Bool

Important

The design and implementation chapter is outdated

Booleans are compressed in bytes, bitflags-style.

#![allow(unused)]
fn main() {
extern crate defmt;
defmt::error!("x: {=bool}, y: {=bool}, z: {=bool}", false, false, true);
// on the wire: [1, 0b001]
//  string index ^  ^^^^^ the booleans: `0bxyz`
}

When mixed with other data, the first {=bool} allocates an output byte that fits up to 7 more bools.

{=bool}s in the formatting string are batched together as follows: Any non-{=bool} arguments are emitted as-is, while {=bool} arguments are collected into a byte and emitted when 8 {=bool}s have been collected. This means that for every set of 8 {=bool}s, the byte containing them will be serialized at the position of their last member. If more than 0 but less than 8 {=bool}s have been encountered at the end of the log frame, a byte containing them will be emitted last.

#![allow(unused)]
fn main() {
extern crate defmt;
defmt::error!("x: {=bool}, y: {=u8}, z: {=bool}", false, 0xff, true);
// on the wire: [1, 0xff, 0b01]
//  string index ^  ^^^^^ ^^^^ the booleans: `0bxz`
//                  |
//                  u8
}

⚠️ If the final parameter is not a {=bool} but there are yet to be compressed {=bool}s present in the format string beforehand, the final output byte containing all compressed booleans will be at the end.

#![allow(unused)]
fn main() {
extern crate defmt;
defmt::error!("x: {=bool}, y: {=u8}", false, 0xff);
// on the wire: [1, 0xff, 0b0,]
//  string index ^  ^^^^^ ^^^^ the booleans: `0bx`
//                  |
//                  u8
}

⚠️ If some {=bool}s are nested inside a struct, they will still be compressed as if they were passed as regular arguments.

#![allow(unused)]
fn main() {
extern crate defmt;
use defmt::Format;
#[derive(Format)]
struct Flags {
         a: bool,
         b: bool,
}

defmt::error!("x: {=bool}, {=?}", false, Flags { a: true, b: false });
// on the wire: [1, 2, 0b010,]
//  string index ^  ^  ^^^^ all booleans: `0bxab`
//                  |
//                  index of "Flags { a: {=bool}, b: {=bool}} "
}