Implementing Format

#[derive(Format)]

The easiest way to implement the Format trait for a struct or enum is to use the derive attribute.

#![allow(unused)]
fn main() {
extern crate defmt;
use defmt::Format;

#[derive(Format)]
struct Header {
    source: u8,
    destination: u8,
    sequence: u16,
}

#[derive(Format)]
struct Descriptor;
#[derive(Format)]
enum Request {
    GetDescriptor { descriptor: Descriptor, length: u16 },
    SetAddress { address: u8 },
}
}

NOTE: Like built-in derives like #[derive(Debug)], #[derive(Format)] will add Format bounds to the generic type parameters of the struct.

NOTE: Do not use the API used by the expansion of the derive(Format) macro; it is unstable.

write!

NOTE write! is available in defmt v0.1.2+

It is also possible to implement the Format trait manually. This trait has a single required method: format. In this method you need to format the value (self) into the given Formatter argument using the defmt::write! macro. Example below:

#![allow(unused)]
fn main() {
extern crate defmt;
// value read from a MMIO register named "CRCCNF"
struct CRCCNF {
   bits: u32,
}

impl defmt::Format for CRCCNF {
    fn format(&self, f: defmt::Formatter) {
        // format the bitfields of the register as struct fields
        defmt::write!(
           f,
           "CRCCNF {{ LEN: {0=0..2}, SKIPADDR: {0=8..10} }}",
           self.bits,
        )
    }
}
}

Newtypes

If you need to implement Format for some "newtype" struct you can delegate the formatting to the inner type. Example below:

#![allow(unused)]
fn main() {
extern crate defmt;
struct MyU8 { inner: u8 }

impl defmt::Format for MyU8 {
    fn format(&self, f: defmt::Formatter) {
        self.inner.format(f)
    }
}
}

Uncompressed adapters

If you quickly want to get some code running and do not care about it being efficient you can use the two adapter types Display2Format and Debug2Format.

These adapters disable compression and use the core::fmt code on-device! You should always prefer defmt::Format over Debug whenever possible.

Note that this always uses {:?} to format the contained value, meaning that any provided defmt display hints will be ignored.