What is the easiest way to pad a string with 0 to the left?

The fmt module documentation describes all the formatting options:

Fill / Alignment

The fill character is provided normally in conjunction with the
width parameter. This indicates that if the value being formatted is
smaller than width some extra characters will be printed around it.
The extra characters are specified by fill, and the alignment can be
one of the following options:

• < – the argument is left-aligned in width columns
• ^ – the argument is center-aligned in width columns
• > – the argument is right-aligned in width columns

assert_eq!("00000110", format!("{:0>8}", "110"));
//                                |||
//                                ||+-- width
//                                |+--- align
//                                +---- fill

See also:

• How can I 0-pad a number by a variable amount when formatting with std::fmt?
• How do I print an integer in binary with leading zeros?
• Hexadecimal formating with padded zeroes
• Convert binary string to hex string with leading zeroes in Rust