python/private/sentinel_impl.bzl

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# Licensed under the Apache License, Version 2.0 (the "License");
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"""A rule to define a target to act as a singleton for label attributes.

Label attributes with defaults cannot accept None, otherwise they fall
back to using the default. A sentinel allows detecting an intended None value.
"""

SentinelInfo = provider(
    doc = "Indicates this was the sentinel target.",
    fields = [],
)

def _sentinel_impl(ctx):
    _ = ctx  # @unused
    return [
        SentinelInfo(),
        # Also output ToolchainInfo to allow it to be used for noop toolchains
        platform_common.ToolchainInfo(),
    ]

sentinel = rule(implementation = _sentinel_impl)