Enum Done 

pub enum Done {
    Nothing,
    Arrived(usize),
    Folders(Vec<(String, usize)>),
    Counts(Vec<usize>),
    Uids(Vec<u32>),
    Folder(String),
    Switched(Box<Facts>),
}

What a job left behind.

Variants

Nothing

Nothing to report but success.

Arrived(usize)

How many messages arrived (a check, or a switch).

Folders(Vec<(String, usize)>)

Counts(Vec<usize>)

The counts asked for, in the order they were asked for.

Uids(Vec<u32>)

Folder(String)

Where an append landed.

Switched(Box<Facts>)

A switch: the facts that came with the new folder. Boxed, since an Account is far bigger than a count.