Format

⚠️ The design and implementation chapter is outdated ⚠️

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) => { defmt::export::make_istr(0) } } trait Format { fn format(&self, fmt: defmt::Formatter); } impl Format for u8 { fn format(&self, fmt: defmt::Formatter) { let t = internp!("{=u8}"); defmt::export::istr(&t); defmt::export::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.