touch WORKSPACE
cat >rules.bzl <<EOF
def _my_flag_impl(ctx):
    return [config_common.FeatureFlagInfo(value = ctx.build_setting_value)]

my_flag = rule(
    implementation = _my_flag_impl,
    build_setting = config.string_list(flag = True),
)

def _my_transition_impl(inputs, outputs):
    result = {}

    for platform in inputs["//:my_flag"]:
        result[platform] = {"//command_line_option:platforms": [platform]}

    return result

_my_transition = transition(
    implementation = _my_transition_impl,
    inputs = ["//:my_flag"],
    outputs = [
	"//command_line_option:platforms",
    ],
)

def _my_rule_impl(ctx):
    return []

my_rule = rule(
    implementation = _my_rule_impl,
    attrs = {
	"deps": attr.label_list(
            cfg = _my_transition,
	),
        "_whitelist_function_transition": attr.label(
            default = "@bazel_tools//tools/whitelists/function_transition_whitelist",
	),
    },
)
EOF

cat > BUILD <<EOF
load(":rules.bzl", "my_flag", "my_rule")

my_flag(
    name = "my_flag",
    build_setting_default = [],
)

cc_binary(
    name = "bin",
    srcs = ["bin.cc"],
)

my_rule(
    name = "my_rule",
    deps = [":bin"],
)
EOF

bazel build --//:my_flag=//:foo,//:bar :my_rule

java.lang.ClassCastException: class java.lang.String cannot be cast to class com.google.devtools.build.lib.cmdline.Label (java.lang.String is in module java.base of loader 'bootstrap'; com.google.devtools.build.lib.cmdline.Label is in unnamed module of loader 'app')
	at com.google.devtools.build.lib.skyframe.PlatformMappingValue.computeMapping(PlatformMappingValue.java:218)
	at com.google.devtools.build.lib.skyframe.PlatformMappingValue.lambda$map$0(PlatformMappingValue.java:191)
	at com.google.common.cache.LocalCache$LocalManualCache$1.load(LocalCache.java:4875)
	at com.google.common.cache.LocalCache$LoadingValueReference.loadFuture(LocalCache.java:3527)
	at com.google.common.cache.LocalCache$Segment.loadSync(LocalCache.java:2276)
	at com.google.common.cache.LocalCache$Segment.lockedGetOrLoad(LocalCache.java:2154)
	at com.google.common.cache.LocalCache$Segment.get(LocalCache.java:2044)
	at com.google.common.cache.LocalCache.get(LocalCache.java:3951)
	at com.google.common.cache.LocalCache$LocalManualCache.get(LocalCache.java:4870)
	at com.google.devtools.build.lib.skyframe.PlatformMappingValue.map(PlatformMappingValue.java:191)
	at com.google.devtools.build.lib.skyframe.BuildConfigurationValue.keyWithPlatformMapping(BuildConfigurationValue.java:68)
	at com.google.devtools.build.lib.analysis.config.ConfigurationResolver.resolveConfigurations(ConfigurationResolver.java:302)
	at com.google.devtools.build.lib.skyframe.ConfiguredTargetFunction.computeDependencies(ConfiguredTargetFunction.java:584)
	at com.google.devtools.build.lib.skyframe.ConfiguredTargetFunction.compute(ConfiguredTargetFunction.java:317)
	at com.google.devtools.build.skyframe.AbstractParallelEvaluator$Evaluate.run(AbstractParallelEvaluator.java:451)