Enum ConfigState 

pub enum ConfigState {
    Loaded(Box<Config>),
    Unreadable(String),
}

What the plan found when it read the config file.

One value rather than a Config and an error beside it, because those two only ever come in two of the four combinations and the other two would be arms nothing could reach.

The config is carried rather than re-read at write time so there is exactly one read: re-opening the file would add an error arm only a race could take, and applying a migration to a document nobody has looked at since the report was printed is exactly the surprise this command exists to avoid.

Variants

Loaded(Box<Config>)

The config as it stands, for the migrations to be applied to. Boxed because a Config is far larger than the message beside it.

Unreadable(String)

It could not be read, and this is why.