async fn check_visibility(
channel: &GuildChannel,
source_channel: &GuildChannel,
guild_id: GuildId,
ctx: &Context,
) -> Result<(), PreviewError>Expand description
Validates that everyone who can view source_channel could also view channel.
The expanded content is posted as a single message that all members of
source_channel can read, so the linked channel must be at least as visible
as the source channel to avoid leaking restricted content.