I actually had a pretty good experience with programming Arduino Uno using C. I used bare AVR library though, writing code in Vim and compiling and flashing it using the Makefile, so without any fancy Arduino-C functions or IDE.

In fact, I had more troubles with setting the Rust toolchain and environment for AVR, than with finding what optimization parameter in avr-gcc caused my program to break :)

If it would be useful for anyone, here's the project: https://git.visil.gw.lt/arduino-embedded-c.git/
Really not much going on here, just a blinking LED with a handy little Makefile to compile and flash it all.