platform_data implementation (#78)

* platform_data implementation
* update doc string for additional clarification
* update doc string
* move BUILD.bazel under experimental/platform_data
* add load statement to example

experimental/platform_data/BUILD.bazel

@@ -0,0 +1,3 @@
+exports_files([
+    "defs.bzl",
+])

experimental/platform_data/defs.bzl

@@ -0,0 +1,85 @@
+"""
+The platform_data rule can be used to change the target platform of a target,
+and then depend on that elsewhere in the build tree. For example:
+load("@platforms//experimental/platform_data:defs.bzl", "platform_data")
+
+cc_binary(name = "foo")
+
+platform_data(
+    name = "foo_embedded",
+    target = ":foo",
+    platform = "//my/new:platform",
+)
+
+py_binary(
+    name = "flasher",
+    srcs = ...,
+    data = [
+        ":foo_embedded",
+    ],
+)
+
+Regardless of what platform the top-level :flasher binary is built for,
+the :foo_embedded target will be built for //my/new:platform. 
+
+Note that if you depend on :foo_embedded it's not exactly the same as depending on :foo, since it won't forward all the same providers. In the future, we can extend this to add some common providers as needed."""
+
+def _target_platform_transition_impl(attr):
+    return {
+        "//command_line_option:platforms": str(attr.platform),
+    }
+
+_target_platform_transition = transition(
+    implementation = _target_platform_transition_impl,
+    inputs = [],
+    outputs = [
+        "//command_line_option:platforms",
+    ],
+)
+
+def _platform_data_impl(ctx):
+    target = ctx.attr.target
+
+    default_info = target[DefaultInfo]
+    files = default_info.files
+    original_executable = default_info.files_to_run.executable
+    runfiles = default_info.default_runfiles
+
+    new_executable = ctx.actions.declare_file(ctx.attr.name)
+
+    ctx.actions.symlink(
+        output = new_executable,
+        target_file = original_executable,
+        is_executable = True,
+    )
+
+    files = depset(direct = [new_executable], transitive = [files])
+    runfiles = runfiles.merge(ctx.runfiles([new_executable]))
+
+    return [
+	DefaultInfo(
+            files = files,
+            runfiles = runfiles,
+            executable = new_executable,
+	)
+    ]
+
+platform_data = rule(
+    implementation = _platform_data_impl,
+    attrs = {
+        "target": attr.label(
+            allow_files = False,
+            executable = True,
+            mandatory = True,
+            cfg = _target_platform_transition,
+        ),
+        "platform": attr.label(
+            mandatory = True,
+        ),
+        "_allowlist_function_transition": attr.label(
+            default = "@bazel_tools//tools/allowlists/function_transition_allowlist",
+        ),
+    },
+    executable = True,
+)
+