Yarg-Lang: A Microcontroller Language Project
yarg-lang.dev hosts a language and tooling specifically for developing firmware for Microcontrollers. The Yarg language supports an interpreted REPL, direct hardware access and interrupt and multi-core based multiprocessing. Of course, it includes lots of other modern language features.

Unlike other options, Yarg is not a cut-down, 'micro' or 'tiny' version of some other language, and it can always put the requirements of Microcontrollers first. There is tooling to support firmware image generation, so that code can be optimised and prepared for production.
Currently it is under active development and it has yet to make a formal release for wide use. A demo is available.
Project Sources
The project is hosted at GitHub, and is open source (MIT licence):
Updates and Newsletter
John uses the yarg-lang tag on their blog for posts about Yarg.
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