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

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