Embedded CPU vendors not shipping C++ compilers is what usually stops people.
[1] https://doc.rust-lang.org/beta/rustc/platform-support.html
However most of the embedded world uses ARM chips and they are Tier 2 like thumbv6m and thumbv7em (there are still odd ones like 8051 or AVR or m68k, many of them lack a good C++ compiler already). They are guaranteed to be built and at the release time the tests still run for them.
However most of the embedded world uses ARM chips
My point exactly.
Quite frankly I'm not sure why you wouldn't given that most are using GCC on common architectures. The chip vendor doesn't have to do any work unless they are working on an obscure architecture.