This ...

use imap_types::command::{Command, CommandBody};

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

... will produce ...

A1 LOGIN alice password

However, ...

use imap_types::command::{Command, CommandBody};

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

... will produce ...

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

Also, the construction ...

use imap_types::command::{Command, CommandBody};

let cmd = 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 ...

use imap_types::{
    command::{Command, CommandBody},
    core::Literal,
};

let cmd = 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:

use imap_types::{
    command::{Command, CommandBody},
    core::{AString, Atom, Tag},
    secret::Secret,
};

// This could be provided by the email application.
struct TagGenerator;

impl TagGenerator {
    fn random() -> Tag<'static> {
        // Make this random :-)
        Tag::unvalidated("A1")
    }
}

let tag = TagGenerator::random();

let cmd = Command {
    tag,
    body: CommandBody::Login {
        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.