add support for defmt

[newAM]

This pull-request adds optional support for defmt.

defmt, like this crate aims to solve a problem in the core library, namely the code-size of the formatting logic.

[PTaylor-us]

That you for the PR. I'm not yet familiar with defmt, but will definitely take a look. @korken89, thoughts?

[korken89]

From my side I think it's a good addition.
I use defmt in my projects and is a real space saver.

[newAM]

That you for the PR. I'm not yet familiar with defmt, but will definitely take a look. @korken89, thoughts?

For more context defmt is developed under the kurling-rs project by ferrous systems.

Advantages

• Enables logging embedded-time types on targets with limited flash. Formatting a simple u8 with core::fmt takes >2k of code size on its own, on targets with 16k of flash that simply means logging is not realistic without defmt.

Disadvantages/Risks

• defmt becomes unmaintained. I think given the backers and continual development since its inception in November 2020 that this is extremely unlikely.
• Additional maintenance cost. The maintenance cost is very low for defmt; there has only ever been one release with breaking changes (0.1.0 -> 0.2.0) in the almost 2 years it has been around.

[Sh3Rm4n]

Ha, I made a draft for implementing defmt as well, but did not had the time to make a proper PR out of it. 😀

So I took the opportunity to look over your implantation :)

LGTM

[PTaylor-us]

Tests are failing. Looks like some undefined references in the defmt. Are we missing some dependencies?

[Sh3Rm4n]

It seems like only windows is failing and the linux build is successful. This might be because link.exe does not support the linkage symbols, which defmt is injecting, which it is using to store all the information / data to defer the strings.

It might just be that defmt does not support windows.

[newAM]

It does support Windows, just not non-embedded targets (knurling-rs/defmt#463). I am surprised the unit tests ran on Linux at all! I pushed a change to skip the defmt feature for the unit tests.