The feature::when_enabled function takes a callback and returns a
listener_registration object. Unless the feature were enabled right from
the start, the listener_registration will be non-null and will keep the
callback registered until the registration is destroyed. If the
registration is destroyed before the feature is enabled, the callback
will not be called. It's easy to make a mistake and forget to keep the
returned registration alive - especially when, in tests, the feature is
enabled early in boot, because in that case when_enabled calls the
callback immediately and returns a null object instead.
In order to prevent issues with prematurely dropped
listener_registration in the future, mark feature::when_enabled with the
[[nodiscard]] attribute.
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