This ...

Command::new(
    "A1",
    CommandBody::login("alice", "password").unwrap(),
).unwrap();

... will produce ...

A1 LOGIN alice password

However, ...

Command::new(
    "A1",
    CommandBody::login("alice\"", b"\xCA\xFE".as_ref()).unwrap(),
)
.unwrap();

... will produce ...

A1 LOGIN "alice\"" {2}
\xCA\xFE

Also, the construction ...

Command::new(
    "A1",
    CommandBody::login("alice\x00", "password").unwrap(),
).unwrap();

... will fail because IMAP doesn't allow NULL bytes in the username (nor password).

You can also use ...

Command::new(
    "A1",
    CommandBody::login(Literal::try_from("alice").unwrap(), "password").unwrap(),
)
.unwrap();

... to produce ...

A1 LOGIN {5}
alice password

... even though "alice" could be encoded more simply with an atom or quoted string.

Also, you can use Rust literals and resort to unvalidated constructors when you are certain that your input is correct:

// This could be provided by the email application.
let tag = TagGenerator::random();

Command {
    tag,
    body: CommandBody::Login {
        // Note that the "unvalidated" feature must be activated.
        username: AString::from(Atom::unvalidated("alice")),
        password: Secret::new(AString::from(Atom::unvalidated("password"))),
    },
};

In this case, imap-codec won't stand in your way. However, it won't guarantee that you produce correct messages, either.