Since Rust does not yet really work on ARM Cortex or AVR it's still a long way from taking over IoT. Only devices with an OS, like a RaspberryPi, are properly supported by Rust at this point. That's a huge limitation. GCC can target literally hundreds of bare metal devices with C and even C++. Developers have been using C and C++ successfully in IoT devices for years now.
Rust may be the bees knees but it has many hurdles to overcome before it dominates a language that has been around for 46 years.
ARM (thumb 2, Cortex M0-M4) is by far the most important target. Second is cores like Cortex A7 and A53.
I think AVR is pretty minor outside hobbyist circles. After ARM support, I'd rather first see MSP430 (amazing for low power), some FPGA softcores (like NIOS, microblaze, etc., common in low volume, industrial & medical devices) and even 8051 (these buggers seem to still be everywhere, but I guess targeting Rust for this arch is the ultimate challenge.).
I guess RISC-V would be cool as well, I can see this getting more popular in the future, eating ARM market share.
Embedded use cases including ARM Cortex-M and AVR support seems to be in the focus on the Rust 2018 roadmap: "We want embedded programming to reach first-class status this year." https://blog.rust-lang.org/2018/03/12/roadmap.html
Rust may be the bees knees but it has many hurdles to overcome before it dominates a language that has been around for 46 years.