API allows passing a vector for the buffer, but it doesn't work properly.

[vext01]

The example, with one small adjustment: the buffer is a Vec:

fn main() -> io::Result<()> {
    let mut cli = AskPass::new(Vec::new());
    let pass = cli.askpass("Password:")?;

    let mut stdout = io::stdout();
    write!(&mut stdout, "Your password is ")?;
    stdout.write_all(pass)?;
    stdout.flush()?;

    Ok(())
}

This compiles and runs, but askpass() returns an empty password:

$ cargo run
   Compiling tty v0.1.0 (/tmp/tty)
    Finished dev [unoptimized + debuginfo] target(s) in 0.66s
     Running `target/debug/tty`
Your password is
$

Is that intentional?

[quininer]

Yes, the length of buffer is maximum length of password. You provide a zero-length buffer, so it always returns empty.