Interned strings

The {:istr} formatting parameter is used for interned strings. Compared to the {:str} parameter, which transmits a complete string, {:istr} saves bandwidth by sending only a string index. The {:istr} parameter expects an argument with type defmt::Str. A Str value is created using the intern! macro; the argument to this macro must be a string literal.


#![allow(unused)]
fn main() {
extern crate defmt;
let s = "The quick brown fox jumps over the lazy dog";
defmt::info!("{:str}", s);
//                      ^ bandwidth-use = 43 bytes

use defmt::Str;
let interned: Str = defmt::intern!("The quick brown fox jumps over the lazy dog");
defmt::info!("{:istr}", interned);
//                       ^^^^^^^^ bandwidth-use <= 2 bytes
}

This was a contrived example to show the difference in bandwidth use. In practice you should use:


#![allow(unused)]
fn main() {
extern crate defmt;
defmt::info!("The quick brown fox jumps over the lazy dog");
}

which also interns the log string and uses as little bandwidth as the {:istr} version.