Format

The untyped argument (:?) requires one level of indirection during serialization.

First let's see how a primitive implements the Format trait:


#![allow(unused)]
fn main() {
extern crate defmt;
macro_rules! internp { ($l:literal) => { 0 } }
trait Format { fn format(&self, fmt: &mut defmt::Formatter); }
impl Format for u8 {
    fn format(&self, fmt: &mut defmt::Formatter) {
        if fmt.needs_tag() {
            let t = internp!("{:u8}");
            fmt.u8(&t);
        }
        fmt.u8(self)
        // on the wire: [1, 42]
        //  string index ^  ^^ `self`
    }
}
}

Format will use the write! macro. This will send the string index of {:u8} followed by the one-byte data. In general, write! can use {:?} so Format nesting is possible.

Now let's look into a log invocation:


#![allow(unused)]
fn main() {
extern crate defmt;
defmt::error!("The answer is {:?}!", 42u8);
// on the wire: [2, 1, 42]
//  string index ^  ^^^^^ `42u8.format(/*..*/)`
//  ^ = intern("The answer is {:?}!")
}

This will send the string index of "The answer is {:?}!" and invoke the argument's Format::format method.