,6 +2,11 @@
 # https://github.com/bazelbuild/rules_scala/issues/1482
 common --enable_workspace --noenable_bzlmod
 
+# Remove once proto toolchainization becomes the default
+# - https://bazel.build/reference/command-line-reference#flag--incompatible_enable_proto_toolchain_resolution
+# - https://docs.google.com/document/d/1CE6wJHNfKbUPBr7-mmk_0Yo3a4TaqcTPE0OWNuQkhPs/edit
+common --incompatible_enable_proto_toolchain_resolution
+
 build --enable_platform_specific_config
 
 #Windows needs --worker_quit_after_build due to workers not being shut down when the compiler tools need to be rebuilt (resulting in 'file in use' errors). See Bazel Issue#10498.

README.md

@@ -62,6 +62,13 @@ load("@rules_scala//scala:deps.bzl", "rules_scala_dependencies")
 
 rules_scala_dependencies()
 
+# 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`.
+load("@platforms//host:extension.bzl", "host_platform_repo")
+
+host_platform_repo(name = "host_platform")
+
 load("@rules_java//java:rules_java_deps.bzl", "rules_java_dependencies")
 
 rules_java_dependencies()
@@ -168,6 +175,40 @@ load(
 )
 ```
 
+### <a id="protoc"></a>Using a precompiled protocol compiler
+
+`rules_scala` now supports
+[`--incompatible_enable_proto_toolchain_resolution`][]. When using this flag,
+`rules_scala` will use a precompiled protocol compiler binary by default. To set
+it in your `.bazelrc` file:
+
+[`--incompatible_enable_proto_toolchain_resolution`]: https://bazel.build/reference/command-line-reference#flag--incompatible_enable_proto_toolchain_resolution
+
+```txt
+common --incompatible_enable_proto_toolchain_resolution
+```
+
+Set the `protoc_platforms` attribute of `scala_toolchains()` if you need to
+configure protocol compilers for platforms other than the host platform.
+
+Windows builds now require the precompiled protocol compiler toolchain. See the
+[Windows MSVC builds of protobuf broken by default](#protoc-msvc) section
+below for details.
+
+More background on proto toolchainization:
+
+- [Proto Toolchainisation Design Doc](
+    https://docs.google.com/document/d/1CE6wJHNfKbUPBr7-mmk_0Yo3a4TaqcTPE0OWNuQkhPs/edit)
+
+- [bazelbuild/bazel#7095: Protobuf repo recompilation sensitivity](
+    https://github.com/bazelbuild/bazel/issues/7095)
+
+- [bazelbuild/rules_proto#179: Implement proto toolchainisation](
+    https://github.com/bazelbuild/rules_proto/issues/179)
+
+- [rules_proto 6.0.0 release notes mentioning Protobuf Toolchainization](
+    https://github.com/bazelbuild/rules_proto/releases/tag/6.0.0)
+
 ### Persistent workers
 
 To run with a persistent worker (much faster), add the following to
@@ -567,6 +608,25 @@ http_archive(
 )
 ```
 
+### <a id="protoc-msvc"></a>Windows MSVC builds of `protobuf` broken by default
+
+MSVC builds of recent `protobuf` versions started failing, as first noted in
+bazelbuild/rules_scala#1710. On top of that, `protobuf` is planning to stop
+supporting Bazel + MSVC builds per:
+
+- [protocolbuffers/protobuf#12947: src build on windows not working](
+    https://github.com/protocolbuffers/protobuf/issues/12947)
+
+- [protobuf.dev News Announcements for Version 30.x:Poison MSVC + Bazel](
+    https://protobuf.dev/news/v30/#poison-msvc--bazel)
+
+- [protocolbuffers/protobuf#20085: Breaking Change: Dropping support for
+    Bazel+MSVC](https://github.com/protocolbuffers/protobuf/issues/20085)
+
+Enable [protocol compiler toolchainization](#protoc) to avoid recompiling
+`@com_google_protobuf//:protoc` on Windows platforms and fix broken Windows
+builds.
+
 ### Bzlmod configuration (coming soon!)
 
