The tracking issue for this feature is: #43874

Retains only the characters specified by the predicate.

In other words, remove all characters c such that f(c) returns false. This method operates in place and preserves the order of the retained characters.

#![allow(unused_variables)]
#![feature(string_retain)]

fn main() {
let mut s = String::from("f_o_ob_ar");

s.retain(|c| c != '_');

assert_eq!(s, "foobar");
}