defmt is a highly efficient logging framework that targets resource-constrained devices, like microcontrollers. defmt stands for "deferred formatting".

Rather than performing formatting input like 255u8 into "The temperature is 255°C" on the embedded device, the formatting is deferred to the host that will output the logs. This way, only the relevant data needs to be sent to the host instead of the entire format string. Additionally the transmitted data is compressed, for example by compressing several booleans into one byte.

This means that defmt:

• reduces compiled binary size, since it only has to contain indices to log strings saved by the host instead of the string itself
• reduces computation efforts on the target, because the formatting of e.g. 255u8 to "255" happens on the host that displays the logging output, not the target
• reduces delays and log buffer usage, since there is less data sent from the embedded device to the host

For more details on how it works, check out the defmt book.

flip-link adds zero-cost stack overflow protection for your embedded programs – no MPU or stack probe support needed!

It does this by flipping the standard memory layout of ARM Cortex-M programs.

With this inverted memory layout, the stack "overflows" instead of corrupting memory when it hits the boundary of the RAM region. This boundary collision raises a hardware exception (usually the "hard fault" exception), which by default halts the program.

For more details, check out our blog post introducing flip-link.

The app-template is a Cargo project template, so you can hit the ground running with probe-run, defmt and flip-link. Using the knurling app-template, and cargo-generate, you can start your embedded project by just running

$ cargo generate \
    --git https://github.com/knurling-rs/app-template \
    --branch main \
    --name my-app

and specifying your desired HAL and compilation target.

defmt-test is an embedded test harness that lets you write and run unit tests as if you were using the built-in #[test] attribute, but they'll run on your embedded target.

Of course, defmt-test also gives you an #[init] attribute for initialization functions needed to set up your peripherals etc.

For more details, check out our blog post introducing defmt-test. Also check our blog post series on testing embedded Rust code.