Using format!, I can create a String from a format string, but what if I already have a String that I'd like to append to? I would like to avoid allocating the second string just to copy it and throw away the allocation.
let s = "hello ".to_string();
append!(s, "{}", 5); // Doesn't exist
A close equivalent in C/C++ would be snprintf.
I see now that String implements Write, so we can use write!:
use std::fmt::Write;
pub fn main() {
let mut a = "hello ".to_string();
write!(a, "{}", 5).unwrap();
println!("{}", a);
assert_eq!("hello 5", a);
}
It is impossible for this write! call to return an Err, at least as of Rust 1.47, so the unwrap should not cause concern.
fmt::Write and io::Write are unrelated.
unwrap() the write! that never fails. The whole idea of something as simple that requires unsafe call is ludicrous. The deleted answer below is safe however.