Is it possible to use a variable as the fill argument in the format! macro?

I wanted to imitate Python's rjust, ljust, and center functions using the format! macro, but I was only able to work out a solution where you can pass in the string and the width. If you want to pass in the fill-argument it doesn't work.

The documentation tells me that it is possible to provide variables to format! and for the width argument it works just fine. When I try to use a variable for fill, the compiler does not recognize the pattern.

Just the width as a variable works:

fn rjust(text: &str, width: usize, fill: Option<char>) -> String {
    format!("{text:>width$}", text = text, width = width)
}
println!("{}", rjust("Hello", 10)); // "     Hello"

Providing the fill as a variable does not work:

fn rjust(text: &str, width: usize, fill: char) -> String {
    format!(
        "{text:fill>width$}",
        text = text,
        fill = fill,
        width = width
    )
}
println!("{}", rjust("Hello", 20, '*'));

The error message:

error: invalid format string: expected `'}'`, found `'>'`
 --> src/lib.rs:4:24
  |
4 |             "{text:fill>width$}",
  |              -         ^ expected `}` in format string
  |              |
  |              because of this opening brace
  |
  = note: if you intended to print `{`, you can escape it using `{{`

If I provide a single character instead of the fill-variable, it works fine. Notice the * character:

fn rjust(text: &str, width: usize, fill: char) -> String {
    format!("{text:*>width$}", text = text, width = width)
}
println!("{}", rjust("Hello", 20, '_')); // ***************Hello

I expected the fill-variable version to work the same as the hardcoded * character version.

A workaround is to subtract the length of the text from the width and then create a String of that length (fill-length) made of fill-characters and concatenate them:

fn rjust(text: &str, width: usize, fill: char) -> String {
    let fill_len = width - text.len();
    let fill_str: String = (0..fill_len).map(|_| fill).collect();
    String::from(fill_str + text)
}
println!("{}", rjust("Hello", 20, '*')); // ***************Hello