See this example:
fn concat<T: std::fmt::Display>(s: &mut String, thing: T) {
// TODO
}
fn main() {
let mut s = "Hello ".into();
concat(&mut s, 42);
assert_eq!(&s, "Hello 42");
}
I know that I can use this:
s.push_str(&format!("{}", thing))
but this is not the most efficient, because format! allocate a String that is not necessary.
The most efficient is to write directly the string representation of the displayable item into the String buffer. How to do this?
There are multiple formatting macros, and in your case you want the write! macro:
use std::fmt::{Display, Write};
fn concat<T: Display>(s: &mut String, thing: &T) {
write!(s, "{}", thing).unwrap();
}
fn main() {
let mut s = "Hello ".into();
concat(&mut s, &42);
assert_eq!(&s, "Hello 42");
}
Anything that implements one of the Write trait (and String does) is a valid target for write!.
Note: actually anything that implements a write_fmt method, as macro don't care much about semantics; which is why either fmt::Write or io::Write work.
Write traits (std::io and std::fmt).
write! macro documentation that I linked, but since you felt the need for it, it now is :)