 The upcoming Bzlmod implementation will funnel through the `scala_toolchains()`

WORKSPACE

@@ -5,6 +5,13 @@ load("//scala:deps.bzl", "rules_scala_dependencies")
 
 rules_scala_dependencies()
 
+# 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`.
+load("@platforms//host:extension.bzl", "host_platform_repo")
+
+host_platform_repo(name = "host_platform")
+
 load("@rules_java//java:rules_java_deps.bzl", "rules_java_dependencies")
 
 rules_java_dependencies()

dt_patches/test_dt_patches/WORKSPACE

@@ -11,6 +11,10 @@ load("@rules_scala//scala:deps.bzl", "rules_scala_dependencies")
 
 rules_scala_dependencies()
 
+load("@platforms//host:extension.bzl", "host_platform_repo")
+
+host_platform_repo(name = "host_platform")
+
 load("@rules_java//java:rules_java_deps.bzl", "rules_java_dependencies")
 
 rules_java_dependencies()

dt_patches/test_dt_patches_user_srcjar/WORKSPACE

@@ -11,6 +11,10 @@ load("@rules_scala//scala:deps.bzl", "rules_scala_dependencies")
 
 rules_scala_dependencies()
 
+load("@platforms//host:extension.bzl", "host_platform_repo")
+
+host_platform_repo(name = "host_platform")
+
 load("@rules_java//java:rules_java_deps.bzl", "rules_java_dependencies")
 
 rules_java_dependencies()

examples/crossbuild/WORKSPACE

@@ -11,6 +11,10 @@ load("@rules_scala//scala:deps.bzl", "rules_scala_dependencies")
 
 rules_scala_dependencies()
 
+load("@platforms//host:extension.bzl", "host_platform_repo")
+
+host_platform_repo(name = "host_platform")
+
 load("@rules_java//java:rules_java_deps.bzl", "rules_java_dependencies")
 
 rules_java_dependencies()

examples/overridden_artifacts/WORKSPACE

@@ -11,6 +11,10 @@ load("@rules_scala//scala:deps.bzl", "rules_scala_dependencies")
 
 rules_scala_dependencies()
 
+load("@platforms//host:extension.bzl", "host_platform_repo")
+
+host_platform_repo(name = "host_platform")
+
 load("@rules_java//java:rules_java_deps.bzl", "rules_java_dependencies")
 
 rules_java_dependencies()

examples/scala3/WORKSPACE

@@ -11,6 +11,10 @@ load("@rules_scala//scala:deps.bzl", "rules_scala_dependencies")
 
 rules_scala_dependencies()
 
+load("@platforms//host:extension.bzl", "host_platform_repo")
+
+host_platform_repo(name = "host_platform")
+
 load("@rules_java//java:rules_java_deps.bzl", "rules_java_dependencies")
 
 rules_java_dependencies()

examples/semanticdb/WORKSPACE

@@ -11,6 +11,10 @@ load("@rules_scala//scala:deps.bzl", "rules_scala_dependencies")
 
 rules_scala_dependencies()
 
+load("@platforms//host:extension.bzl", "host_platform_repo")
+
+host_platform_repo(name = "host_platform")
+
 load("@rules_java//java:rules_java_deps.bzl", "rules_java_dependencies")
 
 rules_java_dependencies()

examples/testing/multi_frameworks_toolchain/WORKSPACE

@@ -11,6 +11,10 @@ load("@rules_scala//scala:deps.bzl", "rules_scala_dependencies")
 
 rules_scala_dependencies()
 
+load("@platforms//host:extension.bzl", "host_platform_repo")
+
+host_platform_repo(name = "host_platform")
+
 load("@rules_java//java:rules_java_deps.bzl", "rules_java_dependencies")
 
 rules_java_dependencies()