feat: platform_transition_filegroup (#55)

* feat: platform_transition_filegroup

Lifted from https://github.com/aspect-build/gcc-toolchain/pull/8/files
See also https://github.com/fmeum/rules_meta/blob/main/meta/internal/meta.bzl#L4

* test: add tests for transition filegroup

docs/BUILD.bazel

@@ -53,4 +53,9 @@ stardoc_with_diff_test(
     bzl_library_target = "//lib:output_files",
 )
 
+stardoc_with_diff_test(
+    name = "transitions",
+    bzl_library_target = "//lib:transitions",
+)
+
 update_docs()

docs/transitions.md

@@ -0,0 +1,24 @@
+<!-- Generated with Stardoc: http://skydoc.bazel.build -->
+
+Rules for working with transitions.
+
+<a id="#platform_transition_filegroup"></a>
+
+## platform_transition_filegroup
+
+<pre>
+platform_transition_filegroup(<a href="#platform_transition_filegroup-name">name</a>, <a href="#platform_transition_filegroup-srcs">srcs</a>, <a href="#platform_transition_filegroup-target_platform">target_platform</a>)
+</pre>
+
+Transitions the srcs to use the provided platform. The filegroup will contain artifacts for the target platform.
+
+**ATTRIBUTES**
+
+
+| Name  | Description | Type | Mandatory | Default |
+| :------------- | :------------- | :------------- | :------------- | :------------- |
+| <a id="platform_transition_filegroup-name"></a>name |  A unique name for this target.   | <a href="https://bazel.build/docs/build-ref.html#name">Name</a> | required |  |
+| <a id="platform_transition_filegroup-srcs"></a>srcs |  The input to be transitioned to the target platform.   | <a href="https://bazel.build/docs/build-ref.html#labels">List of labels</a> | optional | [] |
+| <a id="platform_transition_filegroup-target_platform"></a>target_platform |  The target platform to transition the srcs.   | <a href="https://bazel.build/docs/build-ref.html#labels">Label</a> | required |  |
+
+

lib/BUILD.bazel

@@ -43,12 +43,6 @@ bzl_library(
     deps = ["//lib/private:utils"],
 )
 
-bzl_library(
-    name = "windows_utils",
-    srcs = ["windows_utils.bzl"],
-    visibility = ["//visibility:public"],
-)
-
 bzl_library(
     name = "jq",
     srcs = ["jq.bzl"],
@@ -96,7 +90,19 @@ bzl_library(
 
 bzl_library(
     name = "diff_test",
-    srcs = ["diff_test.bzl"],
+    srcs = ["diff_test.bzl"],  # keep
     visibility = ["//visibility:public"],
     deps = ["//lib/private:diff_test"],
 )
+
+bzl_library(
+    name = "transitions",
+    srcs = ["transitions.bzl"],
+    visibility = ["//visibility:public"],
+)
+
+bzl_library(
+    name = "windows_utils",
+    srcs = ["windows_utils.bzl"],
+    visibility = ["//visibility:public"],
+)

lib/tests/BUILD.bazel

@@ -1,5 +1,6 @@
 "tests for libs"
 
+load("@bazel_skylib//:bzl_library.bzl", "bzl_library")
 load("@bazel_skylib//rules:write_file.bzl", "write_file")
 load("//lib:expand_make_vars.bzl", "expand_template")
 load(":expand_make_vars_test.bzl", "expand_make_vars_test_suite")
@@ -40,3 +41,9 @@ sh_test(
     name = "expand_template_test",
     srcs = ["expand_template"],
 )
+
+bzl_library(
+    name = "generate_outputs",
+    srcs = ["generate_outputs.bzl"],
+    visibility = ["//visibility:public"],
+)

lib/tests/transitions/BUILD.bazel

@@ -0,0 +1,59 @@
+load("//lib:transitions.bzl", "platform_transition_filegroup")
+load("@bazel_skylib//rules:diff_test.bzl", "diff_test")
+
+platform(
+    name = "armv7_linux",
+    constraint_values = [
+        "@platforms//os:linux",
+        "@platforms//cpu:armv7",
+    ],
+)
+
+platform(
+    name = "x86_64_linux",
+    constraint_values = [
+        "@platforms//os:linux",
+        "@platforms//cpu:x86_64",
+    ],
+)
+
+config_setting(
+    name = "is_x86",
+    constraint_values = [
+        "@platforms//cpu:x86_64",
+    ],
+)
+
+# Simple test fixture that produces something different depending on the
+# target platform.
+filegroup(
+    name = "platform_description",
+    srcs = select({
+        ":is_x86": ["linux_x86.txt"],
+        "//conditions:default": ["linux_arm.txt"],
+    }),
+)
+
+platform_transition_filegroup(
+    name = "for_x86",
+    srcs = ["platform_description"],
+    target_platform = ":x86_64_linux",
+)
+
+platform_transition_filegroup(
+    name = "for_arm",
+    srcs = ["platform_description"],
+    target_platform = ":armv7_linux",
+)
+
+diff_test(
+    name = "test_x86",
+    file1 = "for_x86",
+    file2 = "linux_x86.txt",
+)
+
+diff_test(
+    name = "test_arm",
+    file1 = "for_arm",
+    file2 = "linux_arm.txt",
+)

lib/tests/transitions/linux_arm.txt

@@ -0,0 +1 @@
+Linux ARMv7

lib/tests/transitions/linux_x86.txt

@@ -0,0 +1 @@
+Linux x86

lib/transitions.bzl

@@ -0,0 +1,44 @@
+"Rules for working with transitions."
+
+def _transition_platform_impl(_, attr):
+    return {"//command_line_option:platforms": str(attr.target_platform)}
+
+# Transition from any input configuration to one that includes the
+# --platforms command-line flag.
+_transition_platform = transition(
+    implementation = _transition_platform_impl,
+    inputs = [],
+    outputs = ["//command_line_option:platforms"],
+)
+
+def _platform_transition_filegroup_impl(ctx):
+    files = []
+    runfiles = ctx.runfiles()
+    for src in ctx.attr.srcs:
+        files.append(src[DefaultInfo].files)
+
+    runfiles = runfiles.merge_all([src[DefaultInfo].default_runfiles for src in ctx.attr.srcs])
+    return [DefaultInfo(
+        files = depset(transitive = files),
+        runfiles = runfiles,
+    )]
+
+platform_transition_filegroup = rule(
+    _platform_transition_filegroup_impl,
+    attrs = {
+        # Required to Opt-in to the transitions feature.
+        "_allowlist_function_transition": attr.label(
+            default = "@bazel_tools//tools/allowlists/function_transition_allowlist",
+        ),
+        "target_platform": attr.label(
+            doc = "The target platform to transition the srcs.",
+            mandatory = True,
+        ),
+        "srcs": attr.label_list(
+            allow_empty = False,
+            cfg = _transition_platform,
+            doc = "The input to be transitioned to the target platform.",
+        ),
+    },
+    doc = "Transitions the srcs to use the provided platform. The filegroup will contain artifacts for the target platform.",
+)