Specifying the argument types

As with much of the string handling in the C family of languages, it is possible to create a string based on a format string (for example, {0:##.###} would give a formatted output of the form xy.abc).

Similar things can be done in Rust, as follows:

let my_number = format!("{:.3}", 3.1415927); 

In the format string, the colon says we're requesting formatting for the value. Dot and 3 says that we want the number formatted to three decimal points. The formatter rounds the value for us, so the output will be 3.142.